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1 | .. SPDX-License-Identifier: GPL-2.0 |
2 | ||
3 | ========================================== | |
98e4da8c | 4 | WHAT IS Flash-Friendly File System (F2FS)? |
89272ca1 | 5 | ========================================== |
98e4da8c JK |
6 | |
7 | NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have | |
8 | been equipped on a variety systems ranging from mobile to server systems. Since | |
9 | they are known to have different characteristics from the conventional rotating | |
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. | |
12 | ||
13 | F2FS is a file system exploiting NAND flash memory-based storage devices, which | |
14 | is based on Log-structured File System (LFS). The design has been focused on | |
15 | addressing the fundamental issues in LFS, which are snowball effect of wandering | |
16 | tree and high cleaning overhead. | |
17 | ||
18 | Since a NAND flash memory-based storage device shows different characteristic | |
19 | according to its internal geometry or flash memory management scheme, namely FTL, | |
20 | F2FS and its tools support various parameters not only for configuring on-disk | |
21 | layout, but also for selecting allocation and cleaning algorithms. | |
22 | ||
d51a7fba CL |
23 | The following git tree provides the file system formatting tool (mkfs.f2fs), |
24 | a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs). | |
89272ca1 MCC |
25 | |
26 | - git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git | |
5bb446a2 | 27 | |
398bb30d | 28 | For sending patches, please use the following mailing list: |
98e4da8c | 29 | |
89272ca1 MCC |
30 | - linux-f2fs-devel@lists.sourceforge.net |
31 | ||
398bb30d CY |
32 | For reporting bugs, please use the following f2fs bug tracker link: |
33 | ||
34 | - https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs | |
35 | ||
89272ca1 MCC |
36 | Background and Design issues |
37 | ============================ | |
98e4da8c JK |
38 | |
39 | Log-structured File System (LFS) | |
40 | -------------------------------- | |
41 | "A log-structured file system writes all modifications to disk sequentially in | |
42 | a log-like structure, thereby speeding up both file writing and crash recovery. | |
43 | The log is the only structure on disk; it contains indexing information so that | |
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 log into segments and use a | |
46 | segment cleaner to compress the live information from heavily fragmented | |
47 | segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and | |
48 | implementation of a log-structured file system", ACM Trans. Computer Systems | |
49 | 10, 1, 26–52. | |
50 | ||
51 | Wandering Tree Problem | |
52 | ---------------------- | |
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 location. Then the indirect pointer | |
55 | block is also updated due to the direct pointer block update. In this manner, | |
56 | the upper index structures such as inode, inode map, and checkpoint block are | |
57 | also updated recursively. This problem is called as wandering tree problem [1], | |
58 | and in order to enhance the performance, it should eliminate or relax the update | |
59 | propagation as much as possible. | |
60 | ||
61 | [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/ | |
62 | ||
63 | Cleaning Overhead | |
64 | ----------------- | |
65 | Since LFS is based on out-of-place writes, it produces so many obsolete blocks | |
66 | scattered across the whole storage. In order to serve new empty log space, it | |
67 | needs to reclaim these obsolete blocks seamlessly to users. This job is called | |
68 | as a cleaning process. | |
69 | ||
70 | The process consists of three operations as follows. | |
89272ca1 | 71 | |
98e4da8c JK |
72 | 1. A victim segment is selected through referencing segment usage table. |
73 | 2. It loads parent index structures of all the data in the victim identified by | |
74 | segment summary blocks. | |
75 | 3. It checks the cross-reference between the data and its parent index structure. | |
76 | 4. It moves valid data selectively. | |
77 | ||
78 | This cleaning job may cause unexpected long delays, so the most important goal | |
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 them quickly as well. | |
81 | ||
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82 | Key Features |
83 | ============ | |
98e4da8c JK |
84 | |
85 | Flash Awareness | |
86 | --------------- | |
87 | - Enlarge the random write area for better performance, but provide the high | |
88 | spatial locality | |
89 | - Align FS data structures to the operational units in FTL as best efforts | |
90 | ||
91 | Wandering Tree Problem | |
92 | ---------------------- | |
93 | - Use a term, “node”, that represents inodes as well as various pointer blocks | |
94 | - Introduce Node Address Table (NAT) containing the locations of all the “node” | |
95 | blocks; this will cut off the update propagation. | |
96 | ||
97 | Cleaning Overhead | |
98 | ----------------- | |
99 | - Support a background cleaning process | |
100 | - Support greedy and cost-benefit algorithms for victim selection policies | |
101 | - Support multi-head logs for static/dynamic hot and cold data separation | |
102 | - Introduce adaptive logging for efficient block allocation | |
103 | ||
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104 | Mount Options |
105 | ============= | |
98e4da8c | 106 | |
89272ca1 | 107 | |
9aa1ccb4 JC |
108 | ======================== ============================================================ |
109 | background_gc=%s Turn on/off cleaning operations, namely garbage | |
110 | collection, triggered in background when I/O subsystem is | |
111 | idle. If background_gc=on, it will turn on the garbage | |
112 | collection and if background_gc=off, garbage collection | |
113 | will be turned off. If background_gc=sync, it will turn | |
114 | on synchronous garbage collection running in background. | |
