1 .. SPDX-License-Identifier: GPL-2.0 1 .. SPDX-License-Identifier: GPL-2.0
2 2
3 ========================================== 3 ==========================================
4 WHAT IS Flash-Friendly File System (F2FS)? 4 WHAT IS Flash-Friendly File System (F2FS)?
5 ========================================== 5 ==========================================
6 6
7 NAND flash memory-based storage devices, such 7 NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
8 been equipped on a variety systems ranging fro 8 been equipped on a variety systems ranging from mobile to server systems. Since
9 they are known to have different characteristi 9 they are known to have different characteristics from the conventional rotating
10 disks, a file system, an upper layer to the st 10 disks, a file system, an upper layer to the storage device, should adapt to the
11 changes from the sketch in the design level. 11 changes from the sketch in the design level.
12 12
13 F2FS is a file system exploiting NAND flash me 13 F2FS is a file system exploiting NAND flash memory-based storage devices, which
14 is based on Log-structured File System (LFS). 14 is based on Log-structured File System (LFS). The design has been focused on
15 addressing the fundamental issues in LFS, whic 15 addressing the fundamental issues in LFS, which are snowball effect of wandering
16 tree and high cleaning overhead. 16 tree and high cleaning overhead.
17 17
18 Since a NAND flash memory-based storage device 18 Since a NAND flash memory-based storage device shows different characteristic
19 according to its internal geometry or flash me 19 according to its internal geometry or flash memory management scheme, namely FTL,
20 F2FS and its tools support various parameters 20 F2FS and its tools support various parameters not only for configuring on-disk
21 layout, but also for selecting allocation and 21 layout, but also for selecting allocation and cleaning algorithms.
22 22
23 The following git tree provides the file syste 23 The following git tree provides the file system formatting tool (mkfs.f2fs),
24 a consistency checking tool (fsck.f2fs), and a 24 a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
25 25
26 - git://git.kernel.org/pub/scm/linux/kernel/gi 26 - git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
27 27
28 For sending patches, please use the following 28 For sending patches, please use the following mailing list:
29 29
30 - linux-f2fs-devel@lists.sourceforge.net 30 - linux-f2fs-devel@lists.sourceforge.net
31 31
32 For reporting bugs, please use the following f 32 For reporting bugs, please use the following f2fs bug tracker link:
33 33
34 - https://bugzilla.kernel.org/enter_bug.cgi?pr 34 - https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs
35 35
36 Background and Design issues 36 Background and Design issues
37 ============================ 37 ============================
38 38
39 Log-structured File System (LFS) 39 Log-structured File System (LFS)
40 -------------------------------- 40 --------------------------------
41 "A log-structured file system writes all modif 41 "A log-structured file system writes all modifications to disk sequentially in
42 a log-like structure, thereby speeding up bot 42 a log-like structure, thereby speeding up both file writing and crash recovery.
43 The log is the only structure on disk; it cont 43 The log is the only structure on disk; it contains indexing information so that
44 files can be read back from the log efficientl 44 files can be read back from the log efficiently. In order to maintain large free
45 areas on disk for fast writing, we divide the 45 areas on disk for fast writing, we divide the log into segments and use a
46 segment cleaner to compress the live informati 46 segment cleaner to compress the live information from heavily fragmented
47 segments." from Rosenblum, M. and Ousterhout, 47 segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
48 implementation of a log-structured file system 48 implementation of a log-structured file system", ACM Trans. Computer Systems
49 10, 1, 26–52. 49 10, 1, 26–52.
50 50
51 Wandering Tree Problem 51 Wandering Tree Problem
52 ---------------------- 52 ----------------------
53 In LFS, when a file data is updated and writte 53 In LFS, when a file data is updated and written to the end of log, its direct
54 pointer block is updated due to the changed lo 54 pointer block is updated due to the changed location. Then the indirect pointer
55 block is also updated due to the direct pointe 55 block is also updated due to the direct pointer block update. In this manner,
56 the upper index structures such as inode, inod 56 the upper index structures such as inode, inode map, and checkpoint block are
57 also updated recursively. This problem is call 57 also updated recursively. This problem is called as wandering tree problem [1],
58 and in order to enhance the performance, it sh 58 and in order to enhance the performance, it should eliminate or relax the update
59 propagation as much as possible. 59 propagation as much as possible.
60 60
61 [1] Bityutskiy, A. 2005. JFFS3 design issues. 61 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
62 62
63 Cleaning Overhead 63 Cleaning Overhead
64 ----------------- 64 -----------------
65 Since LFS is based on out-of-place writes, it 65 Since LFS is based on out-of-place writes, it produces so many obsolete blocks
66 scattered across the whole storage. In order t 66 scattered across the whole storage. In order to serve new empty log space, it
67 needs to reclaim these obsolete blocks seamles 67 needs to reclaim these obsolete blocks seamlessly to users. This job is called
68 as a cleaning process. 68 as a cleaning process.
69 69
70 The process consists of three operations as fo 70 The process consists of three operations as follows.
71 71
72 1. A victim segment is selected through refere 72 1. A victim segment is selected through referencing segment usage table.
73 2. It loads parent index structures of all the 73 2. It loads parent index structures of all the data in the victim identified by
74 segment summary blocks. 74 segment summary blocks.
75 3. It checks the cross-reference between the d 75 3. It checks the cross-reference between the data and its parent index structure.
76 4. It moves valid data selectively. 76 4. It moves valid data selectively.
77 77
78 This cleaning job may cause unexpected long de 78 This cleaning job may cause unexpected long delays, so the most important goal
79 is to hide the latencies to users. And also de 79 is to hide the latencies to users. And also definitely, it should reduce the
80 amount of valid data to be moved, and move the 80 amount of valid data to be moved, and move them quickly as well.
81 81
82 Key Features 82 Key Features
83 ============ 83 ============
84 84
85 Flash Awareness 85 Flash Awareness
86 --------------- 86 ---------------
87 - Enlarge the random write area for better per 87 - Enlarge the random write area for better performance, but provide the high
88 spatial locality 88 spatial locality
89 - Align FS data structures to the operational 89 - Align FS data structures to the operational units in FTL as best efforts
90 90
91 Wandering Tree Problem 91 Wandering Tree Problem
92 ---------------------- 92 ----------------------
93 - Use a term, “node”, that represents inod 93 - Use a term, “node”, that represents inodes as well as various pointer blocks
94 - Introduce Node Address Table (NAT) containin 94 - Introduce Node Address Table (NAT) containing the locations of all the “node”
95 blocks; this will cut off the update propaga 95 blocks; this will cut off the update propagation.
96 96
97 Cleaning Overhead 97 Cleaning Overhead
98 ----------------- 98 -----------------
99 - Support a background cleaning process 99 - Support a background cleaning process
100 - Support greedy and cost-benefit algorithms f 100 - Support greedy and cost-benefit algorithms for victim selection policies
101 - Support multi-head logs for static/dynamic h 101 - Support multi-head logs for static/dynamic hot and cold data separation
102 - Introduce adaptive logging for efficient blo 102 - Introduce adaptive logging for efficient block allocation
103 103
104 Mount Options 104 Mount Options
105 ============= 105 =============
106 106
107 107
108 ======================== ===================== 108 ======================== ============================================================
109 background_gc=%s Turn on/off cleaning 109 background_gc=%s Turn on/off cleaning operations, namely garbage
110 collection, triggered 110 collection, triggered in background when I/O subsystem is
111 idle. If background_g 111 idle. If background_gc=on, it will turn on the garbage
112 collection and if bac 112 collection and if background_gc=off, garbage collection
113 will be turned off. I 113 will be turned off. If background_gc=sync, it will turn
114 on synchronous garbag 114 on synchronous garbage collection running in background.
