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