Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
kernel
os
linux
1.. SPDX-License-Identifier: GPL-2.0
2
3=================================
4Flash-Friendly File System (F2FS)
5=================================
6
7Overview
8========
9
10NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
11been equipped on a variety systems ranging from mobile to server systems. Since
12they are known to have different characteristics from the conventional rotating
13disks, a file system, an upper layer to the storage device, should adapt to the
14changes from the sketch in the design level.
15
16F2FS is a file system exploiting NAND flash memory-based storage devices, which
17is based on Log-structured File System (LFS). The design has been focused on
18addressing the fundamental issues in LFS, which are snowball effect of wandering
19tree and high cleaning overhead.
20
21Since a NAND flash memory-based storage device shows different characteristic
22according to its internal geometry or flash memory management scheme, namely FTL,
23F2FS and its tools support various parameters not only for configuring on-disk
24layout, but also for selecting allocation and cleaning algorithms.
25
26The following git tree provides the file system formatting tool (mkfs.f2fs),
27a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
28
29- git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
30
31For sending patches, please use the following mailing list:
32
33- linux-f2fs-devel@lists.sourceforge.net
34
35For reporting bugs, please use the following f2fs bug tracker link:
36
37- https://bugzilla.kernel.org/enter_bug.cgi?product=File%20System&component=f2fs
38
39Background and Design issues
40============================
41
42Log-structured File System (LFS)
43--------------------------------
44"A log-structured file system writes all modifications to disk sequentially in
45a log-like structure, thereby speeding up both file writing and crash recovery.
46The log is the only structure on disk; it contains indexing information so that
47files can be read back from the log efficiently. In order to maintain large free
48areas on disk for fast writing, we divide the log into segments and use a
49segment cleaner to compress the live information from heavily fragmented
50segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
51implementation of a log-structured file system", ACM Trans. Computer Systems
5210, 1, 26–52.
53
54Wandering Tree Problem
55----------------------
56In LFS, when a file data is updated and written to the end of log, its direct
57pointer block is updated due to the changed location. Then the indirect pointer
58block is also updated due to the direct pointer block update. In this manner,
59the upper index structures such as inode, inode map, and checkpoint block are
60also updated recursively. This problem is called as wandering tree problem [1],
61and in order to enhance the performance, it should eliminate or relax the update
62propagation as much as possible.
63
64[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
65
66Cleaning Overhead
67-----------------
68Since LFS is based on out-of-place writes, it produces so many obsolete blocks
69scattered across the whole storage. In order to serve new empty log space, it
70needs to reclaim these obsolete blocks seamlessly to users. This job is called
71as a cleaning process.
72
73The process consists of three operations as follows.
74
751. A victim segment is selected through referencing segment usage table.
762. It loads parent index structures of all the data in the victim identified by
77 segment summary blocks.
783. It checks the cross-reference between the data and its parent index structure.
794. It moves valid data selectively.
80
81This cleaning job may cause unexpected long delays, so the most important goal
82is to hide the latencies to users. And also definitely, it should reduce the
83amount of valid data to be moved, and move them quickly as well.
84
85Key Features
86============
87
88Flash Awareness
89---------------
90- Enlarge the random write area for better performance, but provide the high
91 spatial locality
92- Align FS data structures to the operational units in FTL as best efforts
93
94Wandering Tree Problem
95----------------------
96- Use a term, “node”, that represents inodes as well as various pointer blocks
97- Introduce Node Address Table (NAT) containing the locations of all the “node”
98 blocks; this will cut off the update propagation.
99
100Cleaning Overhead
101-----------------
102- Support a background cleaning process
103- Support greedy and cost-benefit algorithms for victim selection policies
104- Support multi-head logs for static/dynamic hot and cold data separation
105- Introduce adaptive logging for efficient block allocation
106
107Mount Options
108=============
109
110
111======================== ============================================================
112background_gc=%s Turn on/off cleaning operations, namely garbage
113 collection, triggered in background when I/O subsystem is
114 idle. If background_gc=on, it will turn on the garbage
115 collection and if background_gc=off, garbage collection
116 will be turned off. If background_gc=sync, it will turn
117 on synchronous garbage collection running in background.
118 Default value for this option is on. So garbage
119 collection is on by default.
120gc_merge When background_gc is on, this option can be enabled to
121 let background GC thread to handle foreground GC requests,
122 it can eliminate the sluggish issue caused by slow foreground
123 GC operation when GC is triggered from a process with limited
124 I/O and CPU resources.
125nogc_merge Disable GC merge feature.
126disable_roll_forward Disable the roll-forward recovery routine
127norecovery Disable the roll-forward recovery routine, mounted read-
128 only (i.e., -o ro,disable_roll_forward)
129discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
130 enabled, f2fs will issue discard/TRIM commands when a
131 segment is cleaned.
132heap/no_heap Deprecated.
133nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
134 by default if CONFIG_F2FS_FS_XATTR is selected.
135noacl Disable POSIX Access Control List. Note: acl is enabled
136 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
137active_logs=%u Support configuring the number of active logs. In the
138 current design, f2fs supports only 2, 4, and 6 logs.