115 | Default value for this option is on. So garbage | |
116 | collection is on by default. | |
5911d2d1 CY |
117 | gc_merge When background_gc is on, this option can be enabled to |
118 | let background GC thread to handle foreground GC requests, | |
119 | it can eliminate the sluggish issue caused by slow foreground | |
120 | GC operation when GC is triggered from a process with limited | |
121 | I/O and CPU resources. | |
122 | nogc_merge Disable GC merge feature. | |
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123 | disable_roll_forward Disable the roll-forward recovery routine |
124 | norecovery Disable the roll-forward recovery routine, mounted read- | |
125 | only (i.e., -o ro,disable_roll_forward) | |
126 | discard/nodiscard Enable/disable real-time discard in f2fs, if discard is | |
127 | enabled, f2fs will issue discard/TRIM commands when a | |
128 | segment is cleaned. | |
4e0197f9 | 129 | heap/no_heap Deprecated. |
9aa1ccb4 JC |
130 | nouser_xattr Disable Extended User Attributes. Note: xattr is enabled |
131 | by default if CONFIG_F2FS_FS_XATTR is selected. | |
132 | noacl Disable POSIX Access Control List. Note: acl is enabled | |
133 | by default if CONFIG_F2FS_FS_POSIX_ACL is selected. | |
134 | active_logs=%u Support configuring the number of active logs. In the | |
135 | current design, f2fs supports only 2, 4, and 6 logs. | |
136 | Default number is 6. | |
137 | disable_ext_identify Disable the extension list configured by mkfs, so f2fs | |
ca313c82 | 138 | is not aware of cold files such as media files. |
9aa1ccb4 JC |
139 | inline_xattr Enable the inline xattrs feature. |
140 | noinline_xattr Disable the inline xattrs feature. | |
141 | inline_xattr_size=%u Support configuring inline xattr size, it depends on | |
142 | flexible inline xattr feature. | |
ca313c82 | 143 | inline_data Enable the inline data feature: Newly created small (<~3.4k) |
9aa1ccb4 | 144 | files can be written into inode block. |
ca313c82 | 145 | inline_dentry Enable the inline dir feature: data in newly created |
9aa1ccb4 JC |
146 | directory entries can be written into inode block. The |
147 | space of inode block which is used to store inline | |
148 | dentries is limited to ~3.4k. | |
149 | noinline_dentry Disable the inline dentry feature. | |
150 | flush_merge Merge concurrent cache_flush commands as much as possible | |
151 | to eliminate redundant command issues. If the underlying | |
152 | device handles the cache_flush command relatively slowly, | |
153 | recommend to enable this option. | |
154 | nobarrier This option can be used if underlying storage guarantees | |
155 | its cached data should be written to the novolatile area. | |
156 | If this option is set, no cache_flush commands are issued | |
157 | but f2fs still guarantees the write ordering of all the | |
158 | data writes. | |
a9e292f2 | 159 | barrier If this option is set, cache_flush commands are allowed to be |
6047de54 | 160 | issued. |
9aa1ccb4 JC |
161 | fastboot This option is used when a system wants to reduce mount |
162 | time as much as possible, even though normal performance | |
163 | can be sacrificed. | |
164 | extent_cache Enable an extent cache based on rb-tree, it can cache | |
165 | as many as extent which map between contiguous logical | |
166 | address and physical address per inode, resulting in | |
167 | increasing the cache hit ratio. Set by default. | |
168 | noextent_cache Disable an extent cache based on rb-tree explicitly, see | |
169 | the above extent_cache mount option. | |
170 | noinline_data Disable the inline data feature, inline data feature is | |
171 | enabled by default. | |
172 | data_flush Enable data flushing before checkpoint in order to | |
173 | persist data of regular and symlink. | |
174 | reserve_root=%d Support configuring reserved space which is used for | |
175 | allocation from a privileged user with specified uid or | |
176 | gid, unit: 4KB, the default limit is 0.2% of user blocks. | |
177 | resuid=%d The user ID which may use the reserved blocks. | |
178 | resgid=%d The group ID which may use the reserved blocks. | |
179 | fault_injection=%d Enable fault injection in all supported types with | |
180 | specified injection rate. | |
181 | fault_type=%d Support configuring fault injection type, should be | |
182 | enabled with fault_injection option, fault type value | |
183 | is shown below, it supports single or combined type. | |
89272ca1 | 184 | |
c7115e09 CY |
185 | =========================== =========== |
186 | Type_Name Type_Value | |
187 | =========================== =========== | |
188 | FAULT_KMALLOC 0x000000001 | |
189 | FAULT_KVMALLOC 0x000000002 | |
190 | FAULT_PAGE_ALLOC 0x000000004 | |
191 | FAULT_PAGE_GET 0x000000008 | |
192 | FAULT_ALLOC_BIO 0x000000010 (obsolete) | |
193 | FAULT_ALLOC_NID 0x000000020 | |
194 | FAULT_ORPHAN 0x000000040 | |
195 | FAULT_BLOCK 0x000000080 | |
196 | FAULT_DIR_DEPTH 0x000000100 | |
197 | FAULT_EVICT_INODE 0x000000200 | |
198 | FAULT_TRUNCATE 0x000000400 | |
199 | FAULT_READ_IO 0x000000800 | |
200 | FAULT_CHECKPOINT 0x000001000 | |
201 | FAULT_DISCARD 0x000002000 | |
202 | FAULT_WRITE_IO 0x000004000 | |
203 | FAULT_SLAB_ALLOC 0x000008000 | |
204 | FAULT_DQUOT_INIT 0x000010000 | |
205 | FAULT_LOCK_OP 0x000020000 | |
206 | FAULT_BLKADDR_VALIDITY 0x000040000 | |
207 | FAULT_BLKADDR_CONSISTENCE 0x000080000 | |
8b10d365 | 208 | FAULT_NO_SEGMENT 0x000100000 |
c7115e09 | 209 | =========================== =========== |
9aa1ccb4 JC |
210 | mode=%s Control block allocation mode which supports "adaptive" |
211 | and "lfs". In "lfs" mode, there should be no random | |
212 | writes towards main area. | |
6691d940 DJ |
213 | "fragment:segment" and "fragment:block" are newly added here. |
214 | These are developer options for experiments to simulate filesystem | |
215 | fragmentation/after-GC situation itself. The developers use these | |
216 | modes to understand filesystem fragmentation/after-GC condition well, | |
