1 .. SPDX-License-Identifier: GPL-2.0
3 ==========================================
4 WHAT IS Flash-Friendly File System (F2FS)?
5 ==========================================
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.
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.
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.
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).
26 - git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
28 For reporting bugs and sending patches, please use the following mailing list:
30 - linux-f2fs-devel@lists.sourceforge.net
32 Background and Design issues
33 ============================
35 Log-structured File System (LFS)
36 --------------------------------
37 "A log-structured file system writes all modifications to disk sequentially in
38 a log-like structure, thereby speeding up both file writing and crash recovery.
39 The log is the only structure on disk; it contains indexing information so that
40 files can be read back from the log efficiently. In order to maintain large free
41 areas on disk for fast writing, we divide the log into segments and use a
42 segment cleaner to compress the live information from heavily fragmented
43 segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
44 implementation of a log-structured file system", ACM Trans. Computer Systems
47 Wandering Tree Problem
48 ----------------------
49 In LFS, when a file data is updated and written to the end of log, its direct
50 pointer block is updated due to the changed location. Then the indirect pointer
51 block is also updated due to the direct pointer block update. In this manner,
52 the upper index structures such as inode, inode map, and checkpoint block are
53 also updated recursively. This problem is called as wandering tree problem [1],
54 and in order to enhance the performance, it should eliminate or relax the update
55 propagation as much as possible.
57 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
61 Since LFS is based on out-of-place writes, it produces so many obsolete blocks
62 scattered across the whole storage. In order to serve new empty log space, it
63 needs to reclaim these obsolete blocks seamlessly to users. This job is called
64 as a cleaning process.
66 The process consists of three operations as follows.
68 1. A victim segment is selected through referencing segment usage table.
69 2. It loads parent index structures of all the data in the victim identified by
70 segment summary blocks.
71 3. It checks the cross-reference between the data and its parent index structure.
72 4. It moves valid data selectively.
74 This cleaning job may cause unexpected long delays, so the most important goal
75 is to hide the latencies to users. And also definitely, it should reduce the
76 amount of valid data to be moved, and move them quickly as well.
83 - Enlarge the random write area for better performance, but provide the high
85 - Align FS data structures to the operational units in FTL as best efforts
87 Wandering Tree Problem
88 ----------------------
89 - Use a term, “node”, that represents inodes as well as various pointer blocks
90 - Introduce Node Address Table (NAT) containing the locations of all the “node”
91 blocks; this will cut off the update propagation.
95 - Support a background cleaning process
96 - Support greedy and cost-benefit algorithms for victim selection policies
97 - Support multi-head logs for static/dynamic hot and cold data separation
98 - Introduce adaptive logging for efficient block allocation
104 ======================== ============================================================
105 background_gc=%s Turn on/off cleaning operations, namely garbage
106 collection, triggered in background when I/O subsystem is
107 idle. If background_gc=on, it will turn on the garbage
108 collection and if background_gc=off, garbage collection
109 will be turned off. If background_gc=sync, it will turn
110 on synchronous garbage collection running in background.
111 Default value for this option is on. So garbage
112 collection is on by default.
113 disable_roll_forward Disable the roll-forward recovery routine
114 norecovery Disable the roll-forward recovery routine, mounted read-
115 only (i.e., -o ro,disable_roll_forward)
116 discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
117 enabled, f2fs will issue discard/TRIM commands when a
119 no_heap Disable heap-style segment allocation which finds free
120 segments for data from the beginning of main area, while
121 for node from the end of main area.
122 nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
123 by default if CONFIG_F2FS_FS_XATTR is selected.
124 noacl Disable POSIX Access Control List. Note: acl is enabled
125 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
126 active_logs=%u Support configuring the number of active logs. In the
127 current design, f2fs supports only 2, 4, and 6 logs.
129 disable_ext_identify Disable the extension list configured by mkfs, so f2fs
130 is not aware of cold files such as media files.
131 inline_xattr Enable the inline xattrs feature.
132 noinline_xattr Disable the inline xattrs feature.
133 inline_xattr_size=%u Support configuring inline xattr size, it depends on
134 flexible inline xattr feature.
135 inline_data Enable the inline data feature: Newly created small (<~3.4k)
136 files can be written into inode block.
137 inline_dentry Enable the inline dir feature: data in newly created
138 directory entries can be written into inode block. The
139 space of inode block which is used to store inline
140 dentries is limited to ~3.4k.
141 noinline_dentry Disable the inline dentry feature.