139 Default number is 6.
140disable_ext_identify Disable the extension list configured by mkfs, so f2fs
141 is not aware of cold files such as media files.
142inline_xattr Enable the inline xattrs feature.
143noinline_xattr Disable the inline xattrs feature.
144inline_xattr_size=%u Support configuring inline xattr size, it depends on
145 flexible inline xattr feature.
146inline_data Enable the inline data feature: Newly created small (<~3.4k)
147 files can be written into inode block.
148inline_dentry Enable the inline dir feature: data in newly created
149 directory entries can be written into inode block. The
150 space of inode block which is used to store inline
151 dentries is limited to ~3.4k.
152noinline_dentry Disable the inline dentry feature.
153flush_merge Merge concurrent cache_flush commands as much as possible
154 to eliminate redundant command issues. If the underlying
155 device handles the cache_flush command relatively slowly,
156 recommend to enable this option.
157nobarrier This option can be used if underlying storage guarantees
158 its cached data should be written to the novolatile area.
159 If this option is set, no cache_flush commands are issued
160 but f2fs still guarantees the write ordering of all the
161 data writes.
162barrier If this option is set, cache_flush commands are allowed to be
163 issued.
164fastboot This option is used when a system wants to reduce mount
165 time as much as possible, even though normal performance
166 can be sacrificed.
167extent_cache Enable an extent cache based on rb-tree, it can cache
168 as many as extent which map between contiguous logical
169 address and physical address per inode, resulting in
170 increasing the cache hit ratio. Set by default.
171noextent_cache Disable an extent cache based on rb-tree explicitly, see
172 the above extent_cache mount option.
173noinline_data Disable the inline data feature, inline data feature is
174 enabled by default.
175data_flush Enable data flushing before checkpoint in order to
176 persist data of regular and symlink.
177reserve_root=%d Support configuring reserved space which is used for
178 allocation from a privileged user with specified uid or
179 gid, unit: 4KB, the default limit is 12.5% of user blocks.
180reserve_node=%d Support configuring reserved nodes which are used for
181 allocation from a privileged user with specified uid or
182 gid, the default limit is 12.5% of all nodes.
183resuid=%d The user ID which may use the reserved blocks and nodes.
184resgid=%d The group ID which may use the reserved blocks and nodes.
185fault_injection=%d Enable fault injection in all supported types with
186 specified injection rate.
187fault_type=%d Support configuring fault injection type, should be
188 enabled with fault_injection option, fault type value
189 is shown below, it supports single or combined type.
190
191 .. code-block:: none
192
193 =========================== ==========
194 Type_Name Type_Value
195 =========================== ==========
196 FAULT_KMALLOC 0x00000001
197 FAULT_KVMALLOC 0x00000002
198 FAULT_PAGE_ALLOC 0x00000004
199 FAULT_PAGE_GET 0x00000008
200 FAULT_ALLOC_BIO 0x00000010 (obsolete)
201 FAULT_ALLOC_NID 0x00000020
202 FAULT_ORPHAN 0x00000040
203 FAULT_BLOCK 0x00000080
204 FAULT_DIR_DEPTH 0x00000100
205 FAULT_EVICT_INODE 0x00000200
206 FAULT_TRUNCATE 0x00000400
207 FAULT_READ_IO 0x00000800
208 FAULT_CHECKPOINT 0x00001000
209 FAULT_DISCARD 0x00002000 (obsolete)
210 FAULT_WRITE_IO 0x00004000
211 FAULT_SLAB_ALLOC 0x00008000
212 FAULT_DQUOT_INIT 0x00010000
213 FAULT_LOCK_OP 0x00020000
214 FAULT_BLKADDR_VALIDITY 0x00040000
215 FAULT_BLKADDR_CONSISTENCE 0x00080000
216 FAULT_NO_SEGMENT 0x00100000
217 FAULT_INCONSISTENT_FOOTER 0x00200000
218 FAULT_ATOMIC_TIMEOUT 0x00400000 (1000ms)
219 FAULT_VMALLOC 0x00800000
220 FAULT_LOCK_TIMEOUT 0x01000000 (1000ms)
221 FAULT_SKIP_WRITE 0x02000000
222 =========================== ==========
223mode=%s Control block allocation mode which supports "adaptive"
224 and "lfs". In "lfs" mode, there should be no random
225 writes towards main area.
226 "fragment:segment" and "fragment:block" are newly added here.
227 These are developer options for experiments to simulate filesystem
228 fragmentation/after-GC situation itself. The developers use these
229 modes to understand filesystem fragmentation/after-GC condition well,
230 and eventually get some insights to handle them better.
231 In "fragment:segment", f2fs allocates a new segment in random
232 position. With this, we can simulate the after-GC condition.
233 In "fragment:block", we can scatter block allocation with
234 "max_fragment_chunk" and "max_fragment_hole" sysfs nodes.
235 We added some randomness to both chunk and hole size to make
236 it close to realistic IO pattern. So, in this mode, f2fs will allocate
237 1..<max_fragment_chunk> blocks in a chunk and make a hole in the
238 length of 1..<max_fragment_hole> by turns. With this, the newly
239 allocated blocks will be scattered throughout the whole partition.