217 | and eventually get some insights to handle them better. | |
218 | In "fragment:segment", f2fs allocates a new segment in ramdom | |
219 | position. With this, we can simulate the after-GC condition. | |
220 | In "fragment:block", we can scatter block allocation with | |
221 | "max_fragment_chunk" and "max_fragment_hole" sysfs nodes. | |
222 | We added some randomness to both chunk and hole size to make | |
223 | it close to realistic IO pattern. So, in this mode, f2fs will allocate | |
224 | 1..<max_fragment_chunk> blocks in a chunk and make a hole in the | |
225 | length of 1..<max_fragment_hole> by turns. With this, the newly | |
226 | allocated blocks will be scattered throughout the whole partition. | |
227 | Note that "fragment:block" implicitly enables "fragment:segment" | |
228 | option for more randomness. | |
229 | Please, use these options for your experiments and we strongly | |
230 | recommend to re-format the filesystem after using these options. | |
9aa1ccb4 JC |
231 | usrquota Enable plain user disk quota accounting. |
232 | grpquota Enable plain group disk quota accounting. | |
233 | prjquota Enable plain project quota accounting. | |
234 | usrjquota=<file> Appoint specified file and type during mount, so that quota | |
235 | grpjquota=<file> information can be properly updated during recovery flow, | |
236 | prjjquota=<file> <quota file>: must be in root directory; | |
237 | jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. | |
ca313c82 RD |
238 | offusrjquota Turn off user journalled quota. |
239 | offgrpjquota Turn off group journalled quota. | |
240 | offprjjquota Turn off project journalled quota. | |
9aa1ccb4 JC |
241 | quota Enable plain user disk quota accounting. |
242 | noquota Disable all plain disk quota option. | |
9aa1ccb4 JC |
243 | alloc_mode=%s Adjust block allocation policy, which supports "reuse" |
244 | and "default". | |
245 | fsync_mode=%s Control the policy of fsync. Currently supports "posix", | |
246 | "strict", and "nobarrier". In "posix" mode, which is | |
247 | default, fsync will follow POSIX semantics and does a | |
248 | light operation to improve the filesystem performance. | |
249 | In "strict" mode, fsync will be heavy and behaves in line | |
250 | with xfs, ext4 and btrfs, where xfstest generic/342 will | |
251 | pass, but the performance will regress. "nobarrier" is | |
252 | based on "posix", but doesn't issue flush command for | |
253 | non-atomic files likewise "nobarrier" mount option. | |
ed318a6c EB |
254 | test_dummy_encryption |
255 | test_dummy_encryption=%s | |
9aa1ccb4 JC |
256 | Enable dummy encryption, which provides a fake fscrypt |
257 | context. The fake fscrypt context is used by xfstests. | |
258 | The argument may be either "v1" or "v2", in order to | |
259 | select the corresponding fscrypt policy version. | |
260 | checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" | |
261 | to reenable checkpointing. Is enabled by default. While | |
262 | disabled, any unmounting or unexpected shutdowns will cause | |
263 | the filesystem contents to appear as they did when the | |
264 | filesystem was mounted with that option. | |
562abda4 | 265 | While mounting with checkpoint=disable, the filesystem must |
9aa1ccb4 JC |
266 | run garbage collection to ensure that all available space can |
267 | be used. If this takes too much time, the mount may return | |
268 | EAGAIN. You may optionally add a value to indicate how much | |
269 | of the disk you would be willing to temporarily give up to | |
270 | avoid additional garbage collection. This can be given as a | |
271 | number of blocks, or as a percent. For instance, mounting | |
272 | with checkpoint=disable:100% would always succeed, but it may | |
273 | hide up to all remaining free space. The actual space that | |
274 | would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable | |
275 | This space is reclaimed once checkpoint=enable. | |
261eeb9c DJ |
276 | checkpoint_merge When checkpoint is enabled, this can be used to create a kernel |
277 | daemon and make it to merge concurrent checkpoint requests as | |
278 | much as possible to eliminate redundant checkpoint issues. Plus, | |
279 | we can eliminate the sluggish issue caused by slow checkpoint | |
280 | operation when the checkpoint is done in a process context in | |
281 | a cgroup having low i/o budget and cpu shares. To make this | |
282 | do better, we set the default i/o priority of the kernel daemon | |
283 | to "3", to give one higher priority than other kernel threads. | |
284 | This is the same way to give a I/O priority to the jbd2 | |
285 | journaling thread of ext4 filesystem. | |
286 | nocheckpoint_merge Disable checkpoint merge feature. | |
9aa1ccb4 JC |
287 | compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo", |
288 | "lz4", "zstd" and "lzo-rle" algorithm. | |
3fde13f8 CY |
289 | compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only |
290 | "lz4" and "zstd" support compress level config. | |
291 | algorithm level range | |
292 | lz4 3 - 16 | |
293 | zstd 1 - 22 | |
622d6f19 RD |
294 | compress_log_size=%u Support configuring compress cluster size. The size will |
295 | be 4KB * (1 << %u). The default and minimum sizes are 16KB. | |
9aa1ccb4 JC |
296 | compress_extension=%s Support adding specified extension, so that f2fs can enable |
297 | compression on those corresponding files, e.g. if all files | |
298 | with '.ext' has high compression rate, we can set the '.ext' | |
299 | on compression extension list and enable compression on | |
300 | these file by default rather than to enable it via ioctl. | |
301 | For other files, we can still enable compression via ioctl. | |
086ba2ec LT |
302 | Note that, there is one reserved special extension '*', it |
303 | can be set to enable compression for all files. | |
8c3b0188 | 304 | nocompress_extension=%s Support adding specified extension, so that f2fs can disable |
151b1982 FC |
305 | compression on those corresponding files, just contrary to compression extension. |