142 flush_merge Merge concurrent cache_flush commands as much as possible
143 to eliminate redundant command issues. If the underlying
144 device handles the cache_flush command relatively slowly,
145 recommend to enable this option.
146 nobarrier This option can be used if underlying storage guarantees
147 its cached data should be written to the novolatile area.
148 If this option is set, no cache_flush commands are issued
149 but f2fs still guarantees the write ordering of all the
151 fastboot This option is used when a system wants to reduce mount
152 time as much as possible, even though normal performance
154 extent_cache Enable an extent cache based on rb-tree, it can cache
155 as many as extent which map between contiguous logical
156 address and physical address per inode, resulting in
157 increasing the cache hit ratio. Set by default.
158 noextent_cache Disable an extent cache based on rb-tree explicitly, see
159 the above extent_cache mount option.
160 noinline_data Disable the inline data feature, inline data feature is
162 data_flush Enable data flushing before checkpoint in order to
163 persist data of regular and symlink.
164 reserve_root=%d Support configuring reserved space which is used for
165 allocation from a privileged user with specified uid or
166 gid, unit: 4KB, the default limit is 0.2% of user blocks.
167 resuid=%d The user ID which may use the reserved blocks.
168 resgid=%d The group ID which may use the reserved blocks.
169 fault_injection=%d Enable fault injection in all supported types with
170 specified injection rate.
171 fault_type=%d Support configuring fault injection type, should be
172 enabled with fault_injection option, fault type value
173 is shown below, it supports single or combined type.
175 =================== ===========
177 =================== ===========
178 FAULT_KMALLOC 0x000000001
179 FAULT_KVMALLOC 0x000000002
180 FAULT_PAGE_ALLOC 0x000000004
181 FAULT_PAGE_GET 0x000000008
182 FAULT_ALLOC_NID 0x000000020
183 FAULT_ORPHAN 0x000000040
184 FAULT_BLOCK 0x000000080
185 FAULT_DIR_DEPTH 0x000000100
186 FAULT_EVICT_INODE 0x000000200
187 FAULT_TRUNCATE 0x000000400
188 FAULT_READ_IO 0x000000800
189 FAULT_CHECKPOINT 0x000001000
190 FAULT_DISCARD 0x000002000
191 FAULT_WRITE_IO 0x000004000
192 =================== ===========
193 mode=%s Control block allocation mode which supports "adaptive"
194 and "lfs". In "lfs" mode, there should be no random
195 writes towards main area.
196 io_bits=%u Set the bit size of write IO requests. It should be set
198 usrquota Enable plain user disk quota accounting.
199 grpquota Enable plain group disk quota accounting.
200 prjquota Enable plain project quota accounting.
201 usrjquota=<file> Appoint specified file and type during mount, so that quota
202 grpjquota=<file> information can be properly updated during recovery flow,
203 prjjquota=<file> <quota file>: must be in root directory;
204 jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
205 offusrjquota Turn off user journalled quota.
206 offgrpjquota Turn off group journalled quota.
207 offprjjquota Turn off project journalled quota.
208 quota Enable plain user disk quota accounting.
209 noquota Disable all plain disk quota option.
210 whint_mode=%s Control which write hints are passed down to block
211 layer. This supports "off", "user-based", and
212 "fs-based". In "off" mode (default), f2fs does not pass
213 down hints. In "user-based" mode, f2fs tries to pass
214 down hints given by users. And in "fs-based" mode, f2fs
215 passes down hints with its policy.
216 alloc_mode=%s Adjust block allocation policy, which supports "reuse"
218 fsync_mode=%s Control the policy of fsync. Currently supports "posix",
219 "strict", and "nobarrier". In "posix" mode, which is
220 default, fsync will follow POSIX semantics and does a
221 light operation to improve the filesystem performance.
222 In "strict" mode, fsync will be heavy and behaves in line
223 with xfs, ext4 and btrfs, where xfstest generic/342 will
224 pass, but the performance will regress. "nobarrier" is
225 based on "posix", but doesn't issue flush command for
226 non-atomic files likewise "nobarrier" mount option.
227 test_dummy_encryption
228 test_dummy_encryption=%s
229 Enable dummy encryption, which provides a fake fscrypt
230 context. The fake fscrypt context is used by xfstests.
231 The argument may be either "v1" or "v2", in order to
232 select the corresponding fscrypt policy version.
233 checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable"
234 to reenable checkpointing. Is enabled by default. While
235 disabled, any unmounting or unexpected shutdowns will cause
236 the filesystem contents to appear as they did when the
237 filesystem was mounted with that option.
