1 ================================================================================
2 WHAT IS Flash-Friendly File System (F2FS)?
3 ================================================================================
5 NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
6 been equipped on a variety systems ranging from mobile to server systems. Since
7 they are known to have different characteristics from the conventional rotating
8 disks, a file system, an upper layer to the storage device, should adapt to the
9 changes from the sketch in the design level.
11 F2FS is a file system exploiting NAND flash memory-based storage devices, which
12 is based on Log-structured File System (LFS). The design has been focused on
13 addressing the fundamental issues in LFS, which are snowball effect of wandering
14 tree and high cleaning overhead.
16 Since a NAND flash memory-based storage device shows different characteristic
17 according to its internal geometry or flash memory management scheme, namely FTL,
18 F2FS and its tools support various parameters not only for configuring on-disk
19 layout, but also for selecting allocation and cleaning algorithms.
21 The following git tree provides the file system formatting tool (mkfs.f2fs),
22 a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
23 >> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
25 For reporting bugs and sending patches, please use the following mailing list:
26 >> linux-f2fs-devel@lists.sourceforge.net
28 ================================================================================
29 BACKGROUND AND DESIGN ISSUES
30 ================================================================================
32 Log-structured File System (LFS)
33 --------------------------------
34 "A log-structured file system writes all modifications to disk sequentially in
35 a log-like structure, thereby speeding up both file writing and crash recovery.
36 The log is the only structure on disk; it contains indexing information so that
37 files can be read back from the log efficiently. In order to maintain large free
38 areas on disk for fast writing, we divide the log into segments and use a
39 segment cleaner to compress the live information from heavily fragmented
40 segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
41 implementation of a log-structured file system", ACM Trans. Computer Systems
44 Wandering Tree Problem
45 ----------------------
46 In LFS, when a file data is updated and written to the end of log, its direct
47 pointer block is updated due to the changed location. Then the indirect pointer
48 block is also updated due to the direct pointer block update. In this manner,
49 the upper index structures such as inode, inode map, and checkpoint block are
50 also updated recursively. This problem is called as wandering tree problem [1],
51 and in order to enhance the performance, it should eliminate or relax the update
52 propagation as much as possible.
54 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
58 Since LFS is based on out-of-place writes, it produces so many obsolete blocks
59 scattered across the whole storage. In order to serve new empty log space, it
60 needs to reclaim these obsolete blocks seamlessly to users. This job is called
61 as a cleaning process.
63 The process consists of three operations as follows.
64 1. A victim segment is selected through referencing segment usage table.
65 2. It loads parent index structures of all the data in the victim identified by
66 segment summary blocks.
67 3. It checks the cross-reference between the data and its parent index structure.
68 4. It moves valid data selectively.
70 This cleaning job may cause unexpected long delays, so the most important goal
71 is to hide the latencies to users. And also definitely, it should reduce the
72 amount of valid data to be moved, and move them quickly as well.
74 ================================================================================
76 ================================================================================
80 - Enlarge the random write area for better performance, but provide the high
82 - Align FS data structures to the operational units in FTL as best efforts
84 Wandering Tree Problem
85 ----------------------
86 - Use a term, “node”, that represents inodes as well as various pointer blocks
87 - Introduce Node Address Table (NAT) containing the locations of all the “node”
88 blocks; this will cut off the update propagation.
92 - Support a background cleaning process
93 - Support greedy and cost-benefit algorithms for victim selection policies
94 - Support multi-head logs for static/dynamic hot and cold data separation
95 - Introduce adaptive logging for efficient block allocation
97 ================================================================================
99 ================================================================================
101 background_gc=%s Turn on/off cleaning operations, namely garbage
102 collection, triggered in background when I/O subsystem is
103 idle. If background_gc=on, it will turn on the garbage
104 collection and if background_gc=off, garbage collection
105 will be turned off. If background_gc=sync, it will turn
106 on synchronous garbage collection running in background.
107 Default value for this option is on. So garbage
108 collection is on by default.
109 disable_roll_forward Disable the roll-forward recovery routine
110 norecovery Disable the roll-forward recovery routine, mounted read-
111 only (i.e., -o ro,disable_roll_forward)
112 discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
113 enabled, f2fs will issue discard/TRIM commands when a
115 no_heap Disable heap-style segment allocation which finds free
116 segments for data from the beginning of main area, while
117 for node from the end of main area.
118 nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
119 by default if CONFIG_F2FS_FS_XATTR is selected.
120 noacl Disable POSIX Access Control List. Note: acl is enabled
121 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
122 active_logs=%u Support configuring the number of active logs. In the
123 current design, f2fs supports only 2, 4, and 6 logs.