306 | If you know exactly which files cannot be compressed, you can use this. | |
307 | The same extension name can't appear in both compress and nocompress | |
308 | extension at the same time. | |
309 | If the compress extension specifies all files, the types specified by the | |
310 | nocompress extension will be treated as special cases and will not be compressed. | |
311 | Don't allow use '*' to specifie all file in nocompress extension. | |
312 | After add nocompress_extension, the priority should be: | |
313 | dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag. | |
314 | See more in compression sections. | |
315 | ||
b28f047b | 316 | compress_chksum Support verifying chksum of raw data in compressed cluster. |
602a16d5 DJ |
317 | compress_mode=%s Control file compression mode. This supports "fs" and "user" |
318 | modes. In "fs" mode (default), f2fs does automatic compression | |
319 | on the compression enabled files. In "user" mode, f2fs disables | |
320 | the automaic compression and gives the user discretion of | |
321 | choosing the target file and the timing. The user can do manual | |
322 | compression/decompression on the compression enabled files using | |
323 | ioctls. | |
6ce19aff CY |
324 | compress_cache Support to use address space of a filesystem managed inode to |
325 | cache compressed block, in order to improve cache hit ratio of | |
326 | random read. | |
2324d50d LT |
327 | inlinecrypt When possible, encrypt/decrypt the contents of encrypted |
328 | files using the blk-crypto framework rather than | |
329 | filesystem-layer encryption. This allows the use of | |
330 | inline encryption hardware. The on-disk format is | |
331 | unaffected. For more details, see | |
332 | Documentation/block/inline-encryption.rst. | |
093749e2 CY |
333 | atgc Enable age-threshold garbage collection, it provides high |
334 | effectiveness and efficiency on background GC. | |
4f993264 CY |
335 | discard_unit=%s Control discard unit, the argument can be "block", "segment" |
336 | and "section", issued discard command's offset/size will be | |
337 | aligned to the unit, by default, "discard_unit=block" is set, | |
338 | so that small discard functionality is enabled. | |
339 | For blkzoned device, "discard_unit=section" will be set by | |
340 | default, it is helpful for large sized SMR or ZNS devices to | |
341 | reduce memory cost by getting rid of fs metadata supports small | |
342 | discard. | |
7a8fc586 DJ |
343 | memory=%s Control memory mode. This supports "normal" and "low" modes. |
344 | "low" mode is introduced to support low memory devices. | |
345 | Because of the nature of low memory devices, in this mode, f2fs | |
346 | will try to save memory sometimes by sacrificing performance. | |
347 | "normal" mode is the default mode and same as before. | |
71644dff JK |
348 | age_extent_cache Enable an age extent cache based on rb-tree. It records |
349 | data block update frequency of the extent per inode, in | |
350 | order to provide better temperature hints for data block | |
351 | allocation. | |
b62e71be CY |
352 | errors=%s Specify f2fs behavior on critical errors. This supports modes: |
353 | "panic", "continue" and "remount-ro", respectively, trigger | |
354 | panic immediately, continue without doing anything, and remount | |
355 | the partition in read-only mode. By default it uses "continue" | |
356 | mode. | |
357 | ====================== =============== =============== ======== | |
358 | mode continue remount-ro panic | |
359 | ====================== =============== =============== ======== | |
d56b699d | 360 | access ops normal normal N/A |
b62e71be CY |
361 | syscall errors -EIO -EROFS N/A |
362 | mount option rw ro N/A | |
363 | pending dir write keep keep N/A | |
364 | pending non-dir write drop keep N/A | |
365 | pending node write drop keep N/A | |
366 | pending meta write keep keep N/A | |
367 | ====================== =============== =============== ======== | |
9aa1ccb4 | 368 | ======================== ============================================================ |
98e4da8c | 369 | |
89272ca1 MCC |
370 | Debugfs Entries |
371 | =============== | |
98e4da8c JK |
372 | |
373 | /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as | |
374 | f2fs. Each file shows the whole f2fs information. | |
375 | ||
376 | /sys/kernel/debug/f2fs/status includes: | |
89272ca1 | 377 | |
98e4da8c JK |
378 | - major file system information managed by f2fs currently |
379 | - average SIT information about whole segments | |
380 | - current memory footprint consumed by f2fs. | |
381 | ||
89272ca1 MCC |
382 | Sysfs Entries |
383 | ============= | |
b59d0bae | 384 | |
6de3f12e | 385 | Information about mounted f2fs file systems can be found in |
b59d0bae NJ |
386 | /sys/fs/f2fs. Each mounted filesystem will have a directory in |
387 | /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). | |
388 | The files in each per-device directory are shown in table below. | |
389 | ||
390 | Files in /sys/fs/f2fs/<devname> | |
391 | (see also Documentation/ABI/testing/sysfs-fs-f2fs) | |
5aba5430 | 392 | |
89272ca1 MCC |
393 | Usage |
394 | ===== | |
98e4da8c JK |
395 | |
396 | 1. Download userland tools and compile them. | |
397 | ||
398 | 2. Skip, if f2fs was compiled statically inside kernel. | |
89272ca1 MCC |
399 | Otherwise, insert the f2fs.ko module:: |
400 | ||
401 | # insmod f2fs.ko | |
98e4da8c | 402 | |
ca313c82 | 403 | 3. Create a directory to use when mounting:: |
98e4da8c | 404 | |
89272ca1 MCC |
405 | # mkdir /mnt/f2fs |
406 | ||
407 | 4. Format the block device, and then mount as f2fs:: | |
408 | ||
409 | # mkfs.f2fs -l label /dev/block_device | |
410 | # mount -t f2fs /dev/block_device /mnt/f2fs | |
98e4da8c | 411 | |
d51a7fba CL |
412 | mkfs.f2fs |
413 | --------- | |
414 | The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, | |
415 | which builds a basic on-disk layout. | |
416 | ||
568d2a1e | 417 | The quick options consist of: |
