1 .. SPDX-License-Identifier: GPL-2.0
2
3 ================================================
4 ZoneFS - Zone filesystem for Zoned block devices
5 ================================================
6
7 Introduction
8 ============
9
10 zonefs is a very simple file system exposing each zone of a zoned block device
11 as a file. Unlike a regular POSIX-compliant file system with native zoned block
12 device support (e.g. f2fs), zonefs does not hide the sequential write
13 constraint of zoned block devices to the user. Files representing sequential
14 write zones of the device must be written sequentially starting from the end
15 of the file (append only writes).
16
17 As such, zonefs is in essence closer to a raw block device access interface
18 than to a full-featured POSIX file system. The goal of zonefs is to simplify
19 the implementation of zoned block device support in applications by replacing
20 raw block device file accesses with a richer file API, avoiding relying on
21 direct block device file ioctls which may be more obscure to developers. One
22 example of this approach is the implementation of LSM (log-structured merge)
23 tree structures (such as used in RocksDB and LevelDB) on zoned block devices
24 by allowing SSTables to be stored in a zone file similarly to a regular file
25 system rather than as a range of sectors of the entire disk. The introduction
26 of the higher level construct "one file is one zone" can help reducing the
27 amount of changes needed in the application as well as introducing support for
28 different application programming languages.
29
30 Zoned block devices
31 -------------------
32
33 Zoned storage devices belong to a class of storage devices with an address
34 space that is divided into zones. A zone is a group of consecutive LBAs and all
35 zones are contiguous (there are no LBA gaps). Zones may have different types.
36
37 * Conventional zones: there are no access constraints to LBAs belonging to
38 conventional zones. Any read or write access can be executed, similarly to a
39 regular block device.
40 * Sequential zones: these zones accept random reads but must be written
41 sequentially. Each sequential zone has a write pointer maintained by the
42 device that keeps track of the mandatory start LBA position of the next write
43 to the device. As a result of this write constraint, LBAs in a sequential zone
44 cannot be overwritten. Sequential zones must first be erased using a special
45 command (zone reset) before rewriting.
46
47 Zoned storage devices can be implemented using various recording and media
48 technologies. The most common form of zoned storage today uses the SCSI Zoned
49 Block Commands (ZBC) and Zoned ATA Commands (ZAC) interfaces on Shingled
50 Magnetic Recording (SMR) HDDs.
51
52 Solid State Disks (SSD) storage devices can also implement a zoned interface
53 to, for instance, reduce internal write amplification due to garbage collection.
54 The NVMe Zoned NameSpace (ZNS) is a technical proposal of the NVMe standard
55 committee aiming at adding a zoned storage interface to the NVMe protocol.
56
57 Zonefs Overview
58 ===============
59
60 Zonefs exposes the zones of a zoned block device as files. The files
61 representing zones are grouped by zone type, which are themselves represented
62 by sub-directories. This file structure is built entirely using zone information
63 provided by the device and so does not require any complex on-disk metadata
64 structure.
65
66 On-disk metadata
67 ----------------
68
69 zonefs on-disk metadata is reduced to an immutable super block which
70 persistently stores a magic number and optional feature flags and values. On
71 mount, zonefs uses blkdev_report_zones() to obtain the device zone configuration
72 and populates the mount point with a static file tree solely based on this
73 information. File sizes come from the device zone type and write pointer
74 position managed by the device itself.
75
76 The super block is always written on disk at sector 0. The first zone of the
77 device storing the super block is never exposed as a zone file by zonefs. If
78 the zone containing the super block is a sequential zone, the mkzonefs format
79 tool always "finishes" the zone, that is, it transitions the zone to a full
80 state to make it read-only, preventing any data write.
81
82 Zone type sub-directories
83 -------------------------
84
85 Files representing zones of the same type are grouped together under the same
86 sub-directory automatically created on mount.
87
88 For conventional zones, the sub-directory "cnv" is used. This directory is
89 however created if and only if the device has usable conventional zones. If
90 the device only has a single conventional zone at sector 0, the zone will not
91 be exposed as a file as it will be used to store the zonefs super block. For
92 such devices, the "cnv" sub-directory will not be created.
93
94 For sequential write zones, the sub-directory "seq" is used.
95
96 These two directories are the only directories that exist in zonefs. Users
97 cannot create other directories and cannot rename nor delete the "cnv" and
98 "seq" sub-directories.