125 disable_ext_identify Disable the extension list configured by mkfs, so f2fs
126 does not aware of cold files such as media files.
127 inline_xattr Enable the inline xattrs feature.
128 noinline_xattr Disable the inline xattrs feature.
129 inline_xattr_size=%u Support configuring inline xattr size, it depends on
130 flexible inline xattr feature.
131 inline_data Enable the inline data feature: New created small(<~3.4k)
132 files can be written into inode block.
133 inline_dentry Enable the inline dir feature: data in new created
134 directory entries can be written into inode block. The
135 space of inode block which is used to store inline
136 dentries is limited to ~3.4k.
137 noinline_dentry Disable the inline dentry feature.
138 flush_merge Merge concurrent cache_flush commands as much as possible
139 to eliminate redundant command issues. If the underlying
140 device handles the cache_flush command relatively slowly,
141 recommend to enable this option.
142 nobarrier This option can be used if underlying storage guarantees
143 its cached data should be written to the novolatile area.
144 If this option is set, no cache_flush commands are issued
145 but f2fs still guarantees the write ordering of all the
147 fastboot This option is used when a system wants to reduce mount
148 time as much as possible, even though normal performance
150 extent_cache Enable an extent cache based on rb-tree, it can cache
151 as many as extent which map between contiguous logical
152 address and physical address per inode, resulting in
153 increasing the cache hit ratio. Set by default.
154 noextent_cache Disable an extent cache based on rb-tree explicitly, see
155 the above extent_cache mount option.
156 noinline_data Disable the inline data feature, inline data feature is
158 data_flush Enable data flushing before checkpoint in order to
159 persist data of regular and symlink.
160 reserve_root=%d Support configuring reserved space which is used for
161 allocation from a privileged user with specified uid or
162 gid, unit: 4KB, the default limit is 0.2% of user blocks.
163 resuid=%d The user ID which may use the reserved blocks.
164 resgid=%d The group ID which may use the reserved blocks.
165 fault_injection=%d Enable fault injection in all supported types with
166 specified injection rate.
167 fault_type=%d Support configuring fault injection type, should be
168 enabled with fault_injection option, fault type value
169 is shown below, it supports single or combined type.
171 FAULT_KMALLOC 0x000000001
172 FAULT_KVMALLOC 0x000000002
173 FAULT_PAGE_ALLOC 0x000000004
174 FAULT_PAGE_GET 0x000000008
175 FAULT_ALLOC_BIO 0x000000010
176 FAULT_ALLOC_NID 0x000000020
177 FAULT_ORPHAN 0x000000040
178 FAULT_BLOCK 0x000000080
179 FAULT_DIR_DEPTH 0x000000100
180 FAULT_EVICT_INODE 0x000000200
181 FAULT_TRUNCATE 0x000000400
182 FAULT_READ_IO 0x000000800
183 FAULT_CHECKPOINT 0x000001000
184 FAULT_DISCARD 0x000002000
185 FAULT_WRITE_IO 0x000004000
186 mode=%s Control block allocation mode which supports "adaptive"
187 and "lfs". In "lfs" mode, there should be no random
188 writes towards main area.
189 io_bits=%u Set the bit size of write IO requests. It should be set
191 usrquota Enable plain user disk quota accounting.
192 grpquota Enable plain group disk quota accounting.
193 prjquota Enable plain project quota accounting.
194 usrjquota=<file> Appoint specified file and type during mount, so that quota
195 grpjquota=<file> information can be properly updated during recovery flow,
196 prjjquota=<file> <quota file>: must be in root directory;
197 jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
198 offusrjquota Turn off user journelled quota.
199 offgrpjquota Turn off group journelled quota.
200 offprjjquota Turn off project journelled quota.
201 quota Enable plain user disk quota accounting.
202 noquota Disable all plain disk quota option.
203 whint_mode=%s Control which write hints are passed down to block
204 layer. This supports "off", "user-based", and
205 "fs-based". In "off" mode (default), f2fs does not pass
206 down hints. In "user-based" mode, f2fs tries to pass
207 down hints given by users. And in "fs-based" mode, f2fs
208 passes down hints with its policy.
209 alloc_mode=%s Adjust block allocation policy, which supports "reuse"
211 fsync_mode=%s Control the policy of fsync. Currently supports "posix",
212 "strict", and "nobarrier". In "posix" mode, which is
213 default, fsync will follow POSIX semantics and does a
214 light operation to improve the filesystem performance.
