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_BIO 0x000000010
183 FAULT_ALLOC_NID 0x000000020
184 FAULT_ORPHAN 0x000000040
185 FAULT_BLOCK 0x000000080
186 FAULT_DIR_DEPTH 0x000000100
187 FAULT_EVICT_INODE 0x000000200
188 FAULT_TRUNCATE 0x000000400
189 FAULT_READ_IO 0x000000800
190 FAULT_CHECKPOINT 0x000001000
191 FAULT_DISCARD 0x000002000
192 FAULT_WRITE_IO 0x000004000
193 =================== ===========
194 mode=%s Control block allocation mode which supports "adaptive"
195 and "lfs". In "lfs" mode, there should be no random
196 writes towards main area.
197 io_bits=%u Set the bit size of write IO requests. It should be set
199 usrquota Enable plain user disk quota accounting.
200 grpquota Enable plain group disk quota accounting.
201 prjquota Enable plain project quota accounting.
202 usrjquota=<file> Appoint specified file and type during mount, so that quota
203 grpjquota=<file> information can be properly updated during recovery flow,
204 prjjquota=<file> <quota file>: must be in root directory;
205 jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
206 offusrjquota Turn off user journalled quota.
207 offgrpjquota Turn off group journalled quota.
208 offprjjquota Turn off project journalled quota.
209 quota Enable plain user disk quota accounting.
210 noquota Disable all plain disk quota option.
211 whint_mode=%s Control which write hints are passed down to block
212 layer. This supports "off", "user-based", and
213 "fs-based". In "off" mode (default), f2fs does not pass
214 down hints. In "user-based" mode, f2fs tries to pass
215 down hints given by users. And in "fs-based" mode, f2fs
216 passes down hints with its policy.
217 alloc_mode=%s Adjust block allocation policy, which supports "reuse"
219 fsync_mode=%s Control the policy of fsync. Currently supports "posix",
220 "strict", and "nobarrier". In "posix" mode, which is
221 default, fsync will follow POSIX semantics and does a
222 light operation to improve the filesystem performance.
223 In "strict" mode, fsync will be heavy and behaves in line
224 with xfs, ext4 and btrfs, where xfstest generic/342 will
225 pass, but the performance will regress. "nobarrier" is
226 based on "posix", but doesn't issue flush command for
227 non-atomic files likewise "nobarrier" mount option.
228 test_dummy_encryption
229 test_dummy_encryption=%s
230 Enable dummy encryption, which provides a fake fscrypt
231 context. The fake fscrypt context is used by xfstests.
232 The argument may be either "v1" or "v2", in order to
233 select the corresponding fscrypt policy version.
234 checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable"
235 to reenable checkpointing. Is enabled by default. While
236 disabled, any unmounting or unexpected shutdowns will cause
237 the filesystem contents to appear as they did when the
238 filesystem was mounted with that option.
239 While mounting with checkpoint=disabled, the filesystem must
240 run garbage collection to ensure that all available space can
241 be used. If this takes too much time, the mount may return
242 EAGAIN. You may optionally add a value to indicate how much
243 of the disk you would be willing to temporarily give up to
244 avoid additional garbage collection. This can be given as a
245 number of blocks, or as a percent. For instance, mounting
246 with checkpoint=disable:100% would always succeed, but it may
247 hide up to all remaining free space. The actual space that
248 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
249 This space is reclaimed once checkpoint=enable.
250 compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo",
251 "lz4", "zstd" and "lzo-rle" algorithm.
252 compress_log_size=%u Support configuring compress cluster size, the size will
253 be 4KB * (1 << %u), 16KB is minimum size, also it's
255 compress_extension=%s Support adding specified extension, so that f2fs can enable
256 compression on those corresponding files, e.g. if all files
257 with '.ext' has high compression rate, we can set the '.ext'
258 on compression extension list and enable compression on
259 these file by default rather than to enable it via ioctl.
260 For other files, we can still enable compression via ioctl.
261 Note that, there is one reserved special extension '*', it
262 can be set to enable compression for all files.
263 inlinecrypt When possible, encrypt/decrypt the contents of encrypted
264 files using the blk-crypto framework rather than
265 filesystem-layer encryption. This allows the use of
266 inline encryption hardware. The on-disk format is
267 unaffected. For more details, see
268 Documentation/block/inline-encryption.rst.
269 atgc Enable age-threshold garbage collection, it provides high
270 effectiveness and efficiency on background GC.
271 ======================== ============================================================
276 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
277 f2fs. Each file shows the whole f2fs information.
279 /sys/kernel/debug/f2fs/status includes:
281 - major file system information managed by f2fs currently
282 - average SIT information about whole segments
283 - current memory footprint consumed by f2fs.