99
100 The size of the directories indicated by the st_size field of struct stat,
101 obtained with the stat() or fstat() system calls, indicates the number of files
102 existing under the directory.
103
104 Zone files
105 ----------
106
107 Zone files are named using the number of the zone they represent within the set
108 of zones of a particular type. That is, both the "cnv" and "seq" directories
109 contain files named "0", "1", "2", ... The file numbers also represent
110 increasing zone start sector on the device.
111
112 All read and write operations to zone files are not allowed beyond the file
113 maximum size, that is, beyond the zone capacity. Any access exceeding the zone
114 capacity is failed with the -EFBIG error.
115
116 Creating, deleting, renaming or modifying any attribute of files and
117 sub-directories is not allowed.
118
119 The number of blocks of a file as reported by stat() and fstat() indicates the
120 capacity of the zone file, or in other words, the maximum file size.
121
122 Conventional zone files
123 -----------------------
124
125 The size of conventional zone files is fixed to the size of the zone they
126 represent. Conventional zone files cannot be truncated.
127
128 These files can be randomly read and written using any type of I/O operation:
129 buffered I/Os, direct I/Os, memory mapped I/Os (mmap), etc. There are no I/O
130 constraint for these files beyond the file size limit mentioned above.
131
132 Sequential zone files
133 ---------------------
134
135 The size of sequential zone files grouped in the "seq" sub-directory represents
136 the file's zone write pointer position relative to the zone start sector.
137
138 Sequential zone files can only be written sequentially, starting from the file
139 end, that is, write operations can only be append writes. Zonefs makes no
140 attempt at accepting random writes and will fail any write request that has a
141 start offset not corresponding to the end of the file, or to the end of the last
142 write issued and still in-flight (for asynchronous I/O operations).
143
144 Since dirty page writeback by the page cache does not guarantee a sequential
145 write pattern, zonefs prevents buffered writes and writeable shared mappings
146 on sequential files. Only direct I/O writes are accepted for these files.
147 zonefs relies on the sequential delivery of write I/O requests to the device
148 implemented by the block layer elevator. An elevator implementing the sequential
149 write feature for zoned block device (ELEVATOR_F_ZBD_SEQ_WRITE elevator feature)
150 must be used. This type of elevator (e.g. mq-deadline) is set by default
151 for zoned block devices on device initialization.
152
153 There are no restrictions on the type of I/O used for read operations in
154 sequential zone files. Buffered I/Os, direct I/Os and shared read mappings are
155 all accepted.
156
157 Truncating sequential zone files is allowed only down to 0, in which case, the
158 zone is reset to rewind the file zone write pointer position to the start of
159 the zone, or up to the zone capacity, in which case the file's zone is
160 transitioned to the FULL state (finish zone operation).
161
162 Format options
163 --------------
164
165 Several optional features of zonefs can be enabled at format time.
166
167 * Conventional zone aggregation: ranges of contiguous conventional zones can be
168 aggregated into a single larger file instead of the default one file per zone.
169 * File ownership: The owner UID and GID of zone files is by default 0 (root)
170 but can be changed to any valid UID/GID.
171 * File access permissions: the default 640 access permissions can be changed.
172
173 IO error handling
174 -----------------
175
176 Zoned block devices may fail I/O requests for reasons similar to regular block
177 devices, e.g. due to bad sectors. However, in addition to such known I/O
178 failure pattern, the standards governing zoned block devices behavior define
179 additional conditions that result in I/O errors.
180
181 * A zone may transition to the read-only condition (BLK_ZONE_COND_READONLY):
182 While the data already written in the zone is still readable, the zone can
183 no longer be written. No user action on the zone (zone management command or
184 read/write access) can change the zone condition back to a normal read/write
185 state. While the reasons for the device to transition a zone to read-only
186 state are not defined by the standards, a typical cause for such transition
187 would be a defective write head on an HDD (all zones under this head are
188 changed to read-only).
189
190 * A zone may transition to the offline condition (BLK_ZONE_COND_OFFLINE):
191 An offline zone cannot be read nor written. No user action can transition an
192 offline zone back to an operational good state. Similarly to zone read-only
193 transitions, the reasons for a drive to transition a zone to the offline
194 condition are undefined. A typical cause would be a defective read-write head
195 on an HDD causing all zones on the platter under the broken head to be
196 inaccessible.