215 In "strict" mode, fsync will be heavy and behaves in line
216 with xfs, ext4 and btrfs, where xfstest generic/342 will
217 pass, but the performance will regress. "nobarrier" is
218 based on "posix", but doesn't issue flush command for
219 non-atomic files likewise "nobarrier" mount option.
220 test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt
221 context. The fake fscrypt context is used by xfstests.
222 checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable"
223 to reenable checkpointing. Is enabled by default. While
224 disabled, any unmounting or unexpected shutdowns will cause
225 the filesystem contents to appear as they did when the
226 filesystem was mounted with that option.
227 While mounting with checkpoint=disabled, the filesystem must
228 run garbage collection to ensure that all available space can
229 be used. If this takes too much time, the mount may return
230 EAGAIN. You may optionally add a value to indicate how much
231 of the disk you would be willing to temporarily give up to
232 avoid additional garbage collection. This can be given as a
233 number of blocks, or as a percent. For instance, mounting
234 with checkpoint=disable:100% would always succeed, but it may
235 hide up to all remaining free space. The actual space that
236 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
237 This space is reclaimed once checkpoint=enable.
238 compress_algorithm=%s Control compress algorithm, currently f2fs supports "lzo"
240 compress_log_size=%u Support configuring compress cluster size, the size will
241 be 4KB * (1 << %u), 16KB is minimum size, also it's
243 compress_extension=%s Support adding specified extension, so that f2fs can enable
244 compression on those corresponding files, e.g. if all files
245 with '.ext' has high compression rate, we can set the '.ext'
246 on compression extension list and enable compression on
247 these file by default rather than to enable it via ioctl.
248 For other files, we can still enable compression via ioctl.
250 ================================================================================
252 ================================================================================
254 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
255 f2fs. Each file shows the whole f2fs information.
257 /sys/kernel/debug/f2fs/status includes:
258 - major file system information managed by f2fs currently
259 - average SIT information about whole segments
260 - current memory footprint consumed by f2fs.
262 ================================================================================
264 ================================================================================
266 Information about mounted f2fs file systems can be found in
267 /sys/fs/f2fs. Each mounted filesystem will have a directory in
268 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
269 The files in each per-device directory are shown in table below.
271 Files in /sys/fs/f2fs/<devname>
272 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
274 ================================================================================
276 ================================================================================
278 1. Download userland tools and compile them.
280 2. Skip, if f2fs was compiled statically inside kernel.
281 Otherwise, insert the f2fs.ko module.
284 3. Create a directory trying to mount
287 4. Format the block device, and then mount as f2fs
288 # mkfs.f2fs -l label /dev/block_device
289 # mount -t f2fs /dev/block_device /mnt/f2fs
293 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
294 which builds a basic on-disk layout.
296 The options consist of:
297 -l [label] : Give a volume label, up to 512 unicode name.
298 -a [0 or 1] : Split start location of each area for heap-based allocation.
299 1 is set by default, which performs this.
300 -o [int] : Set overprovision ratio in percent over volume size.
302 -s [int] : Set the number of segments per section.
304 -z [int] : Set the number of sections per zone.
306 -e [str] : Set basic extension list. e.g. "mp3,gif,mov"
307 -t [0 or 1] : Disable discard command or not.
308 1 is set by default, which conducts discard.
312 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
313 partition, which examines whether the filesystem metadata and user-made data
314 are cross-referenced correctly or not.
315 Note that, initial version of the tool does not fix any inconsistency.
317 The options consist of:
318 -d debug level [default:0]
322 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
323 file. Each file is dump_ssa and dump_sit.
325 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
326 It shows on-disk inode information recognized by a given inode number, and is
327 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
328 ./dump_sit respectively.
330 The options consist of:
331 -d debug level [default:0]
333 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
334 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
337 # dump.f2fs -i [ino] /dev/sdx
338 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
339 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
341 ================================================================================
343 ================================================================================
348 F2FS divides the whole volume into a number of segments, each of which is fixed
349 to 2MB in size. A section is composed of consecutive segments, and a zone
350 consists of a set of sections. By default, section and zone sizes are set to one
351 segment size identically, but users can easily modify the sizes by mkfs.
353 F2FS splits the entire volume into six areas, and all the areas except superblock
354 consists of multiple segments as described below.
356 align with the zone size <-|
357 |-> align with the segment size
358 _________________________________________________________________________
359 | | | Segment | Node | Segment | |
360 | Superblock | Checkpoint | Info. | Address | Summary | Main |
361 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
362 |____________|_____2______|______N______|______N______|______N_____|__N___|
366 ._________________________________________.
367 |_Segment_|_..._|_Segment_|_..._|_Segment_|
376 : It is located at the beginning of the partition, and there exist two copies
377 to avoid file system crash. It contains basic partition information and some
378 default parameters of f2fs.