115 Default value for thi 115 Default value for this option is on. So garbage
116 collection is on by d 116 collection is on by default.
117 gc_merge When background_gc is 117 gc_merge When background_gc is on, this option can be enabled to
118 let background GC thr 118 let background GC thread to handle foreground GC requests,
119 it can eliminate the 119 it can eliminate the sluggish issue caused by slow foreground
120 GC operation when GC 120 GC operation when GC is triggered from a process with limited
121 I/O and CPU resources 121 I/O and CPU resources.
122 nogc_merge Disable GC merge feat 122 nogc_merge Disable GC merge feature.
123 disable_roll_forward Disable the roll-forw 123 disable_roll_forward Disable the roll-forward recovery routine
124 norecovery Disable the roll-forw 124 norecovery Disable the roll-forward recovery routine, mounted read-
125 only (i.e., -o ro,dis 125 only (i.e., -o ro,disable_roll_forward)
126 discard/nodiscard Enable/disable real-t 126 discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
127 enabled, f2fs will is 127 enabled, f2fs will issue discard/TRIM commands when a
128 segment is cleaned. 128 segment is cleaned.
129 heap/no_heap Deprecated. !! 129 no_heap Disable heap-style segment allocation which finds free
>> 130 segments for data from the beginning of main area, while
>> 131 for node from the end of main area.
130 nouser_xattr Disable Extended User 132 nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
131 by default if CONFIG_ 133 by default if CONFIG_F2FS_FS_XATTR is selected.
132 noacl Disable POSIX Access 134 noacl Disable POSIX Access Control List. Note: acl is enabled
133 by default if CONFIG_ 135 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
134 active_logs=%u Support configuring t 136 active_logs=%u Support configuring the number of active logs. In the
135 current design, f2fs 137 current design, f2fs supports only 2, 4, and 6 logs.
136 Default number is 6. 138 Default number is 6.
137 disable_ext_identify Disable the extension 139 disable_ext_identify Disable the extension list configured by mkfs, so f2fs
138 is not aware of cold 140 is not aware of cold files such as media files.
139 inline_xattr Enable the inline xat 141 inline_xattr Enable the inline xattrs feature.
140 noinline_xattr Disable the inline xa 142 noinline_xattr Disable the inline xattrs feature.
141 inline_xattr_size=%u Support configuring i 143 inline_xattr_size=%u Support configuring inline xattr size, it depends on
142 flexible inline xattr 144 flexible inline xattr feature.
143 inline_data Enable the inline dat 145 inline_data Enable the inline data feature: Newly created small (<~3.4k)
144 files can be written 146 files can be written into inode block.
145 inline_dentry Enable the inline dir 147 inline_dentry Enable the inline dir feature: data in newly created
146 directory entries can 148 directory entries can be written into inode block. The
147 space of inode block 149 space of inode block which is used to store inline
148 dentries is limited t 150 dentries is limited to ~3.4k.
149 noinline_dentry Disable the inline de 151 noinline_dentry Disable the inline dentry feature.
150 flush_merge Merge concurrent cach 152 flush_merge Merge concurrent cache_flush commands as much as possible
151 to eliminate redundan 153 to eliminate redundant command issues. If the underlying
152 device handles the ca 154 device handles the cache_flush command relatively slowly,
153 recommend to enable t 155 recommend to enable this option.
154 nobarrier This option can be us 156 nobarrier This option can be used if underlying storage guarantees
155 its cached data shoul 157 its cached data should be written to the novolatile area.
156 If this option is set 158 If this option is set, no cache_flush commands are issued
157 but f2fs still guaran 159 but f2fs still guarantees the write ordering of all the
158 data writes. 160 data writes.
159 barrier If this option is set 161 barrier If this option is set, cache_flush commands are allowed to be
160 issued. 162 issued.
161 fastboot This option is used w 163 fastboot This option is used when a system wants to reduce mount
162 time as much as possi 164 time as much as possible, even though normal performance
163 can be sacrificed. 165 can be sacrificed.
164 extent_cache Enable an extent cach 166 extent_cache Enable an extent cache based on rb-tree, it can cache
165 as many as extent whi 167 as many as extent which map between contiguous logical
166 address and physical 168 address and physical address per inode, resulting in
167 increasing the cache 169 increasing the cache hit ratio. Set by default.
168 noextent_cache Disable an extent cac 170 noextent_cache Disable an extent cache based on rb-tree explicitly, see
169 the above extent_cach 171 the above extent_cache mount option.
170 noinline_data Disable the inline da 172 noinline_data Disable the inline data feature, inline data feature is
171 enabled by default. 173 enabled by default.
172 data_flush Enable data flushing 174 data_flush Enable data flushing before checkpoint in order to
173 persist data of regul 175 persist data of regular and symlink.
174 reserve_root=%d Support configuring r 176 reserve_root=%d Support configuring reserved space which is used for
175 allocation from a pri 177 allocation from a privileged user with specified uid or
176 gid, unit: 4KB, the d 178 gid, unit: 4KB, the default limit is 0.2% of user blocks.
177 resuid=%d The user ID which may 179 resuid=%d The user ID which may use the reserved blocks.
178 resgid=%d The group ID which ma 180 resgid=%d The group ID which may use the reserved blocks.
179 fault_injection=%d Enable fault injectio 181 fault_injection=%d Enable fault injection in all supported types with
180 specified injection r 182 specified injection rate.
181 fault_type=%d Support configuring f 183 fault_type=%d Support configuring fault injection type, should be
182 enabled with fault_in 184 enabled with fault_injection option, fault type value
183 is shown below, it su 185 is shown below, it supports single or combined type.
184 186
185 ===================== !! 187 =================== ===========
186 Type_Name !! 188 Type_Name Type_Value
187 ===================== !! 189 =================== ===========
188 FAULT_KMALLOC !! 190 FAULT_KMALLOC 0x000000001
189 FAULT_KVMALLOC !! 191 FAULT_KVMALLOC 0x000000002
190 FAULT_PAGE_ALLOC !! 192 FAULT_PAGE_ALLOC 0x000000004
191 FAULT_PAGE_GET !! 193 FAULT_PAGE_GET 0x000000008
192 FAULT_ALLOC_BIO !! 194 FAULT_ALLOC_BIO 0x000000010 (obsolete)
193 FAULT_ALLOC_NID !! 195 FAULT_ALLOC_NID 0x000000020
194 FAULT_ORPHAN !! 196 FAULT_ORPHAN 0x000000040
195 FAULT_BLOCK !! 197 FAULT_BLOCK 0x000000080
196 FAULT_DIR_DEPTH !! 198 FAULT_DIR_DEPTH 0x000000100
197 FAULT_EVICT_INODE !! 199 FAULT_EVICT_INODE 0x000000200
198 FAULT_TRUNCATE !! 200 FAULT_TRUNCATE 0x000000400
199 FAULT_READ_IO !! 201 FAULT_READ_IO 0x000000800
200 FAULT_CHECKPOINT !! 202 FAULT_CHECKPOINT 0x000001000
201 FAULT_DISCARD !! 203 FAULT_DISCARD 0x000002000
202 FAULT_WRITE_IO !! 204 FAULT_WRITE_IO 0x000004000
203 FAULT_SLAB_ALLOC !! 205 FAULT_SLAB_ALLOC 0x000008000
204 FAULT_DQUOT_INIT !! 206 FAULT_DQUOT_INIT 0x000010000
205 FAULT_LOCK_OP !! 207 FAULT_LOCK_OP 0x000020000
206 FAULT_BLKADDR_VALIDIT !! 208 FAULT_BLKADDR 0x000040000
207 FAULT_BLKADDR_CONSIST !! 209 =================== ===========
208 FAULT_NO_SEGMENT <<
209 ===================== <<
210 mode=%s Control block allocat 210 mode=%s Control block allocation mode which supports "adaptive"
211 and "lfs". In "lfs" m 211 and "lfs". In "lfs" mode, there should be no random
212 writes towards main a 212 writes towards main area.