288 Information about mounted f2fs file systems can be found in
289 /sys/fs/f2fs. Each mounted filesystem will have a directory in
290 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
291 The files in each per-device directory are shown in table below.
293 Files in /sys/fs/f2fs/<devname>
294 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
299 1. Download userland tools and compile them.
301 2. Skip, if f2fs was compiled statically inside kernel.
302 Otherwise, insert the f2fs.ko module::
306 3. Create a directory to use when mounting::
310 4. Format the block device, and then mount as f2fs::
312 # mkfs.f2fs -l label /dev/block_device
313 # mount -t f2fs /dev/block_device /mnt/f2fs
317 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
318 which builds a basic on-disk layout.
320 The quick options consist of:
322 =============== ===========================================================
323 ``-l [label]`` Give a volume label, up to 512 unicode name.
324 ``-a [0 or 1]`` Split start location of each area for heap-based allocation.
326 1 is set by default, which performs this.
327 ``-o [int]`` Set overprovision ratio in percent over volume size.
330 ``-s [int]`` Set the number of segments per section.
333 ``-z [int]`` Set the number of sections per zone.
336 ``-e [str]`` Set basic extension list. e.g. "mp3,gif,mov"
337 ``-t [0 or 1]`` Disable discard command or not.
339 1 is set by default, which conducts discard.
340 =============== ===========================================================
342 Note: please refer to the manpage of mkfs.f2fs(8) to get full option list.
346 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
347 partition, which examines whether the filesystem metadata and user-made data
348 are cross-referenced correctly or not.
349 Note that, initial version of the tool does not fix any inconsistency.
351 The quick options consist of::
353 -d debug level [default:0]
355 Note: please refer to the manpage of fsck.f2fs(8) to get full option list.
359 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
360 file. Each file is dump_ssa and dump_sit.
362 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
363 It shows on-disk inode information recognized by a given inode number, and is
364 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
365 ./dump_sit respectively.
367 The options consist of::
369 -d debug level [default:0]
371 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
372 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
376 # dump.f2fs -i [ino] /dev/sdx
377 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
378 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
380 Note: please refer to the manpage of dump.f2fs(8) to get full option list.
384 The sload.f2fs gives a way to insert files and directories in the exisiting disk
385 image. This tool is useful when building f2fs images given compiled files.
387 Note: please refer to the manpage of sload.f2fs(8) to get full option list.
391 The resize.f2fs lets a user resize the f2fs-formatted disk image, while preserving
392 all the files and directories stored in the image.
394 Note: please refer to the manpage of resize.f2fs(8) to get full option list.
398 The defrag.f2fs can be used to defragment scattered written data as well as
399 filesystem metadata across the disk. This can improve the write speed by giving
400 more free consecutive space.
402 Note: please refer to the manpage of defrag.f2fs(8) to get full option list.
406 The f2fs_io is a simple tool to issue various filesystem APIs as well as
407 f2fs-specific ones, which is very useful for QA tests.
409 Note: please refer to the manpage of f2fs_io(8) to get full option list.
417 F2FS divides the whole volume into a number of segments, each of which is fixed
418 to 2MB in size. A section is composed of consecutive segments, and a zone
419 consists of a set of sections. By default, section and zone sizes are set to one
420 segment size identically, but users can easily modify the sizes by mkfs.
422 F2FS splits the entire volume into six areas, and all the areas except superblock
423 consist of multiple segments as described below::
425 align with the zone size <-|
426 |-> align with the segment size
427 _________________________________________________________________________
428 | | | Segment | Node | Segment | |
429 | Superblock | Checkpoint | Info. | Address | Summary | Main |
430 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
431 |____________|_____2______|______N______|______N______|______N_____|__N___|
435 ._________________________________________.
436 |_Segment_|_..._|_Segment_|_..._|_Segment_|
445 It is located at the beginning of the partition, and there exist two copies
446 to avoid file system crash. It contains basic partition information and some
447 default parameters of f2fs.
450 It contains file system information, bitmaps for valid NAT/SIT sets, orphan
451 inode lists, and summary entries of current active segments.
453 - Segment Information Table (SIT)
454 It contains segment information such as valid block count and bitmap for the
455 validity of all the blocks.
457 - Node Address Table (NAT)
458 It is composed of a block address table for all the node blocks stored in
461 - Segment Summary Area (SSA)
462 It contains summary entries which contains the owner information of all the
463 data and node blocks stored in Main area.