381 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
382 inode lists, and summary entries of current active segments.
384 - Segment Information Table (SIT)
385 : It contains segment information such as valid block count and bitmap for the
386 validity of all the blocks.
388 - Node Address Table (NAT)
389 : It is composed of a block address table for all the node blocks stored in
392 - Segment Summary Area (SSA)
393 : It contains summary entries which contains the owner information of all the
394 data and node blocks stored in Main area.
397 : It contains file and directory data including their indices.
399 In order to avoid misalignment between file system and flash-based storage, F2FS
400 aligns the start block address of CP with the segment size. Also, it aligns the
401 start block address of Main area with the zone size by reserving some segments
404 Reference the following survey for additional technical details.
405 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
407 File System Metadata Structure
408 ------------------------------
410 F2FS adopts the checkpointing scheme to maintain file system consistency. At
411 mount time, F2FS first tries to find the last valid checkpoint data by scanning
412 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
413 One of them always indicates the last valid data, which is called as shadow copy
414 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
416 For file system consistency, each CP points to which NAT and SIT copies are
417 valid, as shown as below.
419 +--------+----------+---------+
421 +--------+----------+---------+
425 +-------+-------+--------+--------+--------+--------+
426 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
427 +-------+-------+--------+--------+--------+--------+
430 `----------------------------------------'
435 The key data structure to manage the data locations is a "node". Similar to
436 traditional file structures, F2FS has three types of node: inode, direct node,
437 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
438 indices, two direct node pointers, two indirect node pointers, and one double
439 indirect node pointer as described below. One direct node block contains 1018
440 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
441 one inode block (i.e., a file) covers:
443 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
450 | `- direct node (1018)
452 `- double indirect node (1)
453 `- indirect node (1018)
454 `- direct node (1018)
457 Note that, all the node blocks are mapped by NAT which means the location of
458 each node is translated by the NAT table. In the consideration of the wandering
459 tree problem, F2FS is able to cut off the propagation of node updates caused by
465 A directory entry occupies 11 bytes, which consists of the following attributes.
467 - hash hash value of the file name
469 - len the length of file name
470 - type file type such as directory, symlink, etc
472 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
473 used to represent whether each dentry is valid or not. A dentry block occupies
474 4KB with the following composition.
476 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
477 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
480 +--------------------------------+
481 |dentry block 1 | dentry block 2 |
482 +--------------------------------+
485 . [Dentry Block Structure: 4KB] .
486 +--------+----------+----------+------------+
487 | bitmap | reserved | dentries | file names |
488 +--------+----------+----------+------------+
489 [Dentry Block: 4KB] . .
492 +------+------+-----+------+
493 | hash | ino | len | type |
494 +------+------+-----+------+
495 [Dentry Structure: 11 bytes]
497 F2FS implements multi-level hash tables for directory structure. Each level has
498 a hash table with dedicated number of hash buckets as shown below. Note that
499 "A(2B)" means a bucket includes 2 data blocks.
501 ----------------------
504 N : MAX_DIR_HASH_DEPTH
505 ----------------------
509 level #1 | A(2B) - A(2B)
511 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
513 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
515 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
517 The number of blocks and buckets are determined by,
519 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
520 # of blocks in level #n = |
523 ,- 2^(n + dir_level),
524 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
525 # of buckets in level #n = |
526 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
529 When F2FS finds a file name in a directory, at first a hash value of the file
530 name is calculated. Then, F2FS scans the hash table in level #0 to find the
531 dentry consisting of the file name and its inode number. If not found, F2FS
532 scans the next hash table in level #1. In this way, F2FS scans hash tables in
533 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
534 one bucket determined by the following equation, which shows O(log(# of files))
537 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
539 In the case of file creation, F2FS finds empty consecutive slots that cover the
540 file name. F2FS searches the empty slots in the hash tables of whole levels from
541 1 to N in the same way as the lookup operation.
543 The following figure shows an example of two cases holding children.
544 --------------> Dir <--------------
548 child - child [hole] - child
550 child - child - child [hole] - [hole] - child
553 Number of children = 6, Number of children = 3,
554 File size = 7 File size = 7
556 Default Block Allocation
557 ------------------------
559 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
560 and Hot/Warm/Cold data.
562 - Hot node contains direct node blocks of directories.
563 - Warm node contains direct node blocks except hot node blocks.