89272ca1 MCC |
418 | |
419 | =============== =========================================================== | |
420 | ``-l [label]`` Give a volume label, up to 512 unicode name. | |
421 | ``-a [0 or 1]`` Split start location of each area for heap-based allocation. | |
422 | ||
423 | 1 is set by default, which performs this. | |
424 | ``-o [int]`` Set overprovision ratio in percent over volume size. | |
425 | ||
426 | 5 is set by default. | |
427 | ``-s [int]`` Set the number of segments per section. | |
428 | ||
429 | 1 is set by default. | |
430 | ``-z [int]`` Set the number of sections per zone. | |
431 | ||
432 | 1 is set by default. | |
433 | ``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov" | |
434 | ``-t [0 or 1]`` Disable discard command or not. | |
435 | ||
436 | 1 is set by default, which conducts discard. | |
437 | =============== =========================================================== | |
98e4da8c | 438 | |
ca313c82 | 439 | Note: please refer to the manpage of mkfs.f2fs(8) to get full option list. |
568d2a1e | 440 | |
d51a7fba CL |
441 | fsck.f2fs |
442 | --------- | |
443 | The fsck.f2fs is a tool to check the consistency of an f2fs-formatted | |
444 | partition, which examines whether the filesystem metadata and user-made data | |
445 | are cross-referenced correctly or not. | |
446 | Note that, initial version of the tool does not fix any inconsistency. | |
447 | ||
568d2a1e | 448 | The quick options consist of:: |
89272ca1 | 449 | |
d51a7fba CL |
450 | -d debug level [default:0] |
451 | ||
ca313c82 | 452 | Note: please refer to the manpage of fsck.f2fs(8) to get full option list. |
568d2a1e | 453 | |
d51a7fba CL |
454 | dump.f2fs |
455 | --------- | |
456 | 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. | |
458 | ||
459 | The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. | |
4bb9998d | 460 | It shows on-disk inode information recognized by a given inode number, and is |
d51a7fba CL |
461 | able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and |
462 | ./dump_sit respectively. | |
463 | ||
89272ca1 MCC |
464 | The options consist of:: |
465 | ||
d51a7fba CL |
466 | -d debug level [default:0] |
467 | -i inode no (hex) | |
468 | -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] | |
469 | -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] | |
470 | ||
89272ca1 MCC |
471 | Examples:: |
472 | ||
473 | # dump.f2fs -i [ino] /dev/sdx | |
474 | # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) | |
475 | # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) | |
d51a7fba | 476 | |
ca313c82 | 477 | Note: please refer to the manpage of dump.f2fs(8) to get full option list. |
568d2a1e JK |
478 | |
479 | sload.f2fs | |
480 | ---------- | |
d56b699d | 481 | The sload.f2fs gives a way to insert files and directories in the existing disk |
568d2a1e JK |
482 | image. This tool is useful when building f2fs images given compiled files. |
483 | ||
ca313c82 | 484 | Note: please refer to the manpage of sload.f2fs(8) to get full option list. |
568d2a1e JK |
485 | |
486 | resize.f2fs | |
487 | ----------- | |
ca313c82 | 488 | The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving |
568d2a1e JK |
489 | all the files and directories stored in the image. |
490 | ||
ca313c82 | 491 | Note: please refer to the manpage of resize.f2fs(8) to get full option list. |
568d2a1e JK |
492 | |
493 | defrag.f2fs | |
494 | ----------- | |
ca313c82 | 495 | The defrag.f2fs can be used to defragment scattered written data as well as |
568d2a1e JK |
496 | filesystem metadata across the disk. This can improve the write speed by giving |
497 | more free consecutive space. | |
498 | ||
ca313c82 | 499 | Note: please refer to the manpage of defrag.f2fs(8) to get full option list. |
568d2a1e JK |
500 | |
501 | f2fs_io | |
502 | ------- | |
503 | The f2fs_io is a simple tool to issue various filesystem APIs as well as | |
504 | f2fs-specific ones, which is very useful for QA tests. | |
505 | ||
ca313c82 | 506 | Note: please refer to the manpage of f2fs_io(8) to get full option list. |
568d2a1e | 507 | |
89272ca1 MCC |
508 | Design |
509 | ====== | |
98e4da8c JK |
510 | |
511 | On-disk Layout | |
512 | -------------- | |
513 | ||
514 | 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 consecutive segments, and a zone | |
516 | consists of a set of sections. By default, section and zone sizes are set to one | |
517 | segment size identically, but users can easily modify the sizes by mkfs. | |
518 | ||
519 | F2FS splits the entire volume into six areas, and all the areas except superblock | |
ca313c82 | 520 | consist of multiple segments as described below:: |
98e4da8c JK |
521 | |
522 | align with the zone size <-| | |
523 | |-> align with the segment size | |
524 | _________________________________________________________________________ | |
9268cc35 HL |
525 | | | | Segment | Node | Segment | | |
526 | | Superblock | Checkpoint | Info. | Address | Summary | Main | | |
527 | | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | | |
98e4da8c JK |
528 | |____________|_____2______|______N______|______N______|______N_____|__N___| |
529 | . . | |
530 | . . | |
531 | . . | |
532 | ._________________________________________. | |
533 | |_Segment_|_..._|_Segment_|_..._|_Segment_| | |
534 | . . | |
535 | ._________._________ | |
536 | |_section_|__...__|_ | |
537 | . . | |
538 | .________. | |
539 | |__zone__| | |
540 | ||
541 | - Superblock (SB) | |
89272ca1 | 542 | It is located at the beginning of the partition, and there exist two copies |
98e4da8c JK |
543 | to avoid file system crash. It contains basic partition information and some |
544 | default parameters of f2fs. | |
545 | ||
546 | - Checkpoint (CP) | |
89272ca1 | 547 | It contains file system information, bitmaps for valid NAT/SIT sets, orphan |
98e4da8c JK |
548 | inode lists, and summary entries of current active segments. |
549 | ||
98e4da8c | 550 | - Segment Information Table (SIT) |