466 It contains file and directory data including their indices.
468 In order to avoid misalignment between file system and flash-based storage, F2FS
469 aligns the start block address of CP with the segment size. Also, it aligns the
470 start block address of Main area with the zone size by reserving some segments
473 Reference the following survey for additional technical details.
474 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
476 File System Metadata Structure
477 ------------------------------
479 F2FS adopts the checkpointing scheme to maintain file system consistency. At
480 mount time, F2FS first tries to find the last valid checkpoint data by scanning
481 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
482 One of them always indicates the last valid data, which is called as shadow copy
483 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
485 For file system consistency, each CP points to which NAT and SIT copies are
486 valid, as shown as below::
488 +--------+----------+---------+
490 +--------+----------+---------+
494 +-------+-------+--------+--------+--------+--------+
495 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
496 +-------+-------+--------+--------+--------+--------+
499 `----------------------------------------'
504 The key data structure to manage the data locations is a "node". Similar to
505 traditional file structures, F2FS has three types of node: inode, direct node,
506 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
507 indices, two direct node pointers, two indirect node pointers, and one double
508 indirect node pointer as described below. One direct node block contains 1018
509 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
510 one inode block (i.e., a file) covers::
512 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
519 | `- direct node (1018)
521 `- double indirect node (1)
522 `- indirect node (1018)
523 `- direct node (1018)
526 Note that all the node blocks are mapped by NAT which means the location of
527 each node is translated by the NAT table. In the consideration of the wandering
528 tree problem, F2FS is able to cut off the propagation of node updates caused by
534 A directory entry occupies 11 bytes, which consists of the following attributes.
536 - hash hash value of the file name
538 - len the length of file name
539 - type file type such as directory, symlink, etc
541 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
542 used to represent whether each dentry is valid or not. A dentry block occupies
543 4KB with the following composition.
547 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
548 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
551 +--------------------------------+
552 |dentry block 1 | dentry block 2 |
553 +--------------------------------+
556 . [Dentry Block Structure: 4KB] .
557 +--------+----------+----------+------------+
558 | bitmap | reserved | dentries | file names |
559 +--------+----------+----------+------------+
560 [Dentry Block: 4KB] . .
563 +------+------+-----+------+
564 | hash | ino | len | type |
565 +------+------+-----+------+
566 [Dentry Structure: 11 bytes]
568 F2FS implements multi-level hash tables for directory structure. Each level has
569 a hash table with dedicated number of hash buckets as shown below. Note that
570 "A(2B)" means a bucket includes 2 data blocks.
574 ----------------------
577 N : MAX_DIR_HASH_DEPTH
578 ----------------------
582 level #1 | A(2B) - A(2B)
584 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
586 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
588 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
590 The number of blocks and buckets are determined by::
592 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
593 # of blocks in level #n = |
596 ,- 2^(n + dir_level),
597 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
598 # of buckets in level #n = |
599 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
602 When F2FS finds a file name in a directory, at first a hash value of the file
603 name is calculated. Then, F2FS scans the hash table in level #0 to find the
604 dentry consisting of the file name and its inode number. If not found, F2FS
605 scans the next hash table in level #1. In this way, F2FS scans hash tables in
606 each levels incrementally from 1 to N. In each level F2FS needs to scan only
607 one bucket determined by the following equation, which shows O(log(# of files))
610 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
612 In the case of file creation, F2FS finds empty consecutive slots that cover the
613 file name. F2FS searches the empty slots in the hash tables of whole levels from
614 1 to N in the same way as the lookup operation.
616 The following figure shows an example of two cases holding children::
618 --------------> Dir <--------------
622 child - child [hole] - child
624 child - child - child [hole] - [hole] - child
627 Number of children = 6, Number of children = 3,
628 File size = 7 File size = 7
630 Default Block Allocation
631 ------------------------
633 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
634 and Hot/Warm/Cold data.
636 - Hot node contains direct node blocks of directories.
637 - Warm node contains direct node blocks except hot node blocks.
638 - Cold node contains indirect node blocks
639 - Hot data contains dentry blocks
640 - Warm data contains data blocks except hot and cold data blocks
641 - Cold data contains multimedia data or migrated data blocks
643 LFS has two schemes for free space management: threaded log and copy-and-compac-
644 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
645 for devices showing very good sequential write performance, since free segments
646 are served all the time for writing new data. However, it suffers from cleaning
647 overhead under high utilization. Contrarily, the threaded log scheme suffers
648 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
649 scheme where the copy-and-compaction scheme is adopted by default, but the
650 policy is dynamically changed to the threaded log scheme according to the file
653 In order to align F2FS with underlying flash-based storage, F2FS allocates a
654 segment in a unit of section. F2FS expects that the section size would be the
655 same as the unit size of garbage collection in FTL. Furthermore, with respect
656 to the mapping granularity in FTL, F2FS allocates each section of the active
657 logs from different zones as much as possible, since FTL can write the data in
658 the active logs into one allocation unit according to its mapping granularity.