213 "fragment:segment" an 213 "fragment:segment" and "fragment:block" are newly added here.
214 These are developer o 214 These are developer options for experiments to simulate filesystem
215 fragmentation/after-G 215 fragmentation/after-GC situation itself. The developers use these
216 modes to understand f 216 modes to understand filesystem fragmentation/after-GC condition well,
217 and eventually get so 217 and eventually get some insights to handle them better.
218 In "fragment:segment" 218 In "fragment:segment", f2fs allocates a new segment in ramdom
219 position. With this, 219 position. With this, we can simulate the after-GC condition.
220 In "fragment:block", 220 In "fragment:block", we can scatter block allocation with
221 "max_fragment_chunk" 221 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes.
222 We added some randomn 222 We added some randomness to both chunk and hole size to make
223 it close to realistic 223 it close to realistic IO pattern. So, in this mode, f2fs will allocate
224 1..<max_fragment_chun 224 1..<max_fragment_chunk> blocks in a chunk and make a hole in the
225 length of 1..<max_fra 225 length of 1..<max_fragment_hole> by turns. With this, the newly
226 allocated blocks will 226 allocated blocks will be scattered throughout the whole partition.
227 Note that "fragment:b 227 Note that "fragment:block" implicitly enables "fragment:segment"
228 option for more rando 228 option for more randomness.
229 Please, use these opt 229 Please, use these options for your experiments and we strongly
230 recommend to re-forma 230 recommend to re-format the filesystem after using these options.
>> 231 io_bits=%u Set the bit size of write IO requests. It should be set
>> 232 with "mode=lfs".
231 usrquota Enable plain user dis 233 usrquota Enable plain user disk quota accounting.
232 grpquota Enable plain group di 234 grpquota Enable plain group disk quota accounting.
233 prjquota Enable plain project 235 prjquota Enable plain project quota accounting.
234 usrjquota=<file> Appoint specified fil 236 usrjquota=<file> Appoint specified file and type during mount, so that quota
235 grpjquota=<file> information can be pr 237 grpjquota=<file> information can be properly updated during recovery flow,
236 prjjquota=<file> <quota file>: must be 238 prjjquota=<file> <quota file>: must be in root directory;
237 jqfmt=<quota type> <quota type>: [vfsold 239 jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
238 offusrjquota Turn off user journal 240 offusrjquota Turn off user journalled quota.
239 offgrpjquota Turn off group journa 241 offgrpjquota Turn off group journalled quota.
240 offprjjquota Turn off project jour 242 offprjjquota Turn off project journalled quota.
241 quota Enable plain user dis 243 quota Enable plain user disk quota accounting.
242 noquota Disable all plain dis 244 noquota Disable all plain disk quota option.
243 alloc_mode=%s Adjust block allocati 245 alloc_mode=%s Adjust block allocation policy, which supports "reuse"
244 and "default". 246 and "default".
245 fsync_mode=%s Control the policy of 247 fsync_mode=%s Control the policy of fsync. Currently supports "posix",
246 "strict", and "nobarr 248 "strict", and "nobarrier". In "posix" mode, which is
247 default, fsync will f 249 default, fsync will follow POSIX semantics and does a
248 light operation to im 250 light operation to improve the filesystem performance.
249 In "strict" mode, fsy 251 In "strict" mode, fsync will be heavy and behaves in line
250 with xfs, ext4 and bt 252 with xfs, ext4 and btrfs, where xfstest generic/342 will
251 pass, but the perform 253 pass, but the performance will regress. "nobarrier" is
252 based on "posix", but 254 based on "posix", but doesn't issue flush command for
253 non-atomic files like 255 non-atomic files likewise "nobarrier" mount option.
254 test_dummy_encryption 256 test_dummy_encryption
255 test_dummy_encryption=%s 257 test_dummy_encryption=%s
256 Enable dummy encrypti 258 Enable dummy encryption, which provides a fake fscrypt
257 context. The fake fsc 259 context. The fake fscrypt context is used by xfstests.
258 The argument may be e 260 The argument may be either "v1" or "v2", in order to
259 select the correspond 261 select the corresponding fscrypt policy version.
260 checkpoint=%s[:%u[%]] Set to "disable" to t 262 checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable"
261 to reenable checkpoin 263 to reenable checkpointing. Is enabled by default. While
262 disabled, any unmount 264 disabled, any unmounting or unexpected shutdowns will cause
263 the filesystem conten 265 the filesystem contents to appear as they did when the
264 filesystem was mounte 266 filesystem was mounted with that option.
265 While mounting with c 267 While mounting with checkpoint=disable, the filesystem must
266 run garbage collectio 268 run garbage collection to ensure that all available space can
267 be used. If this take 269 be used. If this takes too much time, the mount may return
268 EAGAIN. You may optio 270 EAGAIN. You may optionally add a value to indicate how much
269 of the disk you would 271 of the disk you would be willing to temporarily give up to
270 avoid additional garb 272 avoid additional garbage collection. This can be given as a
271 number of blocks, or 273 number of blocks, or as a percent. For instance, mounting
272 with checkpoint=disab 274 with checkpoint=disable:100% would always succeed, but it may
273 hide up to all remain 275 hide up to all remaining free space. The actual space that
274 would be unusable can 276 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
275 This space is reclaim 277 This space is reclaimed once checkpoint=enable.
276 checkpoint_merge When checkpoint is en 278 checkpoint_merge When checkpoint is enabled, this can be used to create a kernel
277 daemon and make it to 279 daemon and make it to merge concurrent checkpoint requests as
278 much as possible to e 280 much as possible to eliminate redundant checkpoint issues. Plus,
279 we can eliminate the 281 we can eliminate the sluggish issue caused by slow checkpoint
280 operation when the ch 282 operation when the checkpoint is done in a process context in
281 a cgroup having low i 283 a cgroup having low i/o budget and cpu shares. To make this
282 do better, we set the 284 do better, we set the default i/o priority of the kernel daemon
283 to "3", to give one h 285 to "3", to give one higher priority than other kernel threads.
284 This is the same way 286 This is the same way to give a I/O priority to the jbd2
285 journaling thread of 287 journaling thread of ext4 filesystem.
286 nocheckpoint_merge Disable checkpoint me 288 nocheckpoint_merge Disable checkpoint merge feature.
287 compress_algorithm=%s Control compress algo 289 compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo",
288 "lz4", "zstd" and "lz 290 "lz4", "zstd" and "lzo-rle" algorithm.
289 compress_algorithm=%s:%d Control compress algo 291 compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only
290 "lz4" and "zstd" supp 292 "lz4" and "zstd" support compress level config.
291 algorithm level 293 algorithm level range
292 lz4 3 - 16 294 lz4 3 - 16
293 zstd 1 - 22 295 zstd 1 - 22
294 compress_log_size=%u Support configuring c 296 compress_log_size=%u Support configuring compress cluster size. The size will
295 be 4KB * (1 << %u). T 297 be 4KB * (1 << %u). The default and minimum sizes are 16KB.
296 compress_extension=%s Support adding specif 298 compress_extension=%s Support adding specified extension, so that f2fs can enable
297 compression on those 299 compression on those corresponding files, e.g. if all files
298 with '.ext' has high 300 with '.ext' has high compression rate, we can set the '.ext'
299 on compression extens 301 on compression extension list and enable compression on
300 these file by default 302 these file by default rather than to enable it via ioctl.
301 For other files, we c 303 For other files, we can still enable compression via ioctl.
302 Note that, there is o 304 Note that, there is one reserved special extension '*', it
303 can be set to enable 305 can be set to enable compression for all files.