240 Note that "fragment:block" implicitly enables "fragment:segment"
241 option for more randomness.
242 Please, use these options for your experiments and we strongly
243 recommend to re-format the filesystem after using these options.
244usrquota Enable plain user disk quota accounting.
245grpquota Enable plain group disk quota accounting.
246prjquota Enable plain project quota accounting.
247usrjquota=<file> Appoint specified file and type during mount, so that quota
248grpjquota=<file> information can be properly updated during recovery flow,
249prjjquota=<file> <quota file>: must be in root directory;
250jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
251usrjquota= Turn off user journalled quota.
252grpjquota= Turn off group journalled quota.
253prjjquota= Turn off project journalled quota.
254quota Enable plain user disk quota accounting.
255noquota Disable all plain disk quota option.
256alloc_mode=%s Adjust block allocation policy, which supports "reuse"
257 and "default".
258fsync_mode=%s Control the policy of fsync. Currently supports "posix",
259 "strict", and "nobarrier". In "posix" mode, which is
260 default, fsync will follow POSIX semantics and does a
261 light operation to improve the filesystem performance.
262 In "strict" mode, fsync will be heavy and behaves in line
263 with xfs, ext4 and btrfs, where xfstest generic/342 will
264 pass, but the performance will regress. "nobarrier" is
265 based on "posix", but doesn't issue flush command for
266 non-atomic files likewise "nobarrier" mount option.
267test_dummy_encryption
268test_dummy_encryption=%s
269 Enable dummy encryption, which provides a fake fscrypt
270 context. The fake fscrypt context is used by xfstests.
271 The argument may be either "v1" or "v2", in order to
272 select the corresponding fscrypt policy version.
273checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable"
274 to re-enable checkpointing. Is enabled by default. While
275 disabled, any unmounting or unexpected shutdowns will cause
276 the filesystem contents to appear as they did when the
277 filesystem was mounted with that option.
278 While mounting with checkpoint=disable, the filesystem must
279 run garbage collection to ensure that all available space can
280 be used. If this takes too much time, the mount may return
281 EAGAIN. You may optionally add a value to indicate how much
282 of the disk you would be willing to temporarily give up to
283 avoid additional garbage collection. This can be given as a
284 number of blocks, or as a percent. For instance, mounting
285 with checkpoint=disable:100% would always succeed, but it may
286 hide up to all remaining free space. The actual space that
287 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
288 This space is reclaimed once checkpoint=enable.
289checkpoint_merge When checkpoint is enabled, this can be used to create a kernel
290 daemon and make it to merge concurrent checkpoint requests as
291 much as possible to eliminate redundant checkpoint issues. Plus,
292 we can eliminate the sluggish issue caused by slow checkpoint
293 operation when the checkpoint is done in a process context in
294 a cgroup having low i/o budget and cpu shares. To make this
295 do better, we set the default i/o priority of the kernel daemon
296 to "3", to give one higher priority than other kernel threads.
297 This is the same way to give a I/O priority to the jbd2
298 journaling thread of ext4 filesystem.
299nocheckpoint_merge Disable checkpoint merge feature.
300compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo",
301 "lz4", "zstd" and "lzo-rle" algorithm.
302compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only
303 "lz4" and "zstd" support compress level config::
304
305 ========= ===========
306 algorithm level range
307 ========= ===========
308 lz4 3 - 16
309 zstd 1 - 22
310 ========= ===========
311
312compress_log_size=%u Support configuring compress cluster size. The size will
313 be 4KB * (1 << %u). The default and minimum sizes are 16KB.
314compress_extension=%s Support adding specified extension, so that f2fs can enable
315 compression on those corresponding files, e.g. if all files
316 with '.ext' has high compression rate, we can set the '.ext'
317 on compression extension list and enable compression on
318 these file by default rather than to enable it via ioctl.
319 For other files, we can still enable compression via ioctl.
320 Note that, there is one reserved special extension '*', it
321 can be set to enable compression for all files.
322nocompress_extension=%s Support adding specified extension, so that f2fs can disable
323 compression on those corresponding files, just contrary to compression extension.
324 If you know exactly which files cannot be compressed, you can use this.
325 The same extension name can't appear in both compress and nocompress
326 extension at the same time.
327 If the compress extension specifies all files, the types specified by the
328 nocompress extension will be treated as special cases and will not be compressed.
329 Don't allow use '*' to specifie all file in nocompress extension.
330 After add nocompress_extension, the priority should be:
331 dir_flag < comp_extention,nocompress_extension < comp_file_flag,no_comp_file_flag.
332 See more in compression sections.
333
334compress_chksum Support verifying chksum of raw data in compressed cluster.
335compress_mode=%s Control file compression mode. This supports "fs" and "user"
336 modes. In "fs" mode (default), f2fs does automatic compression
337 on the compression enabled files. In "user" mode, f2fs disables
338 the automaic compression and gives the user discretion of
339 choosing the target file and the timing. The user can do manual
340 compression/decompression on the compression enabled files using
341 ioctls.
342compress_cache Support to use address space of a filesystem managed inode to
343 cache compressed block, in order to improve cache hit ratio of
344 random read.