197
198 * Unaligned write errors: These errors result from the host issuing write
199 requests with a start sector that does not correspond to a zone write pointer
200 position when the write request is executed by the device. Even though zonefs
201 enforces sequential file write for sequential zones, unaligned write errors
202 may still happen in the case of a partial failure of a very large direct I/O
203 operation split into multiple BIOs/requests or asynchronous I/O operations.
204 If one of the write request within the set of sequential write requests
205 issued to the device fails, all write requests queued after it will
206 become unaligned and fail.
207
208 * Delayed write errors: similarly to regular block devices, if the device side
209 write cache is enabled, write errors may occur in ranges of previously
210 completed writes when the device write cache is flushed, e.g. on fsync().
211 Similarly to the previous immediate unaligned write error case, delayed write
212 errors can propagate through a stream of cached sequential data for a zone
213 causing all data to be dropped after the sector that caused the error.
214
215 All I/O errors detected by zonefs are notified to the user with an error code
216 return for the system call that triggered or detected the error. The recovery
217 actions taken by zonefs in response to I/O errors depend on the I/O type (read
218 vs write) and on the reason for the error (bad sector, unaligned writes or zone
219 condition change).
220
221 * For read I/O errors, zonefs does not execute any particular recovery action,
222 but only if the file zone is still in a good condition and there is no
223 inconsistency between the file inode size and its zone write pointer position.
224 If a problem is detected, I/O error recovery is executed (see below table).
225
226 * For write I/O errors, zonefs I/O error recovery is always executed.
227
228 * A zone condition change to read-only or offline also always triggers zonefs
229 I/O error recovery.
230
231 Zonefs minimal I/O error recovery may change a file size and file access
232 permissions.
233
234 * File size changes:
235 Immediate or delayed write errors in a sequential zone file may cause the file
236 inode size to be inconsistent with the amount of data successfully written in
237 the file zone. For instance, the partial failure of a multi-BIO large write
238 operation will cause the zone write pointer to advance partially, even though
239 the entire write operation will be reported as failed to the user. In such
240 case, the file inode size must be advanced to reflect the zone write pointer
241 change and eventually allow the user to restart writing at the end of the
242 file.
243 A file size may also be reduced to reflect a delayed write error detected on
244 fsync(): in this case, the amount of data effectively written in the zone may
245 be less than originally indicated by the file inode size. After such I/O
246 error, zonefs always fixes the file inode size to reflect the amount of data
247 persistently stored in the file zone.
248
249 * Access permission changes:
250 A zone condition change to read-only is indicated with a change in the file
251 access permissions to render the file read-only. This disables changes to the
252 file attributes and data modification. For offline zones, all permissions
253 (read and write) to the file are disabled.
254
255 Further action taken by zonefs I/O error recovery can be controlled by the user
256 with the "errors=xxx" mount option. The table below summarizes the result of
257 zonefs I/O error processing depending on the mount option and on the zone
258 conditions::
259
260 +--------------+-----------+-----------------------------------------+
261 | | | Post error state |
262 | "errors=xxx" | device | access permissions |
263 | mount | zone | file file device zone |
264 | option | condition | size read write read write |
265 +--------------+-----------+-----------------------------------------+
266 | | good | fixed yes no yes yes |
267 | remount-ro | read-only | as is yes no yes no |
268 | (default) | offline | 0 no no no no |
269 +--------------+-----------+-----------------------------------------+
270 | | good | fixed yes no yes yes |
271 | zone-ro | read-only | as is yes no yes no |
272 | | offline | 0 no no no no |
273 +--------------+-----------+-----------------------------------------+
274 | | good | 0 no no yes yes |
275 | zone-offline | read-only | 0 no no yes no |
276 | | offline | 0 no no no no |
277 +--------------+-----------+-----------------------------------------+
278 | | good | fixed yes yes yes yes |
279 | repair | read-only | as is yes no yes no |
280 | | offline | 0 no no no no |
281 +--------------+-----------+-----------------------------------------+
282
283 Further notes:
284
285 * The "errors=remount-ro" mount option is the default behavior of zonefs I/O
286 error processing if no errors mount option is specified.
287 * With the "errors=remount-ro" mount option, the change of the file access
288 permissions to read-only applies to all files. The file system is remounted
289 read-only.
290 * Access permission and file size changes due to the device transitioning zones
291 to the offline condition are permanent. Remounting or reformatting the device
292 with mkfs.zonefs (mkzonefs) will not change back offline zone files to a good
293 state.