304 nocompress_extension=%s Support adding specif 306 nocompress_extension=%s Support adding specified extension, so that f2fs can disable
305 compression on those 307 compression on those corresponding files, just contrary to compression extension.
306 If you know exactly w 308 If you know exactly which files cannot be compressed, you can use this.
307 The same extension na 309 The same extension name can't appear in both compress and nocompress
308 extension at the same 310 extension at the same time.
309 If the compress exten 311 If the compress extension specifies all files, the types specified by the
310 nocompress extension 312 nocompress extension will be treated as special cases and will not be compressed.
311 Don't allow use '*' t 313 Don't allow use '*' to specifie all file in nocompress extension.
312 After add nocompress_ 314 After add nocompress_extension, the priority should be:
313 dir_flag < comp_exten 315 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag.
314 See more in compressi 316 See more in compression sections.
315 317
316 compress_chksum Support verifying chk 318 compress_chksum Support verifying chksum of raw data in compressed cluster.
317 compress_mode=%s Control file compress 319 compress_mode=%s Control file compression mode. This supports "fs" and "user"
318 modes. In "fs" mode ( 320 modes. In "fs" mode (default), f2fs does automatic compression
319 on the compression en 321 on the compression enabled files. In "user" mode, f2fs disables
320 the automaic compress 322 the automaic compression and gives the user discretion of
321 choosing the target f 323 choosing the target file and the timing. The user can do manual
322 compression/decompres 324 compression/decompression on the compression enabled files using
323 ioctls. 325 ioctls.
324 compress_cache Support to use addres 326 compress_cache Support to use address space of a filesystem managed inode to
325 cache compressed bloc 327 cache compressed block, in order to improve cache hit ratio of
326 random read. 328 random read.
327 inlinecrypt When possible, encryp 329 inlinecrypt When possible, encrypt/decrypt the contents of encrypted
328 files using the blk-c 330 files using the blk-crypto framework rather than
329 filesystem-layer encr 331 filesystem-layer encryption. This allows the use of
330 inline encryption har 332 inline encryption hardware. The on-disk format is
331 unaffected. For more 333 unaffected. For more details, see
332 Documentation/block/i 334 Documentation/block/inline-encryption.rst.
333 atgc Enable age-threshold 335 atgc Enable age-threshold garbage collection, it provides high
334 effectiveness and eff 336 effectiveness and efficiency on background GC.
335 discard_unit=%s Control discard unit, 337 discard_unit=%s Control discard unit, the argument can be "block", "segment"
336 and "section", issued 338 and "section", issued discard command's offset/size will be
337 aligned to the unit, 339 aligned to the unit, by default, "discard_unit=block" is set,
338 so that small discard 340 so that small discard functionality is enabled.
339 For blkzoned device, 341 For blkzoned device, "discard_unit=section" will be set by
340 default, it is helpfu 342 default, it is helpful for large sized SMR or ZNS devices to
341 reduce memory cost by 343 reduce memory cost by getting rid of fs metadata supports small
342 discard. 344 discard.
343 memory=%s Control memory mode. 345 memory=%s Control memory mode. This supports "normal" and "low" modes.
344 "low" mode is introdu 346 "low" mode is introduced to support low memory devices.
345 Because of the nature 347 Because of the nature of low memory devices, in this mode, f2fs
346 will try to save memo 348 will try to save memory sometimes by sacrificing performance.
347 "normal" mode is the 349 "normal" mode is the default mode and same as before.
348 age_extent_cache Enable an age extent 350 age_extent_cache Enable an age extent cache based on rb-tree. It records
349 data block update fre 351 data block update frequency of the extent per inode, in
350 order to provide bett 352 order to provide better temperature hints for data block
351 allocation. 353 allocation.
352 errors=%s Specify f2fs behavior 354 errors=%s Specify f2fs behavior on critical errors. This supports modes:
353 "panic", "continue" a 355 "panic", "continue" and "remount-ro", respectively, trigger
354 panic immediately, co 356 panic immediately, continue without doing anything, and remount
355 the partition in read 357 the partition in read-only mode. By default it uses "continue"
356 mode. 358 mode.
357 ===================== 359 ====================== =============== =============== ========
358 mode 360 mode continue remount-ro panic
359 ===================== 361 ====================== =============== =============== ========
360 access ops 362 access ops normal normal N/A
361 syscall errors 363 syscall errors -EIO -EROFS N/A
362 mount option 364 mount option rw ro N/A
363 pending dir write 365 pending dir write keep keep N/A
364 pending non-dir write 366 pending non-dir write drop keep N/A
365 pending node write 367 pending node write drop keep N/A
366 pending meta write 368 pending meta write keep keep N/A
367 ===================== 369 ====================== =============== =============== ========
368 ======================== ===================== 370 ======================== ============================================================
369 371
370 Debugfs Entries 372 Debugfs Entries
371 =============== 373 ===============
372 374
373 /sys/kernel/debug/f2fs/ contains information a 375 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
374 f2fs. Each file shows the whole f2fs informati 376 f2fs. Each file shows the whole f2fs information.
375 377
376 /sys/kernel/debug/f2fs/status includes: 378 /sys/kernel/debug/f2fs/status includes:
377 379
378 - major file system information managed by f2 380 - major file system information managed by f2fs currently
379 - average SIT information about whole segment 381 - average SIT information about whole segments
380 - current memory footprint consumed by f2fs. 382 - current memory footprint consumed by f2fs.
381 383
382 Sysfs Entries 384 Sysfs Entries
383 ============= 385 =============
384 386
385 Information about mounted f2fs file systems ca 387 Information about mounted f2fs file systems can be found in
386 /sys/fs/f2fs. Each mounted filesystem will ha 388 /sys/fs/f2fs. Each mounted filesystem will have a directory in
387 /sys/fs/f2fs based on its device name (i.e., / 389 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
388 The files in each per-device directory are sho 390 The files in each per-device directory are shown in table below.
389 391
390 Files in /sys/fs/f2fs/<devname> 392 Files in /sys/fs/f2fs/<devname>
391 (see also Documentation/ABI/testing/sysfs-fs-f 393 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
392 394
393 Usage 395 Usage
394 ===== 396 =====
395 397
396 1. Download userland tools and compile them. 398 1. Download userland tools and compile them.
397 399
398 2. Skip, if f2fs was compiled statically insid 400 2. Skip, if f2fs was compiled statically inside kernel.
399 Otherwise, insert the f2fs.ko module:: 401 Otherwise, insert the f2fs.ko module::
400 402
401 # insmod f2fs.ko 403 # insmod f2fs.ko
402 404
403 3. Create a directory to use when mounting:: 405 3. Create a directory to use when mounting::
404 406
405 # mkdir /mnt/f2fs 407 # mkdir /mnt/f2fs
406 408
407 4. Format the block device, and then mount as 409 4. Format the block device, and then mount as f2fs::
408 410
409 # mkfs.f2fs -l label /dev/block_device 411 # mkfs.f2fs -l label /dev/block_device
410 # mount -t f2fs /dev/block_device /mnt 412 # mount -t f2fs /dev/block_device /mnt/f2fs
411 413
412 mkfs.f2fs 414 mkfs.f2fs
413 --------- 415 ---------
414 The mkfs.f2fs is for the use of formatting a p 416 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
415 which builds a basic on-disk layout. 417 which builds a basic on-disk layout.
416 418
417 The quick options consist of: 419 The quick options consist of:
418 420
419 =============== =========================== 421 =============== ===========================================================
420 ``-l [label]`` Give a volume label, up to 422 ``-l [label]`` Give a volume label, up to 512 unicode name.
421 ``-a [0 or 1]`` Split start location of eac 423 ``-a [0 or 1]`` Split start location of each area for heap-based allocation.
422 424
423 1 is set by default, which 425 1 is set by default, which performs this.