345inlinecrypt When possible, encrypt/decrypt the contents of encrypted
346 files using the blk-crypto framework rather than
347 filesystem-layer encryption. This allows the use of
348 inline encryption hardware. The on-disk format is
349 unaffected. For more details, see
350 Documentation/block/inline-encryption.rst.
351atgc Enable age-threshold garbage collection, it provides high
352 effectiveness and efficiency on background GC.
353discard_unit=%s Control discard unit, the argument can be "block", "segment"
354 and "section", issued discard command's offset/size will be
355 aligned to the unit, by default, "discard_unit=block" is set,
356 so that small discard functionality is enabled.
357 For blkzoned device, "discard_unit=section" will be set by
358 default, it is helpful for large sized SMR or ZNS devices to
359 reduce memory cost by getting rid of fs metadata supports small
360 discard.
361memory=%s Control memory mode. This supports "normal" and "low" modes.
362 "low" mode is introduced to support low memory devices.
363 Because of the nature of low memory devices, in this mode, f2fs
364 will try to save memory sometimes by sacrificing performance.
365 "normal" mode is the default mode and same as before.
366age_extent_cache Enable an age extent cache based on rb-tree. It records
367 data block update frequency of the extent per inode, in
368 order to provide better temperature hints for data block
369 allocation.
370errors=%s Specify f2fs behavior on critical errors. This supports modes:
371 "panic", "continue" and "remount-ro", respectively, trigger
372 panic immediately, continue without doing anything, and remount
373 the partition in read-only mode. By default it uses "continue"
374 mode.
375
376 .. code-block:: none
377
378 ====================== =============== =============== ========
379 mode continue remount-ro panic
380 ====================== =============== =============== ========
381 access ops normal normal N/A
382 syscall errors -EIO -EROFS N/A
383 mount option rw ro N/A
384 pending dir write keep keep N/A
385 pending non-dir write drop keep N/A
386 pending node write drop keep N/A
387 pending meta write keep keep N/A
388 ====================== =============== =============== ========
389nat_bits Enable nat_bits feature to enhance full/empty nat blocks access,
390 by default it's disabled.
391lookup_mode=%s Control the directory lookup behavior for casefolded
392 directories. This option has no effect on directories
393 that do not have the casefold feature enabled.
394
395 .. code-block:: none
396
397 ================== ========================================
398 Value Description
399 ================== ========================================
400 perf (Default) Enforces a hash-only lookup.
401 The linear search fallback is always
402 disabled, ignoring the on-disk flag.
403 compat Enables the linear search fallback for
404 compatibility with directory entries
405 created by older kernel that used a
406 different case-folding algorithm.
407 This mode ignores the on-disk flag.
408 auto F2FS determines the mode based on the
409 on-disk `SB_ENC_NO_COMPAT_FALLBACK_FL`
410 flag.
411 ================== ========================================
412======================== ============================================================
413
414Debugfs Entries
415===============
416
417/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
418f2fs. Each file shows the whole f2fs information.
419
420/sys/kernel/debug/f2fs/status includes:
421
422 - major file system information managed by f2fs currently
423 - average SIT information about whole segments
424 - current memory footprint consumed by f2fs.
425
426Sysfs Entries
427=============
428
429Information about mounted f2fs file systems can be found in
430/sys/fs/f2fs. Each mounted filesystem will have a directory in
431/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
432The files in each per-device directory are shown in table below.
433
434Files in /sys/fs/f2fs/<devname>
435(see also Documentation/ABI/testing/sysfs-fs-f2fs)
436
437Usage
438=====
439
4401. Download userland tools and compile them.
441
4422. Skip, if f2fs was compiled statically inside kernel.
443 Otherwise, insert the f2fs.ko module::
444
445 # insmod f2fs.ko
446
4473. Create a directory to use when mounting::
448
449 # mkdir /mnt/f2fs
450
4514. Format the block device, and then mount as f2fs::
452
453 # mkfs.f2fs -l label /dev/block_device
454 # mount -t f2fs /dev/block_device /mnt/f2fs
455
456mkfs.f2fs
457---------
458The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
459which builds a basic on-disk layout.
460
461The quick options consist of:
462
463=============== ===========================================================
464``-l [label]`` Give a volume label, up to 512 unicode name.
465``-a [0 or 1]`` Split start location of each area for heap-based allocation.
466
467 1 is set by default, which performs this.
468``-o [int]`` Set overprovision ratio in percent over volume size.
469
470 5 is set by default.
471``-s [int]`` Set the number of segments per section.
472
473 1 is set by default.
474``-z [int]`` Set the number of sections per zone.
475
476 1 is set by default.
477``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov"
478``-t [0 or 1]`` Disable discard command or not.
479
480 1 is set by default, which conducts discard.
481=============== ===========================================================
482
483Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.
484
485fsck.f2fs
486---------
487The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
488partition, which examines whether the filesystem metadata and user-made data
489are cross-referenced correctly or not.
490Note that, initial version of the tool does not fix any inconsistency.
491
492The quick options consist of::
493
494 -d debug level [default:0]
495
496Note: please refer to the manpage of fsck.f2fs(8) to get full option list.