663 F2FS does cleaning both on demand and in the background. On-demand cleaning is
664 triggered when there are not enough free segments to serve VFS calls. Background
665 cleaner is operated by a kernel thread, and triggers the cleaning job when the
668 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
669 In the greedy algorithm, F2FS selects a victim segment having the smallest number
670 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
671 according to the segment age and the number of valid blocks in order to address
672 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
673 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
676 In order to identify whether the data in the victim segment are valid or not,
677 F2FS manages a bitmap. Each bit represents the validity of a block, and the
678 bitmap is composed of a bit stream covering whole blocks in main area.
683 1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
685 2) whint_mode=user-based. F2FS tries to pass down hints given by
688 ===================== ======================== ===================
690 ===================== ======================== ===================
691 META WRITE_LIFE_NOT_SET
695 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
699 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
700 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
701 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
703 WRITE_LIFE_MEDIUM " "
707 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
708 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
709 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
710 WRITE_LIFE_NONE " WRITE_LIFE_NONE
711 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
712 WRITE_LIFE_LONG " WRITE_LIFE_LONG
713 ===================== ======================== ===================
715 3) whint_mode=fs-based. F2FS passes down hints with its policy.
717 ===================== ======================== ===================
719 ===================== ======================== ===================
720 META WRITE_LIFE_MEDIUM;
721 HOT_NODE WRITE_LIFE_NOT_SET
723 COLD_NODE WRITE_LIFE_NONE
724 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
728 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
729 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
730 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
732 WRITE_LIFE_MEDIUM " "
736 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
737 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
738 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
739 WRITE_LIFE_NONE " WRITE_LIFE_NONE
740 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
741 WRITE_LIFE_LONG " WRITE_LIFE_LONG
742 ===================== ======================== ===================
747 The default policy follows the below POSIX rule.
749 Allocating disk space
750 The default operation (i.e., mode is zero) of fallocate() allocates
751 the disk space within the range specified by offset and len. The
752 file size (as reported by stat(2)) will be changed if offset+len is
753 greater than the file size. Any subregion within the range specified
754 by offset and len that did not contain data before the call will be
755 initialized to zero. This default behavior closely resembles the
756 behavior of the posix_fallocate(3) library function, and is intended
757 as a method of optimally implementing that function.
759 However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
760 fallocate(fd, DEFAULT_MODE), it allocates on-disk block addressess having
761 zero or random data, which is useful to the below scenario where:
764 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
765 3. fallocate(fd, 0, 0, size)
766 4. address = fibmap(fd, offset)
768 6. write(blkdev, address)
770 Compression implementation
771 --------------------------
773 - New term named cluster is defined as basic unit of compression, file can
774 be divided into multiple clusters logically. One cluster includes 4 << n
775 (n >= 0) logical pages, compression size is also cluster size, each of
776 cluster can be compressed or not.
778 - In cluster metadata layout, one special block address is used to indicate
779 a cluster is a compressed one or normal one; for compressed cluster, following
780 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
781 stores data including compress header and compressed data.
783 - In order to eliminate write amplification during overwrite, F2FS only
784 support compression on write-once file, data can be compressed only when
785 all logical blocks in cluster contain valid data and compress ratio of
786 cluster data is lower than specified threshold.
788 - To enable compression on regular inode, there are three ways:
791 * chattr +c dir; touch dir/file
792 * mount w/ -o compress_extension=ext; touch file.ext
794 Compress metadata layout::
797 +-----------------------------------------------+
798 | cluster 1 | cluster 2 | ......... | cluster N |
799 +-----------------------------------------------+
802 . Compressed Cluster . . Normal Cluster .
803 +----------+---------+---------+---------+ +---------+---------+---------+---------+
804 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 |
805 +----------+---------+---------+---------+ +---------+---------+---------+---------+
809 +-------------+-------------+----------+----------------------------+
810 | data length | data chksum | reserved | compressed data |
811 +-------------+-------------+----------+----------------------------+
813 NVMe Zoned Namespace devices
814 ----------------------------
816 - ZNS defines a per-zone capacity which can be equal or less than the
817 zone-size. Zone-capacity is the number of usable blocks in the zone.
818 F2FS checks if zone-capacity is less than zone-size, if it is, then any
819 segment which starts after the zone-capacity is marked as not-free in
820 the free segment bitmap at initial mount time. These segments are marked
821 as permanently used so they are not allocated for writes and
822 consequently are not needed to be garbage collected. In case the
823 zone-capacity is not aligned to default segment size(2MB), then a segment
824 can start before the zone-capacity and span across zone-capacity boundary.
825 Such spanning segments are also considered as usable segments. All blocks
826 past the zone-capacity are considered unusable in these segments.
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