424 ``-o [int]`` Set overprovision ratio in 426 ``-o [int]`` Set overprovision ratio in percent over volume size.
425 427
426 5 is set by default. 428 5 is set by default.
427 ``-s [int]`` Set the number of segments 429 ``-s [int]`` Set the number of segments per section.
428 430
429 1 is set by default. 431 1 is set by default.
430 ``-z [int]`` Set the number of sections 432 ``-z [int]`` Set the number of sections per zone.
431 433
432 1 is set by default. 434 1 is set by default.
433 ``-e [str]`` Set basic extension list. e 435 ``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov"
434 ``-t [0 or 1]`` Disable discard command or 436 ``-t [0 or 1]`` Disable discard command or not.
435 437
436 1 is set by default, which 438 1 is set by default, which conducts discard.
437 =============== =========================== 439 =============== ===========================================================
438 440
439 Note: please refer to the manpage of mkfs.f2fs 441 Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.
440 442
441 fsck.f2fs 443 fsck.f2fs
442 --------- 444 ---------
443 The fsck.f2fs is a tool to check the consisten 445 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
444 partition, which examines whether the filesyst 446 partition, which examines whether the filesystem metadata and user-made data
445 are cross-referenced correctly or not. 447 are cross-referenced correctly or not.
446 Note that, initial version of the tool does no 448 Note that, initial version of the tool does not fix any inconsistency.
447 449
448 The quick options consist of:: 450 The quick options consist of::
449 451
450 -d debug level [default:0] 452 -d debug level [default:0]
451 453
452 Note: please refer to the manpage of fsck.f2fs 454 Note: please refer to the manpage of fsck.f2fs(8) to get full option list.
453 455
454 dump.f2fs 456 dump.f2fs
455 --------- 457 ---------
456 The dump.f2fs shows the information of specifi 458 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
457 file. Each file is dump_ssa and dump_sit. 459 file. Each file is dump_ssa and dump_sit.
458 460
459 The dump.f2fs is used to debug on-disk data st 461 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
460 It shows on-disk inode information recognized 462 It shows on-disk inode information recognized by a given inode number, and is
461 able to dump all the SSA and SIT entries into 463 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
462 ./dump_sit respectively. 464 ./dump_sit respectively.
463 465
464 The options consist of:: 466 The options consist of::
465 467
466 -d debug level [default:0] 468 -d debug level [default:0]
467 -i inode no (hex) 469 -i inode no (hex)
468 -s [SIT dump segno from #1~#2 (decimal), for 470 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
469 -a [SSA dump segno from #1~#2 (decimal), for 471 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
470 472
471 Examples:: 473 Examples::
472 474
473 # dump.f2fs -i [ino] /dev/sdx 475 # dump.f2fs -i [ino] /dev/sdx
474 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 476 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
475 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 477 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
476 478
477 Note: please refer to the manpage of dump.f2fs 479 Note: please refer to the manpage of dump.f2fs(8) to get full option list.
478 480
479 sload.f2fs 481 sload.f2fs
480 ---------- 482 ----------
481 The sload.f2fs gives a way to insert files and 483 The sload.f2fs gives a way to insert files and directories in the existing disk
482 image. This tool is useful when building f2fs 484 image. This tool is useful when building f2fs images given compiled files.
483 485
484 Note: please refer to the manpage of sload.f2f 486 Note: please refer to the manpage of sload.f2fs(8) to get full option list.
485 487
486 resize.f2fs 488 resize.f2fs
487 ----------- 489 -----------
488 The resize.f2fs lets a user resize the f2fs-fo 490 The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
489 all the files and directories stored in the im 491 all the files and directories stored in the image.
490 492
491 Note: please refer to the manpage of resize.f2 493 Note: please refer to the manpage of resize.f2fs(8) to get full option list.
492 494
493 defrag.f2fs 495 defrag.f2fs
494 ----------- 496 -----------
495 The defrag.f2fs can be used to defragment scat 497 The defrag.f2fs can be used to defragment scattered written data as well as
496 filesystem metadata across the disk. This can 498 filesystem metadata across the disk. This can improve the write speed by giving
497 more free consecutive space. 499 more free consecutive space.
498 500
499 Note: please refer to the manpage of defrag.f2 501 Note: please refer to the manpage of defrag.f2fs(8) to get full option list.
500 502
501 f2fs_io 503 f2fs_io
502 ------- 504 -------
503 The f2fs_io is a simple tool to issue various 505 The f2fs_io is a simple tool to issue various filesystem APIs as well as
504 f2fs-specific ones, which is very useful for Q 506 f2fs-specific ones, which is very useful for QA tests.
505 507
506 Note: please refer to the manpage of f2fs_io(8 508 Note: please refer to the manpage of f2fs_io(8) to get full option list.
507 509
508 Design 510 Design
509 ====== 511 ======
510 512
511 On-disk Layout 513 On-disk Layout
512 -------------- 514 --------------
513 515
514 F2FS divides the whole volume into a number of 516 F2FS divides the whole volume into a number of segments, each of which is fixed
515 to 2MB in size. A section is composed of conse 517 to 2MB in size. A section is composed of consecutive segments, and a zone
516 consists of a set of sections. By default, sec 518 consists of a set of sections. By default, section and zone sizes are set to one
517 segment size identically, but users can easily 519 segment size identically, but users can easily modify the sizes by mkfs.
518 520
519 F2FS splits the entire volume into six areas, 521 F2FS splits the entire volume into six areas, and all the areas except superblock
520 consist of multiple segments as described belo 522 consist of multiple segments as described below::
521 523
522 al 524 align with the zone size <-|
523 |-> align with the segment si 525 |-> align with the segment size
524 _________________________________________ 526 _________________________________________________________________________
525 | | | Segment | 527 | | | Segment | Node | Segment | |
526 | Superblock | Checkpoint | Info. | 528 | Superblock | Checkpoint | Info. | Address | Summary | Main |
527 | (SB) | (CP) | Table (SIT) | 529 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
528 |____________|_____2______|______N______|_ 530 |____________|_____2______|______N______|______N______|______N_____|__N___|
529 531 . .
530 532 . .
531 533 . .
532 ._________ 534 ._________________________________________.
533 |_Segment_ 535 |_Segment_|_..._|_Segment_|_..._|_Segment_|
534 . 536 . .
535 ._________ 537 ._________._________
536 |_section_ 538 |_section_|__...__|_
537 . 539 . .
538 .________. 540 .________.
539 |__zone__| 541 |__zone__|
540 542
541 - Superblock (SB) 543 - Superblock (SB)
542 It is located at the beginning of the parti 544 It is located at the beginning of the partition, and there exist two copies
543 to avoid file system crash. It contains bas 545 to avoid file system crash. It contains basic partition information and some
544 default parameters of f2fs. 546 default parameters of f2fs.
545 547
546 - Checkpoint (CP) 548 - Checkpoint (CP)
547 It contains file system information, bitmap 549 It contains file system information, bitmaps for valid NAT/SIT sets, orphan
548 inode lists, and summary entries of current 550 inode lists, and summary entries of current active segments.
549 551
550 - Segment Information Table (SIT) 552 - Segment Information Table (SIT)
551 It contains segment information such as val 553 It contains segment information such as valid block count and bitmap for the
552 validity of all the blocks. 554 validity of all the blocks.
553 555
554 - Node Address Table (NAT) 556 - Node Address Table (NAT)
555 It is composed of a block address table for 557 It is composed of a block address table for all the node blocks stored in
556 Main area. 558 Main area.
557 559
558 - Segment Summary Area (SSA) 560 - Segment Summary Area (SSA)
559 It contains summary entries which contains 561 It contains summary entries which contains the owner information of all the
560 data and node blocks stored in Main area. 562 data and node blocks stored in Main area.