497
498dump.f2fs
499---------
500The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
501file. Each file is dump_ssa and dump_sit.
502
503The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
504It shows on-disk inode information recognized by a given inode number, and is
505able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
506./dump_sit respectively.
507
508The options consist of::
509
510 -d debug level [default:0]
511 -i inode no (hex)
512 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
513 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
514
515Examples::
516
517 # dump.f2fs -i [ino] /dev/sdx
518 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
519 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
520
521Note: please refer to the manpage of dump.f2fs(8) to get full option list.
522
523sload.f2fs
524----------
525The sload.f2fs gives a way to insert files and directories in the existing disk
526image. This tool is useful when building f2fs images given compiled files.
527
528Note: please refer to the manpage of sload.f2fs(8) to get full option list.
529
530resize.f2fs
531-----------
532The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
533all the files and directories stored in the image.
534
535Note: please refer to the manpage of resize.f2fs(8) to get full option list.
536
537defrag.f2fs
538-----------
539The defrag.f2fs can be used to defragment scattered written data as well as
540filesystem metadata across the disk. This can improve the write speed by giving
541more free consecutive space.
542
543Note: please refer to the manpage of defrag.f2fs(8) to get full option list.
544
545f2fs_io
546-------
547The f2fs_io is a simple tool to issue various filesystem APIs as well as
548f2fs-specific ones, which is very useful for QA tests.
549
550Note: please refer to the manpage of f2fs_io(8) to get full option list.
551
552Design
553======
554
555On-disk Layout
556--------------
557
558F2FS divides the whole volume into a number of segments, each of which is fixed
559to 2MB in size. A section is composed of consecutive segments, and a zone
560consists of a set of sections. By default, section and zone sizes are set to one
561segment size identically, but users can easily modify the sizes by mkfs.
562
563F2FS splits the entire volume into six areas, and all the areas except superblock
564consist of multiple segments as described below::
565
566 align with the zone size <-|
567 |-> align with the segment size
568 _________________________________________________________________________
569 | | | Segment | Node | Segment | |
570 | Superblock | Checkpoint | Info. | Address | Summary | Main |
571 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
572 |____________|_____2______|______N______|______N______|______N_____|__N___|
573 . .
574 . .
575 . .
576 ._________________________________________.
577 |_Segment_|_..._|_Segment_|_..._|_Segment_|
578 . .
579 ._________._________
580 |_section_|__...__|_
581 . .
582 .________.
583 |__zone__|
584
585- Superblock (SB)
586 It is located at the beginning of the partition, and there exist two copies
587 to avoid file system crash. It contains basic partition information and some
588 default parameters of f2fs.
589
590- Checkpoint (CP)
591 It contains file system information, bitmaps for valid NAT/SIT sets, orphan
592 inode lists, and summary entries of current active segments.
593
594- Segment Information Table (SIT)
595 It contains segment information such as valid block count and bitmap for the
596 validity of all the blocks.
597
598- Node Address Table (NAT)
599 It is composed of a block address table for all the node blocks stored in
600 Main area.
601
602- Segment Summary Area (SSA)
603 It contains summary entries which contains the owner information of all the
604 data and node blocks stored in Main area.
605
606- Main Area
607 It contains file and directory data including their indices.
608
609In order to avoid misalignment between file system and flash-based storage, F2FS
610aligns the start block address of CP with the segment size. Also, it aligns the
611start block address of Main area with the zone size by reserving some segments
612in SSA area.
613
614Reference the following survey for additional technical details.
615https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
616
617File System Metadata Structure
618------------------------------
619
620F2FS adopts the checkpointing scheme to maintain file system consistency. At
621mount time, F2FS first tries to find the last valid checkpoint data by scanning
622CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
623One of them always indicates the last valid data, which is called as shadow copy
624mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
625
626For file system consistency, each CP points to which NAT and SIT copies are
627valid, as shown as below::
628
629 +--------+----------+---------+
630 | CP | SIT | NAT |
631 +--------+----------+---------+
632 . . . .
633 . . . .
634 . . . .
635 +-------+-------+--------+--------+--------+--------+
636 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
637 +-------+-------+--------+--------+--------+--------+
638 | ^ ^
639 | | |
640 `----------------------------------------'
641
642Index Structure
643---------------
644
645The key data structure to manage the data locations is a "node". Similar to
646traditional file structures, F2FS has three types of node: inode, direct node,
647indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
648indices, two direct node pointers, two indirect node pointers, and one double
649indirect node pointer as described below. One direct node block contains 1018
650data blocks, and one indirect node block contains also 1018 node blocks. Thus,
651one inode block (i.e., a file) covers::
652
653 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
654
655 Inode block (4KB)
656 |- data (923)
657 |- direct node (2)
658 | `- data (1018)
659 |- indirect node (2)
660 | `- direct node (1018)
661 | `- data (1018)
662 `- double indirect node (1)
663 `- indirect node (1018)
664 `- direct node (1018)
665 `- data (1018)
666
667Note that all the node blocks are mapped by NAT which means the location of
668each node is translated by the NAT table. In the consideration of the wandering
669tree problem, F2FS is able to cut off the propagation of node updates caused by
670leaf data writes.