238 While mounting with checkpoint=disabled, the filesystem must
239 run garbage collection to ensure that all available space can
240 be used. If this takes too much time, the mount may return
241 EAGAIN. You may optionally add a value to indicate how much
242 of the disk you would be willing to temporarily give up to
243 avoid additional garbage collection. This can be given as a
244 number of blocks, or as a percent. For instance, mounting
245 with checkpoint=disable:100% would always succeed, but it may
246 hide up to all remaining free space. The actual space that
247 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
248 This space is reclaimed once checkpoint=enable.
249 checkpoint_merge When checkpoint is enabled, this can be used to create a kernel
250 daemon and make it to merge concurrent checkpoint requests as
251 much as possible to eliminate redundant checkpoint issues. Plus,
252 we can eliminate the sluggish issue caused by slow checkpoint
253 operation when the checkpoint is done in a process context in
254 a cgroup having low i/o budget and cpu shares. To make this
255 do better, we set the default i/o priority of the kernel daemon
256 to "3", to give one higher priority than other kernel threads.
257 This is the same way to give a I/O priority to the jbd2
258 journaling thread of ext4 filesystem.
259 nocheckpoint_merge Disable checkpoint merge feature.
260 compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo",
261 "lz4", "zstd" and "lzo-rle" algorithm.
262 compress_algorithm=%s:%d Control compress algorithm and its compress level, now, only
263 "lz4" and "zstd" support compress level config.
264 algorithm level range
267 compress_log_size=%u Support configuring compress cluster size, the size will
268 be 4KB * (1 << %u), 16KB is minimum size, also it's
270 compress_extension=%s Support adding specified extension, so that f2fs can enable
271 compression on those corresponding files, e.g. if all files
272 with '.ext' has high compression rate, we can set the '.ext'
273 on compression extension list and enable compression on
274 these file by default rather than to enable it via ioctl.
275 For other files, we can still enable compression via ioctl.
276 Note that, there is one reserved special extension '*', it
277 can be set to enable compression for all files.
278 compress_chksum Support verifying chksum of raw data in compressed cluster.
279 compress_mode=%s Control file compression mode. This supports "fs" and "user"
280 modes. In "fs" mode (default), f2fs does automatic compression
281 on the compression enabled files. In "user" mode, f2fs disables
282 the automaic compression and gives the user discretion of
283 choosing the target file and the timing. The user can do manual
284 compression/decompression on the compression enabled files using
286 inlinecrypt When possible, encrypt/decrypt the contents of encrypted
287 files using the blk-crypto framework rather than
288 filesystem-layer encryption. This allows the use of
289 inline encryption hardware. The on-disk format is
290 unaffected. For more details, see
291 Documentation/block/inline-encryption.rst.
292 atgc Enable age-threshold garbage collection, it provides high
293 effectiveness and efficiency on background GC.
294 ======================== ============================================================
299 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
300 f2fs. Each file shows the whole f2fs information.
302 /sys/kernel/debug/f2fs/status includes:
304 - major file system information managed by f2fs currently
305 - average SIT information about whole segments
306 - current memory footprint consumed by f2fs.
311 Information about mounted f2fs file systems can be found in
312 /sys/fs/f2fs. Each mounted filesystem will have a directory in
313 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
314 The files in each per-device directory are shown in table below.
316 Files in /sys/fs/f2fs/<devname>
317 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
322 1. Download userland tools and compile them.
324 2. Skip, if f2fs was compiled statically inside kernel.
325 Otherwise, insert the f2fs.ko module::
329 3. Create a directory to use when mounting::
333 4. Format the block device, and then mount as f2fs::
335 # mkfs.f2fs -l label /dev/block_device
336 # mount -t f2fs /dev/block_device /mnt/f2fs
340 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
341 which builds a basic on-disk layout.
343 The quick options consist of:
345 =============== ===========================================================
346 ``-l [label]`` Give a volume label, up to 512 unicode name.
347 ``-a [0 or 1]`` Split start location of each area for heap-based allocation.
349 1 is set by default, which performs this.
350 ``-o [int]`` Set overprovision ratio in percent over volume size.
353 ``-s [int]`` Set the number of segments per section.
356 ``-z [int]`` Set the number of sections per zone.
359 ``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov"
360 ``-t [0 or 1]`` Disable discard command or not.
362 1 is set by default, which conducts discard.
363 =============== ===========================================================
365 Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.