561 563
562 - Main Area 564 - Main Area
563 It contains file and directory data includi 565 It contains file and directory data including their indices.
564 566
565 In order to avoid misalignment between file sy 567 In order to avoid misalignment between file system and flash-based storage, F2FS
566 aligns the start block address of CP with the 568 aligns the start block address of CP with the segment size. Also, it aligns the
567 start block address of Main area with the zone 569 start block address of Main area with the zone size by reserving some segments
568 in SSA area. 570 in SSA area.
569 571
570 Reference the following survey for additional 572 Reference the following survey for additional technical details.
571 https://wiki.linaro.org/WorkingGroups/Kernel/P 573 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
572 574
573 File System Metadata Structure 575 File System Metadata Structure
574 ------------------------------ 576 ------------------------------
575 577
576 F2FS adopts the checkpointing scheme to mainta 578 F2FS adopts the checkpointing scheme to maintain file system consistency. At
577 mount time, F2FS first tries to find the last 579 mount time, F2FS first tries to find the last valid checkpoint data by scanning
578 CP area. In order to reduce the scanning time, 580 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
579 One of them always indicates the last valid da 581 One of them always indicates the last valid data, which is called as shadow copy
580 mechanism. In addition to CP, NAT and SIT also 582 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
581 583
582 For file system consistency, each CP points to 584 For file system consistency, each CP points to which NAT and SIT copies are
583 valid, as shown as below:: 585 valid, as shown as below::
584 586
585 +--------+----------+---------+ 587 +--------+----------+---------+
586 | CP | SIT | NAT | 588 | CP | SIT | NAT |
587 +--------+----------+---------+ 589 +--------+----------+---------+
588 . . . . 590 . . . .
589 . . . . 591 . . . .
590 . . . 592 . . . .
591 +-------+-------+--------+--------+--------+ 593 +-------+-------+--------+--------+--------+--------+
592 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | 594 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
593 +-------+-------+--------+--------+--------+ 595 +-------+-------+--------+--------+--------+--------+
594 | ^ 596 | ^ ^
595 | | 597 | | |
596 `---------------------------------------- 598 `----------------------------------------'
597 599
598 Index Structure 600 Index Structure
599 --------------- 601 ---------------
600 602
601 The key data structure to manage the data loca 603 The key data structure to manage the data locations is a "node". Similar to
602 traditional file structures, F2FS has three ty 604 traditional file structures, F2FS has three types of node: inode, direct node,
603 indirect node. F2FS assigns 4KB to an inode bl 605 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
604 indices, two direct node pointers, two indirec 606 indices, two direct node pointers, two indirect node pointers, and one double
605 indirect node pointer as described below. One 607 indirect node pointer as described below. One direct node block contains 1018
606 data blocks, and one indirect node block conta 608 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
607 one inode block (i.e., a file) covers:: 609 one inode block (i.e., a file) covers::
608 610
609 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 10 611 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
610 612
611 Inode block (4KB) 613 Inode block (4KB)
612 |- data (923) 614 |- data (923)
613 |- direct node (2) 615 |- direct node (2)
614 | `- data (1018) 616 | `- data (1018)
615 |- indirect node (2) 617 |- indirect node (2)
616 | `- direct node (1018) 618 | `- direct node (1018)
617 | `- data (1018) 619 | `- data (1018)
618 `- double indirect node (1) 620 `- double indirect node (1)
619 `- indirect node (101 621 `- indirect node (1018)
620 `- direc 622 `- direct node (1018)
621 623 `- data (1018)
622 624
623 Note that all the node blocks are mapped by NA 625 Note that all the node blocks are mapped by NAT which means the location of
624 each node is translated by the NAT table. In t 626 each node is translated by the NAT table. In the consideration of the wandering
625 tree problem, F2FS is able to cut off the prop 627 tree problem, F2FS is able to cut off the propagation of node updates caused by
626 leaf data writes. 628 leaf data writes.
627 629
628 Directory Structure 630 Directory Structure
629 ------------------- 631 -------------------
630 632
631 A directory entry occupies 11 bytes, which con 633 A directory entry occupies 11 bytes, which consists of the following attributes.
632 634
633 - hash hash value of the file name 635 - hash hash value of the file name
634 - ino inode number 636 - ino inode number
635 - len the length of file name 637 - len the length of file name
636 - type file type such as directory, s 638 - type file type such as directory, symlink, etc
637 639
638 A dentry block consists of 214 dentry slots an 640 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
639 used to represent whether each dentry is valid 641 used to represent whether each dentry is valid or not. A dentry block occupies
640 4KB with the following composition. 642 4KB with the following composition.
641 643
642 :: 644 ::
643 645
644 Dentry Block(4 K) = bitmap (27 bytes) + rese 646 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
645 dentries(11 * 214 bytes) 647 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
646 648
647 [Bucket] 649 [Bucket]
648 +-------------------------------- 650 +--------------------------------+
649 |dentry block 1 | dentry block 2 651 |dentry block 1 | dentry block 2 |
650 +-------------------------------- 652 +--------------------------------+
651 . . 653 . .
652 . . 654 . .
653 . [Dentry Block Structure: 4KB] 655 . [Dentry Block Structure: 4KB] .
654 +--------+----------+----------+------------ 656 +--------+----------+----------+------------+
655 | bitmap | reserved | dentries | file names 657 | bitmap | reserved | dentries | file names |
656 +--------+----------+----------+------------ 658 +--------+----------+----------+------------+
657 [Dentry Block: 4KB] . . 659 [Dentry Block: 4KB] . .
658 . . 660 . .
659 . . 661 . .
660 +------+------+-----+------+ 662 +------+------+-----+------+
661 | hash | ino | len | type | 663 | hash | ino | len | type |
662 +------+------+-----+------+ 664 +------+------+-----+------+
663 [Dentry Structure: 11 bytes] 665 [Dentry Structure: 11 bytes]
664 666
665 F2FS implements multi-level hash tables for di 667 F2FS implements multi-level hash tables for directory structure. Each level has
666 a hash table with dedicated number of hash buc 668 a hash table with dedicated number of hash buckets as shown below. Note that
667 "A(2B)" means a bucket includes 2 data blocks. 669 "A(2B)" means a bucket includes 2 data blocks.
668 670
669 :: 671 ::
670 672
671 ---------------------- 673 ----------------------
672 A : bucket 674 A : bucket
673 B : block 675 B : block
674 N : MAX_DIR_HASH_DEPTH 676 N : MAX_DIR_HASH_DEPTH
675 ---------------------- 677 ----------------------
676 678
677 level #0 | A(2B) 679 level #0 | A(2B)
678 | 680 |
679 level #1 | A(2B) - A(2B) 681 level #1 | A(2B) - A(2B)
680 | 682 |
681 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 683 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
682 . | . . . . 684 . | . . . .
683 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) 685 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
684 . | . . . . 686 . | . . . .
685 level #N | A(4B) - A(4B) - A(4B) - A(4B) 687 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
686 688
687 The number of blocks and buckets are determine 689 The number of blocks and buckets are determined by::
688 690
689 ,- 2, if n < MAX_D 691 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
690 # of blocks in level #n = | 692 # of blocks in level #n = |
691 `- 4, Otherwise 693 `- 4, Otherwise
692 694
693 ,- 2^(n + dir_lev 695 ,- 2^(n + dir_level),
694 | if n + d 696 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
695 # of buckets in level #n = | 697 # of buckets in level #n = |
696 `- 2^((MAX_DIR_HA 698 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
697 Otherwis 699 Otherwise
698 700
699 When F2FS finds a file name in a directory, at 701 When F2FS finds a file name in a directory, at first a hash value of the file
700 name is calculated. Then, F2FS scans the hash 702 name is calculated. Then, F2FS scans the hash table in level #0 to find the
701 dentry consisting of the file name and its ino 703 dentry consisting of the file name and its inode number. If not found, F2FS
702 scans the next hash table in level #1. In this 704 scans the next hash table in level #1. In this way, F2FS scans hash tables in
703 each levels incrementally from 1 to N. In each 705 each levels incrementally from 1 to N. In each level F2FS needs to scan only
704 one bucket determined by the following equatio 706 one bucket determined by the following equation, which shows O(log(# of files))
705 complexity:: 707 complexity::
706 708
707 bucket number to scan in level #n = (hash va 709 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
708 710
709 In the case of file creation, F2FS finds empty 711 In the case of file creation, F2FS finds empty consecutive slots that cover the
710 file name. F2FS searches the empty slots in th 712 file name. F2FS searches the empty slots in the hash tables of whole levels from
711 1 to N in the same way as the lookup operation 713 1 to N in the same way as the lookup operation.