671
672Directory Structure
673-------------------
674
675A directory entry occupies 11 bytes, which consists of the following attributes.
676
677- hash hash value of the file name
678- ino inode number
679- len the length of file name
680- type file type such as directory, symlink, etc
681
682A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
683used to represent whether each dentry is valid or not. A dentry block occupies
6844KB with the following composition.
685
686::
687
688 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
689 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
690
691 [Bucket]
692 +--------------------------------+
693 |dentry block 1 | dentry block 2 |
694 +--------------------------------+
695 . .
696 . .
697 . [Dentry Block Structure: 4KB] .
698 +--------+----------+----------+------------+
699 | bitmap | reserved | dentries | file names |
700 +--------+----------+----------+------------+
701 [Dentry Block: 4KB] . .
702 . .
703 . .
704 +------+------+-----+------+
705 | hash | ino | len | type |
706 +------+------+-----+------+
707 [Dentry Structure: 11 bytes]
708
709F2FS implements multi-level hash tables for directory structure. Each level has
710a hash table with dedicated number of hash buckets as shown below. Note that
711"A(2B)" means a bucket includes 2 data blocks.
712
713::
714
715 ----------------------
716 A : bucket
717 B : block
718 N : MAX_DIR_HASH_DEPTH
719 ----------------------
720
721 level #0 | A(2B)
722 |
723 level #1 | A(2B) - A(2B)
724 |
725 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
726 . | . . . .
727 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
728 . | . . . .
729 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
730
731The number of blocks and buckets are determined by::
732
733 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
734 # of blocks in level #n = |
735 `- 4, Otherwise
736
737 ,- 2^(n + dir_level),
738 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
739 # of buckets in level #n = |
740 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
741 Otherwise
742
743When F2FS finds a file name in a directory, at first a hash value of the file
744name is calculated. Then, F2FS scans the hash table in level #0 to find the
745dentry consisting of the file name and its inode number. If not found, F2FS
746scans the next hash table in level #1. In this way, F2FS scans hash tables in
747each levels incrementally from 1 to N. In each level F2FS needs to scan only
748one bucket determined by the following equation, which shows O(log(# of files))
749complexity::
750
751 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
752
753In the case of file creation, F2FS finds empty consecutive slots that cover the
754file name. F2FS searches the empty slots in the hash tables of whole levels from
7551 to N in the same way as the lookup operation.
756
757The following figure shows an example of two cases holding children::
758
759 --------------> Dir <--------------
760 | |
761 child child
762
763 child - child [hole] - child
764
765 child - child - child [hole] - [hole] - child
766
767 Case 1: Case 2:
768 Number of children = 6, Number of children = 3,
769 File size = 7 File size = 7
770
771Default Block Allocation
772------------------------
773
774At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
775and Hot/Warm/Cold data.
776
777- Hot node contains direct node blocks of directories.
778- Warm node contains direct node blocks except hot node blocks.
779- Cold node contains indirect node blocks
780- Hot data contains dentry blocks
781- Warm data contains data blocks except hot and cold data blocks
782- Cold data contains multimedia data or migrated data blocks
783
784LFS has two schemes for free space management: threaded log and copy-and-compac-
785tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
786for devices showing very good sequential write performance, since free segments
787are served all the time for writing new data. However, it suffers from cleaning
788overhead under high utilization. Contrarily, the threaded log scheme suffers
789from random writes, but no cleaning process is needed. F2FS adopts a hybrid
790scheme where the copy-and-compaction scheme is adopted by default, but the
791policy is dynamically changed to the threaded log scheme according to the file
792system status.
793
794In order to align F2FS with underlying flash-based storage, F2FS allocates a
795segment in a unit of section. F2FS expects that the section size would be the
796same as the unit size of garbage collection in FTL. Furthermore, with respect
797to the mapping granularity in FTL, F2FS allocates each section of the active
798logs from different zones as much as possible, since FTL can write the data in
799the active logs into one allocation unit according to its mapping granularity.
800
801Cleaning process
802----------------
803
804F2FS does cleaning both on demand and in the background. On-demand cleaning is
805triggered when there are not enough free segments to serve VFS calls. Background
806cleaner is operated by a kernel thread, and triggers the cleaning job when the
807system is idle.
808
809F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
810In the greedy algorithm, F2FS selects a victim segment having the smallest number
811of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
812according to the segment age and the number of valid blocks in order to address
813log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
814algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
815algorithm.
816
817In order to identify whether the data in the victim segment are valid or not,
818F2FS manages a bitmap. Each bit represents the validity of a block, and the
819bitmap is composed of a bit stream covering whole blocks in main area.
820
821Write-hint Policy
822-----------------
823
824F2FS sets the whint all the time with the below policy.