369 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
370 partition, which examines whether the filesystem metadata and user-made data
371 are cross-referenced correctly or not.
372 Note that, initial version of the tool does not fix any inconsistency.
374 The quick options consist of::
376 -d debug level [default:0]
378 Note: please refer to the manpage of fsck.f2fs(8) to get full option list.
382 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
383 file. Each file is dump_ssa and dump_sit.
385 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
386 It shows on-disk inode information recognized by a given inode number, and is
387 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
388 ./dump_sit respectively.
390 The options consist of::
392 -d debug level [default:0]
394 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
395 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
399 # dump.f2fs -i [ino] /dev/sdx
400 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
401 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
403 Note: please refer to the manpage of dump.f2fs(8) to get full option list.
407 The sload.f2fs gives a way to insert files and directories in the exisiting disk
408 image. This tool is useful when building f2fs images given compiled files.
410 Note: please refer to the manpage of sload.f2fs(8) to get full option list.
414 The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
415 all the files and directories stored in the image.
417 Note: please refer to the manpage of resize.f2fs(8) to get full option list.
421 The defrag.f2fs can be used to defragment scattered written data as well as
422 filesystem metadata across the disk. This can improve the write speed by giving
423 more free consecutive space.
425 Note: please refer to the manpage of defrag.f2fs(8) to get full option list.
429 The f2fs_io is a simple tool to issue various filesystem APIs as well as
430 f2fs-specific ones, which is very useful for QA tests.
432 Note: please refer to the manpage of f2fs_io(8) to get full option list.
440 F2FS divides the whole volume into a number of segments, each of which is fixed
441 to 2MB in size. A section is composed of consecutive segments, and a zone
442 consists of a set of sections. By default, section and zone sizes are set to one
443 segment size identically, but users can easily modify the sizes by mkfs.
445 F2FS splits the entire volume into six areas, and all the areas except superblock
446 consist of multiple segments as described below::
448 align with the zone size <-|
449 |-> align with the segment size
450 _________________________________________________________________________
451 | | | Segment | Node | Segment | |
452 | Superblock | Checkpoint | Info. | Address | Summary | Main |
453 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
454 |____________|_____2______|______N______|______N______|______N_____|__N___|
458 ._________________________________________.
459 |_Segment_|_..._|_Segment_|_..._|_Segment_|
468 It is located at the beginning of the partition, and there exist two copies
469 to avoid file system crash. It contains basic partition information and some
470 default parameters of f2fs.
473 It contains file system information, bitmaps for valid NAT/SIT sets, orphan
474 inode lists, and summary entries of current active segments.
476 - Segment Information Table (SIT)
477 It contains segment information such as valid block count and bitmap for the
478 validity of all the blocks.
480 - Node Address Table (NAT)
481 It is composed of a block address table for all the node blocks stored in
484 - Segment Summary Area (SSA)
485 It contains summary entries which contains the owner information of all the
486 data and node blocks stored in Main area.
489 It contains file and directory data including their indices.
491 In order to avoid misalignment between file system and flash-based storage, F2FS
492 aligns the start block address of CP with the segment size. Also, it aligns the
493 start block address of Main area with the zone size by reserving some segments
496 Reference the following survey for additional technical details.
497 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
499 File System Metadata Structure
500 ------------------------------
502 F2FS adopts the checkpointing scheme to maintain file system consistency. At
503 mount time, F2FS first tries to find the last valid checkpoint data by scanning
504 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
505 One of them always indicates the last valid data, which is called as shadow copy
506 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
508 For file system consistency, each CP points to which NAT and SIT copies are
509 valid, as shown as below::
511 +--------+----------+---------+
513 +--------+----------+---------+
517 +-------+-------+--------+--------+--------+--------+
518 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
519 +-------+-------+--------+--------+--------+--------+
522 `----------------------------------------'
527 The key data structure to manage the data locations is a "node". Similar to
528 traditional file structures, F2FS has three types of node: inode, direct node,
529 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
530 indices, two direct node pointers, two indirect node pointers, and one double
531 indirect node pointer as described below. One direct node block contains 1018
532 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
533 one inode block (i.e., a file) covers::
535 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
542 | `- direct node (1018)
544 `- double indirect node (1)
545 `- indirect node (1018)
546 `- direct node (1018)
549 Note that all the node blocks are mapped by NAT which means the location of
550 each node is translated by the NAT table. In the consideration of the wandering
551 tree problem, F2FS is able to cut off the propagation of node updates caused by
557 A directory entry occupies 11 bytes, which consists of the following attributes.
559 - hash hash value of the file name
561 - len the length of file name
562 - type file type such as directory, symlink, etc
564 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
565 used to represent whether each dentry is valid or not. A dentry block occupies
566 4KB with the following composition.