89272ca1 | 551 | It contains segment information such as valid block count and bitmap for the |
98e4da8c JK |
552 | validity of all the blocks. |
553 | ||
9268cc35 | 554 | - Node Address Table (NAT) |
89272ca1 | 555 | It is composed of a block address table for all the node blocks stored in |
9268cc35 HL |
556 | Main area. |
557 | ||
98e4da8c | 558 | - Segment Summary Area (SSA) |
89272ca1 | 559 | It contains summary entries which contains the owner information of all the |
98e4da8c JK |
560 | data and node blocks stored in Main area. |
561 | ||
562 | - Main Area | |
89272ca1 | 563 | It contains file and directory data including their indices. |
98e4da8c JK |
564 | |
565 | In order to avoid misalignment between file system and flash-based storage, F2FS | |
566 | 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 size by reserving some segments | |
568 | in SSA area. | |
569 | ||
570 | Reference the following survey for additional technical details. | |
571 | https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey | |
572 | ||
573 | File System Metadata Structure | |
574 | ------------------------------ | |
575 | ||
576 | F2FS adopts the checkpointing scheme to maintain file system consistency. At | |
577 | mount time, F2FS first tries to find the last valid checkpoint data by scanning | |
578 | 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 data, which is called as shadow copy | |
580 | mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. | |
581 | ||
582 | For file system consistency, each CP points to which NAT and SIT copies are | |
89272ca1 | 583 | valid, as shown as below:: |
98e4da8c JK |
584 | |
585 | +--------+----------+---------+ | |
9268cc35 | 586 | | CP | SIT | NAT | |
98e4da8c JK |
587 | +--------+----------+---------+ |
588 | . . . . | |
589 | . . . . | |
590 | . . . . | |
591 | +-------+-------+--------+--------+--------+--------+ | |
9268cc35 | 592 | | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | |
98e4da8c JK |
593 | +-------+-------+--------+--------+--------+--------+ |
594 | | ^ ^ | |
595 | | | | | |
596 | `----------------------------------------' | |
597 | ||
598 | Index Structure | |
599 | --------------- | |
600 | ||
601 | The key data structure to manage the data locations is a "node". Similar to | |
602 | traditional file structures, F2FS has three types of node: inode, direct node, | |
d08ab08d | 603 | indirect node. F2FS assigns 4KB to an inode block which contains 923 data block |
98e4da8c JK |
604 | indices, two direct node pointers, two indirect node pointers, and one double |
605 | indirect node pointer as described below. One direct node block contains 1018 | |
606 | data blocks, and one indirect node block contains also 1018 node blocks. Thus, | |
89272ca1 | 607 | one inode block (i.e., a file) covers:: |
98e4da8c JK |
608 | |
609 | 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. | |
610 | ||
611 | Inode block (4KB) | |
612 | |- data (923) | |
613 | |- direct node (2) | |
614 | | `- data (1018) | |
615 | |- indirect node (2) | |
616 | | `- direct node (1018) | |
617 | | `- data (1018) | |
618 | `- double indirect node (1) | |
619 | `- indirect node (1018) | |
620 | `- direct node (1018) | |
621 | `- data (1018) | |
622 | ||
ca313c82 | 623 | Note that all the node blocks are mapped by NAT which means the location of |
98e4da8c JK |
624 | each node is translated by the NAT table. In the consideration of the wandering |
625 | tree problem, F2FS is able to cut off the propagation of node updates caused by | |
626 | leaf data writes. | |
627 | ||
628 | Directory Structure | |
629 | ------------------- | |
630 | ||
631 | A directory entry occupies 11 bytes, which consists of the following attributes. | |
632 | ||
633 | - hash hash value of the file name | |
634 | - ino inode number | |
635 | - len the length of file name | |
636 | - type file type such as directory, symlink, etc | |
637 | ||
638 | A dentry block consists of 214 dentry slots and file names. Therein a bitmap is | |
639 | used to represent whether each dentry is valid or not. A dentry block occupies | |
640 | 4KB with the following composition. | |
641 | ||
89272ca1 MCC |
642 | :: |
643 | ||
98e4da8c JK |
644 | Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + |
645 | dentries(11 * 214 bytes) + file name (8 * 214 bytes) | |
646 | ||
647 | [Bucket] | |
648 | +--------------------------------+ | |
649 | |dentry block 1 | dentry block 2 | | |
650 | +--------------------------------+ | |
651 | . . | |
652 | . . | |
653 | . [Dentry Block Structure: 4KB] . | |
654 | +--------+----------+----------+------------+ | |
655 | | bitmap | reserved | dentries | file names | | |
656 | +--------+----------+----------+------------+ | |
657 | [Dentry Block: 4KB] . . | |
658 | . . | |
659 | . . | |
660 | +------+------+-----+------+ | |
661 | | hash | ino | len | type | | |
662 | +------+------+-----+------+ | |
663 | [Dentry Structure: 11 bytes] | |
664 | ||
665 | F2FS implements multi-level hash tables for directory structure. Each level has | |
666 | a hash table with dedicated number of hash buckets as shown below. Note that | |
667 | "A(2B)" means a bucket includes 2 data blocks. | |
668 | ||
89272ca1 MCC |
669 | :: |
670 | ||
671 | ---------------------- | |
672 | A : bucket | |
673 | B : block | |
674 | N : MAX_DIR_HASH_DEPTH | |
675 | ---------------------- | |
98e4da8c | 676 | |
89272ca1 MCC |
677 | level #0 | A(2B) |
678 | | | |
679 | level #1 | A(2B) - A(2B) | |
680 | | | |
681 | level #2 | A(2B) - A(2B) - A(2B) - A(2B) | |
682 | . | . . . . | |
683 | level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) | |
684 | . | . . . . | |
685 | level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) | |
98e4da8c | 686 | |
89272ca1 | 687 | The number of blocks and buckets are determined by:: |
98e4da8c JK |
688 | |
689 | ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, | |
690 | # of blocks in level #n = | | |
691 | `- 4, Otherwise | |
692 | ||
bfec07d0 CY |