712 714
713 The following figure shows an example of two c 715 The following figure shows an example of two cases holding children::
714 716
715 --------------> Dir <-------------- 717 --------------> Dir <--------------
716 | | 718 | |
717 child child 719 child child
718 720
719 child - child [hole] - 721 child - child [hole] - child
720 722
721 child - child - child [hole] - 723 child - child - child [hole] - [hole] - child
722 724
723 Case 1: Case 2: 725 Case 1: Case 2:
724 Number of children = 6, Number of 726 Number of children = 6, Number of children = 3,
725 File size = 7 File size 727 File size = 7 File size = 7
726 728
727 Default Block Allocation 729 Default Block Allocation
728 ------------------------ 730 ------------------------
729 731
730 At runtime, F2FS manages six active logs insid 732 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
731 and Hot/Warm/Cold data. 733 and Hot/Warm/Cold data.
732 734
733 - Hot node contains direct node blocks of 735 - Hot node contains direct node blocks of directories.
734 - Warm node contains direct node blocks ex 736 - Warm node contains direct node blocks except hot node blocks.
735 - Cold node contains indirect node blocks 737 - Cold node contains indirect node blocks
736 - Hot data contains dentry blocks 738 - Hot data contains dentry blocks
737 - Warm data contains data blocks except ho 739 - Warm data contains data blocks except hot and cold data blocks
738 - Cold data contains multimedia data or mi 740 - Cold data contains multimedia data or migrated data blocks
739 741
740 LFS has two schemes for free space management: 742 LFS has two schemes for free space management: threaded log and copy-and-compac-
741 tion. The copy-and-compaction scheme which is 743 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
742 for devices showing very good sequential write 744 for devices showing very good sequential write performance, since free segments
743 are served all the time for writing new data. 745 are served all the time for writing new data. However, it suffers from cleaning
744 overhead under high utilization. Contrarily, t 746 overhead under high utilization. Contrarily, the threaded log scheme suffers
745 from random writes, but no cleaning process is 747 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
746 scheme where the copy-and-compaction scheme is 748 scheme where the copy-and-compaction scheme is adopted by default, but the
747 policy is dynamically changed to the threaded 749 policy is dynamically changed to the threaded log scheme according to the file
748 system status. 750 system status.
749 751
750 In order to align F2FS with underlying flash-b 752 In order to align F2FS with underlying flash-based storage, F2FS allocates a
751 segment in a unit of section. F2FS expects tha 753 segment in a unit of section. F2FS expects that the section size would be the
752 same as the unit size of garbage collection in 754 same as the unit size of garbage collection in FTL. Furthermore, with respect
753 to the mapping granularity in FTL, F2FS alloca 755 to the mapping granularity in FTL, F2FS allocates each section of the active
754 logs from different zones as much as possible, 756 logs from different zones as much as possible, since FTL can write the data in
755 the active logs into one allocation unit accor 757 the active logs into one allocation unit according to its mapping granularity.
756 758
757 Cleaning process 759 Cleaning process
758 ---------------- 760 ----------------
759 761
760 F2FS does cleaning both on demand and in the b 762 F2FS does cleaning both on demand and in the background. On-demand cleaning is
761 triggered when there are not enough free segme 763 triggered when there are not enough free segments to serve VFS calls. Background
762 cleaner is operated by a kernel thread, and tr 764 cleaner is operated by a kernel thread, and triggers the cleaning job when the
763 system is idle. 765 system is idle.
764 766
765 F2FS supports two victim selection policies: g 767 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
766 In the greedy algorithm, F2FS selects a victim 768 In the greedy algorithm, F2FS selects a victim segment having the smallest number
767 of valid blocks. In the cost-benefit algorithm 769 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
768 according to the segment age and the number of 770 according to the segment age and the number of valid blocks in order to address
769 log block thrashing problem in the greedy algo 771 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
770 algorithm for on-demand cleaner, while backgro 772 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
771 algorithm. 773 algorithm.
772 774
773 In order to identify whether the data in the v 775 In order to identify whether the data in the victim segment are valid or not,
774 F2FS manages a bitmap. Each bit represents the 776 F2FS manages a bitmap. Each bit represents the validity of a block, and the
775 bitmap is composed of a bit stream covering wh 777 bitmap is composed of a bit stream covering whole blocks in main area.
776 <<
777 Write-hint Policy <<
778 ----------------- <<
779 <<
780 F2FS sets the whint all the time with the belo <<
781 <<
782 ===================== ======================== <<
783 User F2FS <<
784 ===================== ======================== <<
785 N/A META <<
786 N/A HOT_NODE <<
787 N/A WARM_NODE <<
788 N/A COLD_NODE <<
789 ioctl(COLD) COLD_DATA <<
790 extension list " <<
791 <<
792 -- buffered io <<
793 N/A COLD_DATA <<
794 N/A HOT_DATA <<
795 N/A WARM_DATA <<
796 <<
797 -- direct io <<
798 WRITE_LIFE_EXTREME COLD_DATA <<
799 WRITE_LIFE_SHORT HOT_DATA <<
800 WRITE_LIFE_NOT_SET WARM_DATA <<
801 WRITE_LIFE_NONE " <<
802 WRITE_LIFE_MEDIUM " <<
803 WRITE_LIFE_LONG " <<
804 ===================== ======================== <<
805 778
806 Fallocate(2) Policy 779 Fallocate(2) Policy
807 ------------------- 780 -------------------
808 781
809 The default policy follows the below POSIX rul 782 The default policy follows the below POSIX rule.
810 783
811 Allocating disk space 784 Allocating disk space
812 The default operation (i.e., mode is zero) 785 The default operation (i.e., mode is zero) of fallocate() allocates
813 the disk space within the range specified 786 the disk space within the range specified by offset and len. The
814 file size (as reported by stat(2)) will be 787 file size (as reported by stat(2)) will be changed if offset+len is
815 greater than the file size. Any subregion 788 greater than the file size. Any subregion within the range specified
816 by offset and len that did not contain dat 789 by offset and len that did not contain data before the call will be
817 initialized to zero. This default behavio 790 initialized to zero. This default behavior closely resembles the
818 behavior of the posix_fallocate(3) library 791 behavior of the posix_fallocate(3) library function, and is intended
819 as a method of optimally implementing that 792 as a method of optimally implementing that function.
820 793
821 However, once F2FS receives ioctl(fd, F2FS_IOC 794 However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
822 fallocate(fd, DEFAULT_MODE), it allocates on-d 795 fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having
823 zero or random data, which is useful to the be 796 zero or random data, which is useful to the below scenario where:
824 797
825 1. create(fd) 798 1. create(fd)
826 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) 799 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
827 3. fallocate(fd, 0, 0, size) 800 3. fallocate(fd, 0, 0, size)
828 4. address = fibmap(fd, offset) 801 4. address = fibmap(fd, offset)
829 5. open(blkdev) 802 5. open(blkdev)
830 6. write(blkdev, address) 803 6. write(blkdev, address)
831 804
832 Compression implementation 805 Compression implementation
833 -------------------------- 806 --------------------------
834 807
835 - New term named cluster is defined as basic u 808 - New term named cluster is defined as basic unit of compression, file can
836 be divided into multiple clusters logically. 809 be divided into multiple clusters logically. One cluster includes 4 << n
837 (n >= 0) logical pages, compression size is 810 (n >= 0) logical pages, compression size is also cluster size, each of
838 cluster can be compressed or not. 811 cluster can be compressed or not.