570 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
571 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
574 +--------------------------------+
575 |dentry block 1 | dentry block 2 |
576 +--------------------------------+
579 . [Dentry Block Structure: 4KB] .
580 +--------+----------+----------+------------+
581 | bitmap | reserved | dentries | file names |
582 +--------+----------+----------+------------+
583 [Dentry Block: 4KB] . .
586 +------+------+-----+------+
587 | hash | ino | len | type |
588 +------+------+-----+------+
589 [Dentry Structure: 11 bytes]
591 F2FS implements multi-level hash tables for directory structure. Each level has
592 a hash table with dedicated number of hash buckets as shown below. Note that
593 "A(2B)" means a bucket includes 2 data blocks.
597 ----------------------
600 N : MAX_DIR_HASH_DEPTH
601 ----------------------
605 level #1 | A(2B) - A(2B)
607 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
609 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
611 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
613 The number of blocks and buckets are determined by::
615 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
616 # of blocks in level #n = |
619 ,- 2^(n + dir_level),
620 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
621 # of buckets in level #n = |
622 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
625 When F2FS finds a file name in a directory, at first a hash value of the file
626 name is calculated. Then, F2FS scans the hash table in level #0 to find the
627 dentry consisting of the file name and its inode number. If not found, F2FS
628 scans the next hash table in level #1. In this way, F2FS scans hash tables in
629 each levels incrementally from 1 to N. In each level F2FS needs to scan only
630 one bucket determined by the following equation, which shows O(log(# of files))
633 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
635 In the case of file creation, F2FS finds empty consecutive slots that cover the
636 file name. F2FS searches the empty slots in the hash tables of whole levels from
637 1 to N in the same way as the lookup operation.
639 The following figure shows an example of two cases holding children::
641 --------------> Dir <--------------
645 child - child [hole] - child
647 child - child - child [hole] - [hole] - child
650 Number of children = 6, Number of children = 3,
651 File size = 7 File size = 7
653 Default Block Allocation
654 ------------------------
656 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
657 and Hot/Warm/Cold data.
659 - Hot node contains direct node blocks of directories.
660 - Warm node contains direct node blocks except hot node blocks.
661 - Cold node contains indirect node blocks
662 - Hot data contains dentry blocks
663 - Warm data contains data blocks except hot and cold data blocks
664 - Cold data contains multimedia data or migrated data blocks
666 LFS has two schemes for free space management: threaded log and copy-and-compac-
667 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
668 for devices showing very good sequential write performance, since free segments
669 are served all the time for writing new data. However, it suffers from cleaning
670 overhead under high utilization. Contrarily, the threaded log scheme suffers
671 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
672 scheme where the copy-and-compaction scheme is adopted by default, but the
673 policy is dynamically changed to the threaded log scheme according to the file
676 In order to align F2FS with underlying flash-based storage, F2FS allocates a
677 segment in a unit of section. F2FS expects that the section size would be the
678 same as the unit size of garbage collection in FTL. Furthermore, with respect
679 to the mapping granularity in FTL, F2FS allocates each section of the active
680 logs from different zones as much as possible, since FTL can write the data in
681 the active logs into one allocation unit according to its mapping granularity.
686 F2FS does cleaning both on demand and in the background. On-demand cleaning is
687 triggered when there are not enough free segments to serve VFS calls. Background
688 cleaner is operated by a kernel thread, and triggers the cleaning job when the
691 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
692 In the greedy algorithm, F2FS selects a victim segment having the smallest number
693 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
694 according to the segment age and the number of valid blocks in order to address
695 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
696 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
699 In order to identify whether the data in the victim segment are valid or not,
700 F2FS manages a bitmap. Each bit represents the validity of a block, and the
701 bitmap is composed of a bit stream covering whole blocks in main area.
706 1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
708 2) whint_mode=user-based. F2FS tries to pass down hints given by
711 ===================== ======================== ===================
713 ===================== ======================== ===================
714 META WRITE_LIFE_NOT_SET
718 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
722 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
723 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
724 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
726 WRITE_LIFE_MEDIUM " "
730 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
731 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
732 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
733 WRITE_LIFE_NONE " WRITE_LIFE_NONE
734 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
735 WRITE_LIFE_LONG " WRITE_LIFE_LONG
736 ===================== ======================== ===================
738 3) whint_mode=fs-based. F2FS passes down hints with its policy.