564 - Cold node contains indirect node blocks
565 - Hot data contains dentry blocks
566 - Warm data contains data blocks except hot and cold data blocks
567 - Cold data contains multimedia data or migrated data blocks
569 LFS has two schemes for free space management: threaded log and copy-and-compac-
570 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
571 for devices showing very good sequential write performance, since free segments
572 are served all the time for writing new data. However, it suffers from cleaning
573 overhead under high utilization. Contrarily, the threaded log scheme suffers
574 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
575 scheme where the copy-and-compaction scheme is adopted by default, but the
576 policy is dynamically changed to the threaded log scheme according to the file
579 In order to align F2FS with underlying flash-based storage, F2FS allocates a
580 segment in a unit of section. F2FS expects that the section size would be the
581 same as the unit size of garbage collection in FTL. Furthermore, with respect
582 to the mapping granularity in FTL, F2FS allocates each section of the active
583 logs from different zones as much as possible, since FTL can write the data in
584 the active logs into one allocation unit according to its mapping granularity.
589 F2FS does cleaning both on demand and in the background. On-demand cleaning is
590 triggered when there are not enough free segments to serve VFS calls. Background
591 cleaner is operated by a kernel thread, and triggers the cleaning job when the
594 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
595 In the greedy algorithm, F2FS selects a victim segment having the smallest number
596 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
597 according to the segment age and the number of valid blocks in order to address
598 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
599 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
602 In order to identify whether the data in the victim segment are valid or not,
603 F2FS manages a bitmap. Each bit represents the validity of a block, and the
604 bitmap is composed of a bit stream covering whole blocks in main area.
609 1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
611 2) whint_mode=user-based. F2FS tries to pass down hints given by
616 META WRITE_LIFE_NOT_SET
620 *ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
624 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
625 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
626 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
628 WRITE_LIFE_MEDIUM " "
632 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
633 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
634 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
635 WRITE_LIFE_NONE " WRITE_LIFE_NONE
636 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
637 WRITE_LIFE_LONG " WRITE_LIFE_LONG
639 3) whint_mode=fs-based. F2FS passes down hints with its policy.
643 META WRITE_LIFE_MEDIUM;
644 HOT_NODE WRITE_LIFE_NOT_SET
646 COLD_NODE WRITE_LIFE_NONE
647 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
651 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
652 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
653 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
655 WRITE_LIFE_MEDIUM " "
659 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
660 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
661 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
662 WRITE_LIFE_NONE " WRITE_LIFE_NONE
663 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
664 WRITE_LIFE_LONG " WRITE_LIFE_LONG
669 The default policy follows the below posix rule.
671 Allocating disk space
672 The default operation (i.e., mode is zero) of fallocate() allocates
673 the disk space within the range specified by offset and len. The
674 file size (as reported by stat(2)) will be changed if offset+len is
675 greater than the file size. Any subregion within the range specified
676 by offset and len that did not contain data before the call will be
677 initialized to zero. This default behavior closely resembles the
678 behavior of the posix_fallocate(3) library function, and is intended
679 as a method of optimally implementing that function.
681 However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
682 fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addressess having
683 zero or random data, which is useful to the below scenario where:
685 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
686 3. fallocate(fd, 0, 0, size)
687 4. address = fibmap(fd, offset)
689 6. write(blkdev, address)
691 Compression implementation
692 --------------------------
694 - New term named cluster is defined as basic unit of compression, file can
695 be divided into multiple clusters logically. One cluster includes 4 << n
696 (n >= 0) logical pages, compression size is also cluster size, each of
697 cluster can be compressed or not.
699 - In cluster metadata layout, one special block address is used to indicate
700 cluster is compressed one or normal one, for compressed cluster, following
701 metadata maps cluster to [1, 4 << n - 1] physical blocks, in where f2fs
702 stores data including compress header and compressed data.
704 - In order to eliminate write amplification during overwrite, F2FS only
705 support compression on write-once file, data can be compressed only when
706 all logical blocks in file are valid and cluster compress ratio is lower
707 than specified threshold.
709 - To enable compression on regular inode, there are three ways:
711 * chattr +c dir; touch dir/file
712 * mount w/ -o compress_extension=ext; touch file.ext
714 Compress metadata layout:
716 +-----------------------------------------------+
717 | cluster 1 | cluster 2 | ......... | cluster N |
718 +-----------------------------------------------+
721 . Compressed Cluster . . Normal Cluster .
722 +----------+---------+---------+---------+ +---------+---------+---------+---------+
723 |compr flag| block 1 | block 2 | block 3 | | block 1 | block 2 | block 3 | block 4 |
724 +----------+---------+---------+---------+ +---------+---------+---------+---------+
728 +-------------+-------------+----------+----------------------------+
729 | data length | data chksum | reserved | compressed data |
730 +-------------+-------------+----------+----------------------------+