693 | ,- 2^(n + dir_level), |
694 | | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, | |
98e4da8c | 695 | # of buckets in level #n = | |
bfec07d0 CY |
696 | `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), |
697 | Otherwise | |
98e4da8c JK |
698 | |
699 | 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 table in level #0 to find the | |
701 | 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 way, F2FS scans hash tables in | |
ca313c82 | 703 | each levels incrementally from 1 to N. In each level F2FS needs to scan only |
98e4da8c | 704 | one bucket determined by the following equation, which shows O(log(# of files)) |
89272ca1 | 705 | complexity:: |
98e4da8c JK |
706 | |
707 | bucket number to scan in level #n = (hash value) % (# of buckets in level #n) | |
708 | ||
709 | In the case of file creation, F2FS finds empty consecutive slots that cover the | |
710 | 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. | |
712 | ||
89272ca1 MCC |
713 | The following figure shows an example of two cases holding children:: |
714 | ||
98e4da8c JK |
715 | --------------> Dir <-------------- |
716 | | | | |
717 | child child | |
718 | ||
719 | child - child [hole] - child | |
720 | ||
721 | child - child - child [hole] - [hole] - child | |
722 | ||
723 | Case 1: Case 2: | |
724 | Number of children = 6, Number of children = 3, | |
725 | File size = 7 File size = 7 | |
726 | ||
727 | Default Block Allocation | |
728 | ------------------------ | |
729 | ||
730 | At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node | |
731 | and Hot/Warm/Cold data. | |
732 | ||
733 | - Hot node contains direct node blocks of directories. | |
734 | - Warm node contains direct node blocks except hot node blocks. | |
735 | - Cold node contains indirect node blocks | |
736 | - Hot data contains dentry blocks | |
737 | - Warm data contains data blocks except hot and cold data blocks | |
738 | - Cold data contains multimedia data or migrated data blocks | |
739 | ||
740 | LFS has two schemes for free space management: threaded log and copy-and-compac- | |
741 | tion. The copy-and-compaction scheme which is known as cleaning, is well-suited | |
742 | for devices showing very good sequential write performance, since free segments | |
743 | are served all the time for writing new data. However, it suffers from cleaning | |
744 | overhead under high utilization. Contrarily, the threaded log scheme suffers | |
745 | from random writes, but no cleaning process is needed. F2FS adopts a hybrid | |
746 | scheme where the copy-and-compaction scheme is adopted by default, but the | |
747 | policy is dynamically changed to the threaded log scheme according to the file | |
748 | system status. | |
749 | ||
750 | In order to align F2FS with underlying flash-based storage, F2FS allocates a | |
751 | 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 FTL. Furthermore, with respect | |
753 | to the mapping granularity in FTL, F2FS allocates each section of the active | |
754 | logs from different zones as much as possible, since FTL can write the data in | |
755 | the active logs into one allocation unit according to its mapping granularity. | |
756 | ||
757 | Cleaning process | |
758 | ---------------- | |
759 | ||
760 | F2FS does cleaning both on demand and in the background. On-demand cleaning is | |
761 | triggered when there are not enough free segments to serve VFS calls. Background | |
762 | cleaner is operated by a kernel thread, and triggers the cleaning job when the | |
763 | system is idle. | |
764 | ||
765 | F2FS supports two victim selection policies: greedy and cost-benefit algorithms. | |
766 | In the greedy algorithm, F2FS selects a victim segment having the smallest number | |
767 | of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment | |
768 | according to the segment age and the number of valid blocks in order to address | |
769 | log block thrashing problem in the greedy algorithm. F2FS adopts the greedy | |
770 | algorithm for on-demand cleaner, while background cleaner adopts cost-benefit | |
771 | algorithm. | |
772 | ||
773 | In order to identify whether the data in the victim segment are valid or not, | |
774 | F2FS manages a bitmap. Each bit represents the validity of a block, and the | |
775 | bitmap is composed of a bit stream covering whole blocks in main area. | |
8b3a0ca0 | 776 | |
cad3836f JK |
777 | Fallocate(2) Policy |
778 | ------------------- | |
779 | ||
ca313c82 | 780 | The default policy follows the below POSIX rule. |
cad3836f JK |
781 | |
782 | Allocating disk space | |
783 | The default operation (i.e., mode is zero) of fallocate() allocates | |
784 | the disk space within the range specified by offset and len. The | |
785 | file size (as reported by stat(2)) will be changed if offset+len is | |
786 | greater than the file size. Any subregion within the range specified | |
787 | by offset and len that did not contain data before the call will be | |
788 | initialized to zero. This default behavior closely resembles the | |
789 | behavior of the posix_fallocate(3) library function, and is intended | |
790 | as a method of optimally implementing that function. | |
791 | ||
792 | However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to | |
d56b699d | 793 | fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having |
cad3836f | 794 | zero or random data, which is useful to the below scenario where: |
89272ca1 | 795 | |
cad3836f JK |
796 | 1. create(fd) |
797 | 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) | |
798 | 3. fallocate(fd, 0, 0, size) | |
799 | 4. address = fibmap(fd, offset) | |
800 | 5. open(blkdev) | |
801 | 6. write(blkdev, address) | |
4c8ff709 CY |
802 | |
803 | Compression implementation | |
804 | -------------------------- | |
805 | ||
806 | - New term named cluster is defined as basic unit of compression, file can | |
89272ca1 MCC |
807 | be divided into multiple clusters logically. One cluster includes 4 << n |