294 * File access permission changes to read-only due to the device transitioning
295 zones to the read-only condition are permanent. Remounting or reformatting
296 the device will not re-enable file write access.
297 * File access permission changes implied by the remount-ro, zone-ro and
298 zone-offline mount options are temporary for zones in a good condition.
299 Unmounting and remounting the file system will restore the previous default
300 (format time values) access rights to the files affected.
301 * The repair mount option triggers only the minimal set of I/O error recovery
302 actions, that is, file size fixes for zones in a good condition. Zones
303 indicated as being read-only or offline by the device still imply changes to
304 the zone file access permissions as noted in the table above.
305
306 Mount options
307 -------------
308
309 zonefs defines several mount options:
310 * errors=<behavior>
311 * explicit-open
312
313 "errors=<behavior>" option
314 ~~~~~~~~~~~~~~~~~~~~~~~~~~
315
316 The "errors=<behavior>" option mount option allows the user to specify zonefs
317 behavior in response to I/O errors, inode size inconsistencies or zone
318 condition changes. The defined behaviors are as follow:
319
320 * remount-ro (default)
321 * zone-ro
322 * zone-offline
323 * repair
324
325 The run-time I/O error actions defined for each behavior are detailed in the
326 previous section. Mount time I/O errors will cause the mount operation to fail.
327 The handling of read-only zones also differs between mount-time and run-time.
328 If a read-only zone is found at mount time, the zone is always treated in the
329 same manner as offline zones, that is, all accesses are disabled and the zone
330 file size set to 0. This is necessary as the write pointer of read-only zones
331 is defined as invalib by the ZBC and ZAC standards, making it impossible to
332 discover the amount of data that has been written to the zone. In the case of a
333 read-only zone discovered at run-time, as indicated in the previous section.
334 The size of the zone file is left unchanged from its last updated value.
335
336 "explicit-open" option
337 ~~~~~~~~~~~~~~~~~~~~~~
338
339 A zoned block device (e.g. an NVMe Zoned Namespace device) may have limits on
340 the number of zones that can be active, that is, zones that are in the
341 implicit open, explicit open or closed conditions. This potential limitation
342 translates into a risk for applications to see write IO errors due to this
343 limit being exceeded if the zone of a file is not already active when a write
344 request is issued by the user.
345
346 To avoid these potential errors, the "explicit-open" mount option forces zones
347 to be made active using an open zone command when a file is opened for writing
348 for the first time. If the zone open command succeeds, the application is then
349 guaranteed that write requests can be processed. Conversely, the
350 "explicit-open" mount option will result in a zone close command being issued
351 to the device on the last close() of a zone file if the zone is not full nor
352 empty.
353
354 Runtime sysfs attributes
355 ------------------------
356
357 zonefs defines several sysfs attributes for mounted devices. All attributes
358 are user readable and can be found in the directory /sys/fs/zonefs/<dev>/,
359 where <dev> is the name of the mounted zoned block device.
360
361 The attributes defined are as follows.
362
363 * **max_wro_seq_files**: This attribute reports the maximum number of
364 sequential zone files that can be open for writing. This number corresponds
365 to the maximum number of explicitly or implicitly open zones that the device
366 supports. A value of 0 means that the device has no limit and that any zone
367 (any file) can be open for writing and written at any time, regardless of the
368 state of other zones. When the *explicit-open* mount option is used, zonefs
369 will fail any open() system call requesting to open a sequential zone file for
370 writing when the number of sequential zone files already open for writing has
371 reached the *max_wro_seq_files* limit.
372 * **nr_wro_seq_files**: This attribute reports the current number of sequential
373 zone files open for writing. When the "explicit-open" mount option is used,
374 this number can never exceed *max_wro_seq_files*. If the *explicit-open*
375 mount option is not used, the reported number can be greater than
376 *max_wro_seq_files*. In such case, it is the responsibility of the
377 application to not write simultaneously more than *max_wro_seq_files*
378 sequential zone files. Failure to do so can result in write errors.
379 * **max_active_seq_files**: This attribute reports the maximum number of
380 sequential zone files that are in an active state, that is, sequential zone
381 files that are partially written (not empty nor full) or that have a zone that
382 is explicitly open (which happens only if the *explicit-open* mount option is
383 used). This number is always equal to the maximum number of active zones that
384 the device supports. A value of 0 means that the mounted device has no limit
385 on the number of sequential zone files that can be active.