839 812
840 - In cluster metadata layout, one special bloc 813 - In cluster metadata layout, one special block address is used to indicate
841 a cluster is a compressed one or normal one; 814 a cluster is a compressed one or normal one; for compressed cluster, following
842 metadata maps cluster to [1, 4 << n - 1] phy 815 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
843 stores data including compress header and co 816 stores data including compress header and compressed data.
844 817
845 - In order to eliminate write amplification du 818 - In order to eliminate write amplification during overwrite, F2FS only
846 support compression on write-once file, data 819 support compression on write-once file, data can be compressed only when
847 all logical blocks in cluster contain valid 820 all logical blocks in cluster contain valid data and compress ratio of
848 cluster data is lower than specified thresho 821 cluster data is lower than specified threshold.
849 822
850 - To enable compression on regular inode, ther 823 - To enable compression on regular inode, there are four ways:
851 824
852 * chattr +c file 825 * chattr +c file
853 * chattr +c dir; touch dir/file 826 * chattr +c dir; touch dir/file
854 * mount w/ -o compress_extension=ext; touch 827 * mount w/ -o compress_extension=ext; touch file.ext
855 * mount w/ -o compress_extension=*; touch an 828 * mount w/ -o compress_extension=*; touch any_file
856 829
857 - To disable compression on regular inode, the 830 - To disable compression on regular inode, there are two ways:
858 831
859 * chattr -c file 832 * chattr -c file
860 * mount w/ -o nocompress_extension=ext; touc 833 * mount w/ -o nocompress_extension=ext; touch file.ext
861 834
862 - Priority in between FS_COMPR_FL, FS_NOCOMP_F 835 - Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions:
863 836
864 * compress_extension=so; nocompress_extensio 837 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch
865 dir/foo.so; touch dir/bar.zip; touch dir/b 838 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt
866 should be compresse, bar.zip should be non 839 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip
867 can enable compress on bar.zip. 840 can enable compress on bar.zip.
868 * compress_extension=so; nocompress_extensio 841 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch
869 dir/foo.so; touch dir/bar.zip; touch dir/b 842 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be
870 compresse, bar.zip and baz.txt should be n 843 compresse, bar.zip and baz.txt should be non-compressed.
871 chattr+c dir/bar.zip; chattr+c dir/baz.txt 844 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip
872 and baz.txt. 845 and baz.txt.
873 846
874 - At this point, compression feature doesn't e 847 - At this point, compression feature doesn't expose compressed space to user
875 directly in order to guarantee potential dat 848 directly in order to guarantee potential data updates later to the space.
876 Instead, the main goal is to reduce data wri 849 Instead, the main goal is to reduce data writes to flash disk as much as
877 possible, resulting in extending disk life t 850 possible, resulting in extending disk life time as well as relaxing IO
878 congestion. Alternatively, we've added ioctl 851 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS)
879 interface to reclaim compressed space and sh 852 interface to reclaim compressed space and show it to user after setting a
880 special flag to the inode. Once the compress 853 special flag to the inode. Once the compressed space is released, the flag
881 will block writing data to the file until ei 854 will block writing data to the file until either the compressed space is
882 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS 855 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is
883 truncated to zero. 856 truncated to zero.
884 857
885 Compress metadata layout:: 858 Compress metadata layout::
886 859
887 [Dnode Structu 860 [Dnode Structure]
888 +----------------------------- 861 +-----------------------------------------------+
889 | cluster 1 | cluster 2 | .... 862 | cluster 1 | cluster 2 | ......... | cluster N |
890 +----------------------------- 863 +-----------------------------------------------+
891 . . 864 . . . .
892 . . 865 . . . .
893 . Compressed Cluster . 866 . Compressed Cluster . . Normal Cluster .
894 +----------+---------+---------+---------+ 867 +----------+---------+---------+---------+ +---------+---------+---------+---------+
895 |compr flag| block 1 | block 2 | block 3 | 868 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 |
896 +----------+---------+---------+---------+ 869 +----------+---------+---------+---------+ +---------+---------+---------+---------+
897 . . 870 . .
898 . 871 . .
899 . 872 . .
900 +-------------+-------------+--------- 873 +-------------+-------------+----------+----------------------------+
901 | data length | data chksum | reserved 874 | data length | data chksum | reserved | compressed data |
902 +-------------+-------------+--------- 875 +-------------+-------------+----------+----------------------------+
903 876
904 Compression mode 877 Compression mode
905 -------------------------- 878 --------------------------
906 879
907 f2fs supports "fs" and "user" compression mode 880 f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
908 With this option, f2fs provides a choice to se 881 With this option, f2fs provides a choice to select the way how to compress the
909 compression enabled files (refer to "Compressi 882 compression enabled files (refer to "Compression implementation" section for how to
910 enable compression on a regular inode). 883 enable compression on a regular inode).
911 884
912 1) compress_mode=fs 885 1) compress_mode=fs
913 This is the default option. f2fs does automati 886 This is the default option. f2fs does automatic compression in the writeback of the
914 compression enabled files. 887 compression enabled files.
915 888
916 2) compress_mode=user 889 2) compress_mode=user
917 This disables the automatic compression and gi 890 This disables the automatic compression and gives the user discretion of choosing the
918 target file and the timing. The user can do ma 891 target file and the timing. The user can do manual compression/decompression on the
919 compression enabled files using F2FS_IOC_DECOM 892 compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
920 ioctls like the below. 893 ioctls like the below.
921 894
922 To decompress a file, 895 To decompress a file,
923 896
924 fd = open(filename, O_WRONLY, 0); 897 fd = open(filename, O_WRONLY, 0);
925 ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); 898 ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);
926 899
927 To compress a file, 900 To compress a file,
928 901
929 fd = open(filename, O_WRONLY, 0); 902 fd = open(filename, O_WRONLY, 0);
930 ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); 903 ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);
931 904
932 NVMe Zoned Namespace devices 905 NVMe Zoned Namespace devices
933 ---------------------------- 906 ----------------------------
934 907
935 - ZNS defines a per-zone capacity which can be 908 - ZNS defines a per-zone capacity which can be equal or less than the
936 zone-size. Zone-capacity is the number of us 909 zone-size. Zone-capacity is the number of usable blocks in the zone.
937 F2FS checks if zone-capacity is less than zo 910 F2FS checks if zone-capacity is less than zone-size, if it is, then any
938 segment which starts after the zone-capacity 911 segment which starts after the zone-capacity is marked as not-free in
939 the free segment bitmap at initial mount tim 912 the free segment bitmap at initial mount time. These segments are marked
940 as permanently used so they are not allocate 913 as permanently used so they are not allocated for writes and
941 consequently are not needed to be garbage co 914 consequently are not needed to be garbage collected. In case the
942 zone-capacity is not aligned to default segm 915 zone-capacity is not aligned to default segment size(2MB), then a segment
943 can start before the zone-capacity and span 916 can start before the zone-capacity and span across zone-capacity boundary.
944 Such spanning segments are also considered a 917 Such spanning segments are also considered as usable segments. All blocks
945 past the zone-capacity are considered unusab 918 past the zone-capacity are considered unusable in these segments.
Linux® is a registered trademark of Linus Torvalds in the United States and other countries.
TOMOYO® is a registered trademark of NTT DATA CORPORATION.