808 | (n >= 0) logical pages, compression size is also cluster size, each of | |
809 | cluster can be compressed or not. | |
4c8ff709 CY |
810 | |
811 | - In cluster metadata layout, one special block address is used to indicate | |
ca313c82 | 812 | a cluster is a compressed one or normal one; for compressed cluster, following |
89272ca1 MCC |
813 | metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs |
814 | stores data including compress header and compressed data. | |
4c8ff709 CY |
815 | |
816 | - In order to eliminate write amplification during overwrite, F2FS only | |
89272ca1 | 817 | support compression on write-once file, data can be compressed only when |
4fc781a3 CY |
818 | all logical blocks in cluster contain valid data and compress ratio of |
819 | cluster data is lower than specified threshold. | |
4c8ff709 | 820 | |
151b1982 | 821 | - To enable compression on regular inode, there are four ways: |
89272ca1 MCC |
822 | |
823 | * chattr +c file | |
824 | * chattr +c dir; touch dir/file | |
825 | * mount w/ -o compress_extension=ext; touch file.ext | |
38740707 CY |
826 | * mount w/ -o compress_extension=*; touch any_file |
827 | ||
151b1982 FC |
828 | - To disable compression on regular inode, there are two ways: |
829 | ||
830 | * chattr -c file | |
831 | * mount w/ -o nocompress_extension=ext; touch file.ext | |
832 | ||
833 | - Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions: | |
834 | ||
835 | * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch | |
836 | dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt | |
837 | should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip | |
838 | can enable compress on bar.zip. | |
839 | * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch | |
840 | dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be | |
841 | compresse, bar.zip and baz.txt should be non-compressed. | |
842 | chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip | |
843 | and baz.txt. | |
844 | ||
38740707 CY |
845 | - At this point, compression feature doesn't expose compressed space to user |
846 | directly in order to guarantee potential data updates later to the space. | |
847 | Instead, the main goal is to reduce data writes to flash disk as much as | |
848 | possible, resulting in extending disk life time as well as relaxing IO | |
4a4fc043 | 849 | congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS) |
d218bee8 CL |
850 | interface to reclaim compressed space and show it to user after setting a |
851 | special flag to the inode. Once the compressed space is released, the flag | |
852 | will block writing data to the file until either the compressed space is | |
853 | reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is | |
854 | truncated to zero. | |
89272ca1 MCC |
855 | |
856 | Compress metadata layout:: | |
857 | ||
858 | [Dnode Structure] | |
859 | +-----------------------------------------------+ | |
860 | | cluster 1 | cluster 2 | ......... | cluster N | | |
861 | +-----------------------------------------------+ | |
862 | . . . . | |
d218bee8 | 863 | . . . . |
89272ca1 MCC |
864 | . Compressed Cluster . . Normal Cluster . |
865 | +----------+---------+---------+---------+ +---------+---------+---------+---------+ | |
866 | |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 | | |
867 | +----------+---------+---------+---------+ +---------+---------+---------+---------+ | |
d218bee8 | 868 | . . |
89272ca1 MCC |
869 | . . |
870 | . . | |
871 | +-------------+-------------+----------+----------------------------+ | |
872 | | data length | data chksum | reserved | compressed data | | |
873 | +-------------+-------------+----------+----------------------------+ | |
de881df9 | 874 | |
602a16d5 DJ |
875 | Compression mode |
876 | -------------------------- | |
877 | ||
878 | f2fs supports "fs" and "user" compression modes with "compression_mode" mount option. | |
879 | With this option, f2fs provides a choice to select the way how to compress the | |
880 | compression enabled files (refer to "Compression implementation" section for how to | |
881 | enable compression on a regular inode). | |
882 | ||
883 | 1) compress_mode=fs | |
884 | This is the default option. f2fs does automatic compression in the writeback of the | |
885 | compression enabled files. | |
886 | ||
887 | 2) compress_mode=user | |
092af2eb | 888 | This disables the automatic compression and gives the user discretion of choosing the |
602a16d5 DJ |
889 | target file and the timing. The user can do manual compression/decompression on the |
890 | compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE | |
891 | ioctls like the below. | |
892 | ||
893 | To decompress a file, | |
894 | ||
895 | fd = open(filename, O_WRONLY, 0); | |
896 | ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE); | |
897 | ||
898 | To compress a file, | |
899 | ||
900 | fd = open(filename, O_WRONLY, 0); | |
901 | ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE); | |
902 | ||
de881df9 AR |
903 | NVMe Zoned Namespace devices |
904 | ---------------------------- | |
905 | ||
906 | - ZNS defines a per-zone capacity which can be equal or less than the | |
907 | zone-size. Zone-capacity is the number of usable blocks in the zone. | |
ca313c82 | 908 | F2FS checks if zone-capacity is less than zone-size, if it is, then any |
de881df9 AR |
909 | segment which starts after the zone-capacity is marked as not-free in |
910 | the free segment bitmap at initial mount time. These segments are marked | |
911 | as permanently used so they are not allocated for writes and | |
912 | consequently are not needed to be garbage collected. In case the | |
913 | zone-capacity is not aligned to default segment size(2MB), then a segment | |
914 | can start before the zone-capacity and span across zone-capacity boundary. | |
915 | Such spanning segments are also considered as usable segments. All blocks | |
916 | past the zone-capacity are considered unusable in these segments. |