386 * **nr_active_seq_files**: This attributes reports the current number of
387 sequential zone files that are active. If *max_active_seq_files* is not 0,
388 then the value of *nr_active_seq_files* can never exceed the value of
389 *nr_active_seq_files*, regardless of the use of the *explicit-open* mount
390 option.
391
392 Zonefs User Space Tools
393 =======================
394
395 The mkzonefs tool is used to format zoned block devices for use with zonefs.
396 This tool is available on Github at:
397
398 https://github.com/damien-lemoal/zonefs-tools
399
400 zonefs-tools also includes a test suite which can be run against any zoned
401 block device, including null_blk block device created with zoned mode.
402
403 Examples
404 --------
405
406 The following formats a 15TB host-managed SMR HDD with 256 MB zones
407 with the conventional zones aggregation feature enabled::
408
409 # mkzonefs -o aggr_cnv /dev/sdX
410 # mount -t zonefs /dev/sdX /mnt
411 # ls -l /mnt/
412 total 0
413 dr-xr-xr-x 2 root root 1 Nov 25 13:23 cnv
414 dr-xr-xr-x 2 root root 55356 Nov 25 13:23 seq
415
416 The size of the zone files sub-directories indicate the number of files
417 existing for each type of zones. In this example, there is only one
418 conventional zone file (all conventional zones are aggregated under a single
419 file)::
420
421 # ls -l /mnt/cnv
422 total 137101312
423 -rw-r----- 1 root root 140391743488 Nov 25 13:23 0
424
425 This aggregated conventional zone file can be used as a regular file::
426
427 # mkfs.ext4 /mnt/cnv/0
428 # mount -o loop /mnt/cnv/0 /data
429
430 The "seq" sub-directory grouping files for sequential write zones has in this
431 example 55356 zones::
432
433 # ls -lv /mnt/seq
434 total 14511243264
435 -rw-r----- 1 root root 0 Nov 25 13:23 0
436 -rw-r----- 1 root root 0 Nov 25 13:23 1
437 -rw-r----- 1 root root 0 Nov 25 13:23 2
438 ...
439 -rw-r----- 1 root root 0 Nov 25 13:23 55354
440 -rw-r----- 1 root root 0 Nov 25 13:23 55355
441
442 For sequential write zone files, the file size changes as data is appended at
443 the end of the file, similarly to any regular file system::
444
445 # dd if=/dev/zero of=/mnt/seq/0 bs=4096 count=1 conv=notrunc oflag=direct
446 1+0 records in
447 1+0 records out
448 4096 bytes (4.1 kB, 4.0 KiB) copied, 0.00044121 s, 9.3 MB/s
449
450 # ls -l /mnt/seq/0
451 -rw-r----- 1 root root 4096 Nov 25 13:23 /mnt/seq/0
452
453 The written file can be truncated to the zone size, preventing any further
454 write operation::
455
456 # truncate -s 268435456 /mnt/seq/0
457 # ls -l /mnt/seq/0
458 -rw-r----- 1 root root 268435456 Nov 25 13:49 /mnt/seq/0
459
460 Truncation to 0 size allows freeing the file zone storage space and restart
461 append-writes to the file::
462
463 # truncate -s 0 /mnt/seq/0
464 # ls -l /mnt/seq/0
465 -rw-r----- 1 root root 0 Nov 25 13:49 /mnt/seq/0
466
467 Since files are statically mapped to zones on the disk, the number of blocks
468 of a file as reported by stat() and fstat() indicates the capacity of the file
469 zone::
470
471 # stat /mnt/seq/0
472 File: /mnt/seq/0
473 Size: 0 Blocks: 524288 IO Block: 4096 regular empty file
474 Device: 870h/2160d Inode: 50431 Links: 1
475 Access: (0640/-rw-r-----) Uid: ( 0/ root) Gid: ( 0/ root)
476 Access: 2019-11-25 13:23:57.048971997 +0900
477 Modify: 2019-11-25 13:52:25.553805765 +0900
478 Change: 2019-11-25 13:52:25.553805765 +0900
479 Birth: -
480
481 The number of blocks of the file ("Blocks") in units of 512B blocks gives the
482 maximum file size of 524288 * 512 B = 256 MB, corresponding to the device zone
483 capacity in this example. Of note is that the "IO block" field always
484 indicates the minimum I/O size for writes and corresponds to the device
485 physical sector size.
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