740 ===================== ======================== ===================
742 ===================== ======================== ===================
743 META WRITE_LIFE_MEDIUM;
744 HOT_NODE WRITE_LIFE_NOT_SET
746 COLD_NODE WRITE_LIFE_NONE
747 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
751 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
752 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
753 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
755 WRITE_LIFE_MEDIUM " "
759 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
760 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
761 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
762 WRITE_LIFE_NONE " WRITE_LIFE_NONE
763 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
764 WRITE_LIFE_LONG " WRITE_LIFE_LONG
765 ===================== ======================== ===================
770 The default policy follows the below POSIX rule.
772 Allocating disk space
773 The default operation (i.e., mode is zero) of fallocate() allocates
774 the disk space within the range specified by offset and len. The
775 file size (as reported by stat(2)) will be changed if offset+len is
776 greater than the file size. Any subregion within the range specified
777 by offset and len that did not contain data before the call will be
778 initialized to zero. This default behavior closely resembles the
779 behavior of the posix_fallocate(3) library function, and is intended
780 as a method of optimally implementing that function.
782 However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
783 fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having
784 zero or random data, which is useful to the below scenario where:
787 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
788 3. fallocate(fd, 0, 0, size)
789 4. address = fibmap(fd, offset)
791 6. write(blkdev, address)
793 Compression implementation
794 --------------------------
796 - New term named cluster is defined as basic unit of compression, file can
797 be divided into multiple clusters logically. One cluster includes 4 << n
798 (n >= 0) logical pages, compression size is also cluster size, each of
799 cluster can be compressed or not.
801 - In cluster metadata layout, one special block address is used to indicate
802 a cluster is a compressed one or normal one; for compressed cluster, following
803 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
804 stores data including compress header and compressed data.
806 - In order to eliminate write amplification during overwrite, F2FS only
807 support compression on write-once file, data can be compressed only when
808 all logical blocks in cluster contain valid data and compress ratio of
809 cluster data is lower than specified threshold.
811 - To enable compression on regular inode, there are three ways:
814 * chattr +c dir; touch dir/file
815 * mount w/ -o compress_extension=ext; touch file.ext
817 Compress metadata layout::
820 +-----------------------------------------------+
821 | cluster 1 | cluster 2 | ......... | cluster N |
822 +-----------------------------------------------+
825 . Compressed Cluster . . Normal Cluster .
826 +----------+---------+---------+---------+ +---------+---------+---------+---------+
827 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 |
828 +----------+---------+---------+---------+ +---------+---------+---------+---------+
832 +-------------+-------------+----------+----------------------------+
833 | data length | data chksum | reserved | compressed data |
834 +-------------+-------------+----------+----------------------------+
837 --------------------------
839 f2fs supports "fs" and "user" compression modes with "compression_mode" mount option.
840 With this option, f2fs provides a choice to select the way how to compress the
841 compression enabled files (refer to "Compression implementation" section for how to
842 enable compression on a regular inode).
845 This is the default option. f2fs does automatic compression in the writeback of the
846 compression enabled files.
848 2) compress_mode=user
849 This disables the automatic compression and gives the user discretion of choosing the
850 target file and the timing. The user can do manual compression/decompression on the
851 compression enabled files using F2FS_IOC_DECOMPRESS_FILE and F2FS_IOC_COMPRESS_FILE
852 ioctls like the below.
854 To decompress a file,
856 fd = open(filename, O_WRONLY, 0);
857 ret = ioctl(fd, F2FS_IOC_DECOMPRESS_FILE);
861 fd = open(filename, O_WRONLY, 0);
862 ret = ioctl(fd, F2FS_IOC_COMPRESS_FILE);
864 NVMe Zoned Namespace devices
865 ----------------------------
867 - ZNS defines a per-zone capacity which can be equal or less than the
868 zone-size. Zone-capacity is the number of usable blocks in the zone.
869 F2FS checks if zone-capacity is less than zone-size, if it is, then any
870 segment which starts after the zone-capacity is marked as not-free in
871 the free segment bitmap at initial mount time. These segments are marked
872 as permanently used so they are not allocated for writes and
873 consequently are not needed to be garbage collected. In case the
874 zone-capacity is not aligned to default segment size(2MB), then a segment
875 can start before the zone-capacity and span across zone-capacity boundary.
876 Such spanning segments are also considered as usable segments. All blocks
877 past the zone-capacity are considered unusable in these segments.