825
826===================== ======================== ===================
827User F2FS Block
828===================== ======================== ===================
829N/A META WRITE_LIFE_NONE|REQ_META
830N/A HOT_NODE WRITE_LIFE_NONE
831N/A WARM_NODE WRITE_LIFE_MEDIUM
832N/A COLD_NODE WRITE_LIFE_LONG
833ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
834extension list " "
835
836-- buffered io
837------------------------------------------------------------------
838N/A COLD_DATA WRITE_LIFE_EXTREME
839N/A HOT_DATA WRITE_LIFE_SHORT
840N/A WARM_DATA WRITE_LIFE_NOT_SET
841
842-- direct io
843------------------------------------------------------------------
844WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
845WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
846WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
847WRITE_LIFE_NONE " WRITE_LIFE_NONE
848WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
849WRITE_LIFE_LONG " WRITE_LIFE_LONG
850===================== ======================== ===================
851
852Fallocate(2) Policy
853-------------------
854
855The default policy follows the below POSIX rule.
856
857Allocating disk space
858 The default operation (i.e., mode is zero) of fallocate() allocates
859 the disk space within the range specified by offset and len. The
860 file size (as reported by stat(2)) will be changed if offset+len is
861 greater than the file size. Any subregion within the range specified
862 by offset and len that did not contain data before the call will be
863 initialized to zero. This default behavior closely resembles the
864 behavior of the posix_fallocate(3) library function, and is intended
865 as a method of optimally implementing that function.
866
867However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
868fallocate(fd, DEFAULT_MODE), it allocates on-disk block addresses having
869zero or random data, which is useful to the below scenario where:
870
871 1. create(fd)
872 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
873 3. fallocate(fd, 0, 0, size)
874 4. address = fibmap(fd, offset)
875 5. open(blkdev)
876 6. write(blkdev, address)
877
878Compression implementation
879--------------------------
880
881- New term named cluster is defined as basic unit of compression, file can
882 be divided into multiple clusters logically. One cluster includes 4 << n
883 (n >= 0) logical pages, compression size is also cluster size, each of
884 cluster can be compressed or not.
885
886- In cluster metadata layout, one special block address is used to indicate
887 a cluster is a compressed one or normal one; for compressed cluster, following
888 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
889 stores data including compress header and compressed data.
890
891- In order to eliminate write amplification during overwrite, F2FS only
892 support compression on write-once file, data can be compressed only when
893 all logical blocks in cluster contain valid data and compress ratio of
894 cluster data is lower than specified threshold.
895
896- To enable compression on regular inode, there are four ways:
897
898 * chattr +c file
899 * chattr +c dir; touch dir/file
900 * mount w/ -o compress_extension=ext; touch file.ext
901 * mount w/ -o compress_extension=*; touch any_file
902
903- To disable compression on regular inode, there are two ways:
904
905 * chattr -c file
906 * mount w/ -o nocompress_extension=ext; touch file.ext
907
908- Priority in between FS_COMPR_FL, FS_NOCOMP_FS, extensions:
909
910 * compress_extension=so; nocompress_extension=zip; chattr +c dir; touch
911 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so and baz.txt
912 should be compresse, bar.zip should be non-compressed. chattr +c dir/bar.zip
913 can enable compress on bar.zip.
914 * compress_extension=so; nocompress_extension=zip; chattr -c dir; touch
915 dir/foo.so; touch dir/bar.zip; touch dir/baz.txt; then foo.so should be
916 compresse, bar.zip and baz.txt should be non-compressed.
917 chattr+c dir/bar.zip; chattr+c dir/baz.txt; can enable compress on bar.zip
918 and baz.txt.
919
920- At this point, compression feature doesn't expose compressed space to user
921 directly in order to guarantee potential data updates later to the space.
922 Instead, the main goal is to reduce data writes to flash disk as much as
923 possible, resulting in extending disk life time as well as relaxing IO
924 congestion. Alternatively, we've added ioctl(F2FS_IOC_RELEASE_COMPRESS_BLOCKS)
925 interface to reclaim compressed space and show it to user after setting a
926 special flag to the inode. Once the compressed space is released, the flag
927 will block writing data to the file until either the compressed space is
928 reserved via ioctl(F2FS_IOC_RESERVE_COMPRESS_BLOCKS) or the file size is
929 truncated to zero.
930
931Compress metadata layout::
932
933 [Dnode Structure]
934 +-----------------------------------------------+
935 | cluster 1 | cluster 2 | ......... | cluster N |
936 +-----------------------------------------------+
937 . . . .
938 . . . .
939 . Compressed Cluster . . Normal Cluster .
940 +----------+---------+---------+---------+ +---------+---------+---------+---------+
941 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 |
942 +----------+---------+---------+---------+ +---------+---------+---------+---------+
943 . .
944 . .
945 . .
946 +-------------+-------------+----------+----------------------------+
947 | data length | data chksum | reserved | compressed data |
948 +-------------+-------------+----------+----------------------------+
949
950Compression mode
951--------------------------
952
953f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
954With this option, f2fs provides a choice to select the way how to compress the
955compression enabled files (refer to "Compression implementation" section for how to
956enable compression on a regular inode).
957
9581) compress_mode=fs
959
960 This is the default option. f2fs does automatic compression in the writeback of the
961 compression enabled files.
962
9632) compress_mode=user
964
965 This disables the automatic compression and gives the user discretion of choosing the
966 target file and the timing. The user can do manual compression/decompression on the
967 compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
968 ioctls like the below.
969
970To decompress a file::
971
972 fd = open(filename, O_WRONLY, 0);
973 ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);
974
975To compress a file::
976
977 fd = open(filename, O_WRONLY, 0);
978 ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);
979
980NVMe Zoned Namespace devices
981----------------------------
982
983- ZNS defines a per-zone capacity which can be equal or less than the
984 zone-size. Zone-capacity is the number of usable blocks in the zone.
985 F2FS checks if zone-capacity is less than zone-size, if it is, then any
986 segment which starts after the zone-capacity is marked as not-free in
987 the free segment bitmap at initial mount time. These segments are marked
988 as permanently used so they are not allocated for writes and
989 consequently are not needed to be garbage collected. In case the
990 zone-capacity is not aligned to default segment size(2MB), then a segment
991 can start before the zone-capacity and span across zone-capacity boundary.
992 Such spanning segments are also considered as usable segments. All blocks
993 past the zone-capacity are considered unusable in these segments.
994
995Device aliasing feature
996-----------------------
997
998f2fs can utilize a special file called a "device aliasing file." This file allows
999the entire storage device to be mapped with a single, large extent, not using
1000the usual f2fs node structures. This mapped area is pinned and primarily intended
1001for holding the space.
1002
1003Essentially, this mechanism allows a portion of the f2fs area to be temporarily
1004reserved and used by another filesystem or for different purposes. Once that
1005external usage is complete, the device aliasing file can be deleted, releasing
1006the reserved space back to F2FS for its own use.
1007
1008.. code-block::
1009
1010 # ls /dev/vd*
1011 /dev/vdb (32GB) /dev/vdc (32GB)
1012 # mkfs.ext4 /dev/vdc
1013 # mkfs.f2fs -c /dev/vdc@vdc.file /dev/vdb
1014 # mount /dev/vdb /mnt/f2fs
1015 # ls -l /mnt/f2fs
1016 vdc.file
1017 # df -h
1018 /dev/vdb 64G 33G 32G 52% /mnt/f2fs
1019
1020 # mount -o loop /dev/vdc /mnt/ext4
1021 # df -h
1022 /dev/vdb 64G 33G 32G 52% /mnt/f2fs
1023 /dev/loop7 32G 24K 30G 1% /mnt/ext4
1024 # umount /mnt/ext4
1025
1026 # f2fs_io getflags /mnt/f2fs/vdc.file
1027 get a flag on /mnt/f2fs/vdc.file ret=0, flags=nocow(pinned),immutable
1028 # f2fs_io setflags noimmutable /mnt/f2fs/vdc.file
1029 get a flag on noimmutable ret=0, flags=800010
1030 set a flag on /mnt/f2fs/vdc.file ret=0, flags=noimmutable
1031 # rm /mnt/f2fs/vdc.file
1032 # df -h
1033 /dev/vdb 64G 753M 64G 2% /mnt/f2fs
1034
1035So, the key idea is, user can do any file operations on /dev/vdc, and
1036reclaim the space after the use, while the space is counted as /data.
1037That doesn't require modifying partition size and filesystem format.
1038
1039Per-file Read-Only Large Folio Support
1040--------------------------------------
1041
1042F2FS implements large folio support on the read path to leverage high-order
1043page allocation for significant performance gains. To minimize code complexity,
1044this support is currently excluded from the write path, which requires handling
1045complex optimizations such as compression and block allocation modes.
1046
1047This optional feature is triggered only when a file's immutable bit is set.
1048Consequently, F2FS will return EOPNOTSUPP if a user attempts to open a cached
1049file with write permissions, even immediately after clearing the bit. Write
1050access is only restored once the cached inode is dropped. The usage flow is
1051demonstrated below:
1052
1053.. code-block::
1054
1055 # f2fs_io setflags immutable /data/testfile_read_seq
1056
1057 /* flush and reload the inode to enable the large folio */
1058 # sync && echo 3 > /proc/sys/vm/drop_caches
1059
1060 /* mmap(MAP_POPULATE) + mlock() */
1061 # f2fs_io read 128 0 1024 mmap 1 0 /data/testfile_read_seq
1062
1063 /* mmap() + fadvise(POSIX_FADV_WILLNEED) + mlock() */
1064 # f2fs_io read 128 0 1024 fadvise 1 0 /data/testfile_read_seq
1065
1066 /* mmap() + mlock2(MLOCK_ONFAULT) + madvise(MADV_POPULATE_READ) */
1067 # f2fs_io read 128 0 1024 madvise 1 0 /data/testfile_read_seq
1068
1069 # f2fs_io clearflags immutable /data/testfile_read_seq
1070
1071 # f2fs_io write 1 0 1 zero buffered /data/testfile_read_seq
1072 Failed to open /mnt/test/test: Operation not supported
1073
1074 /* flush and reload the inode to disable the large folio */
1075 # sync && echo 3 > /proc/sys/vm/drop_caches
1076
1077 # f2fs_io write 1 0 1 zero buffered /data/testfile_read_seq
1078 Written 4096 bytes with pattern = zero, total_time = 29 us, max_latency = 28 us
1079
1080 # rm /data/testfile_read_seq