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.
239 ================================================================================
241 ================================================================================
243 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
244 f2fs. Each file shows the whole f2fs information.
246 /sys/kernel/debug/f2fs/status includes:
247 - major file system information managed by f2fs currently
248 - average SIT information about whole segments
249 - current memory footprint consumed by f2fs.
251 ================================================================================
253 ================================================================================
255 Information about mounted f2fs file systems can be found in
256 /sys/fs/f2fs. Each mounted filesystem will have a directory in
257 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
258 The files in each per-device directory are shown in table below.
260 Files in /sys/fs/f2fs/<devname>
261 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
262 ..............................................................................
265 gc_urgent_sleep_time This parameter controls sleep time for gc_urgent.
266 500 ms is set by default. See above gc_urgent.
268 gc_min_sleep_time This tuning parameter controls the minimum sleep
269 time for the garbage collection thread. Time is
272 gc_max_sleep_time This tuning parameter controls the maximum sleep
273 time for the garbage collection thread. Time is
276 gc_no_gc_sleep_time This tuning parameter controls the default sleep
277 time for the garbage collection thread. Time is
280 gc_idle This parameter controls the selection of victim
281 policy for garbage collection. Setting gc_idle = 0
282 (default) will disable this option. Setting
283 gc_idle = 1 will select the Cost Benefit approach
284 & setting gc_idle = 2 will select the greedy approach.
286 gc_urgent This parameter controls triggering background GCs
287 urgently or not. Setting gc_urgent = 0 [default]
288 makes back to default behavior, while if it is set
289 to 1, background thread starts to do GC by given
290 gc_urgent_sleep_time interval.
292 reclaim_segments This parameter controls the number of prefree
293 segments to be reclaimed. If the number of prefree
294 segments is larger than the number of segments
295 in the proportion to the percentage over total
296 volume size, f2fs tries to conduct checkpoint to
297 reclaim the prefree segments to free segments.
298 By default, 5% over total # of segments.
300 main_blkaddr This value gives the first block address of
301 MAIN area in the partition.
303 max_small_discards This parameter controls the number of discard
304 commands that consist small blocks less than 2MB.
305 The candidates to be discarded are cached until
306 checkpoint is triggered, and issued during the
307 checkpoint. By default, it is disabled with 0.
309 discard_granularity This parameter controls the granularity of discard
310 command size. It will issue discard commands iif
311 the size is larger than given granularity. Its
312 unit size is 4KB, and 4 (=16KB) is set by default.
313 The maximum value is 128 (=512KB).
315 reserved_blocks This parameter indicates the number of blocks that
316 f2fs reserves internally for root.
318 batched_trim_sections This parameter controls the number of sections
319 to be trimmed out in batch mode when FITRIM
320 conducts. 32 sections is set by default.
322 ipu_policy This parameter controls the policy of in-place
323 updates in f2fs. There are five policies:
324 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
325 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
326 0x10: F2FS_IPU_FSYNC.
328 min_ipu_util This parameter controls the threshold to trigger
329 in-place-updates. The number indicates percentage
330 of the filesystem utilization, and used by
331 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
333 min_fsync_blocks This parameter controls the threshold to trigger
334 in-place-updates when F2FS_IPU_FSYNC mode is set.
335 The number indicates the number of dirty pages
336 when fsync needs to flush on its call path. If
337 the number is less than this value, it triggers
340 min_seq_blocks This parameter controls the threshold to serialize
341 write IOs issued by multiple threads in parallel.
343 min_hot_blocks This parameter controls the threshold to allocate
344 a hot data log for pending data blocks to write.
346 min_ssr_sections This parameter adds the threshold when deciding
347 SSR block allocation. If this is large, SSR mode
348 will be enabled early.
350 ram_thresh This parameter controls the memory footprint used
351 by free nids and cached nat entries. By default,
352 1 is set, which indicates 10 MB / 1 GB RAM.
354 ra_nid_pages When building free nids, F2FS reads NAT blocks
355 ahead for speed up. Default is 0.
357 dirty_nats_ratio Given dirty ratio of cached nat entries, F2FS
358 determines flushing them in background.
360 max_victim_search This parameter controls the number of trials to
361 find a victim segment when conducting SSR and
362 cleaning operations. The default value is 4096
363 which covers 8GB block address range.
365 migration_granularity For large-sized sections, F2FS can stop GC given
366 this granularity instead of reclaiming entire
369 dir_level This parameter controls the directory level to
370 support large directory. If a directory has a
371 number of files, it can reduce the file lookup
372 latency by increasing this dir_level value.
373 Otherwise, it needs to decrease this value to
374 reduce the space overhead. The default value is 0.
376 cp_interval F2FS tries to do checkpoint periodically, 60 secs
379 idle_interval F2FS detects system is idle, if there's no F2FS
380 operations during given interval, 5 secs by
383 discard_idle_interval F2FS detects the discard thread is idle, given
384 time interval. Default is 5 secs.
386 gc_idle_interval F2FS detects the GC thread is idle, given time
387 interval. Default is 5 secs.
389 umount_discard_timeout When unmounting the disk, F2FS waits for finishing
390 queued discard commands which can take huge time.
391 This gives time out for it, 5 secs by default.
393 iostat_enable This controls to enable/disable iostat in F2FS.
395 readdir_ra This enables/disabled readahead of inode blocks
396 in readdir, and default is enabled.
398 gc_pin_file_thresh This indicates how many GC can be failed for the
399 pinned file. If it exceeds this, F2FS doesn't
400 guarantee its pinning state. 2048 trials is set
403 extension_list This enables to change extension_list for hot/cold
406 inject_rate This controls injection rate of arbitrary faults.
408 inject_type This controls injection type of arbitrary faults.
410 dirty_segments This shows # of dirty segments.
412 lifetime_write_kbytes This shows # of data written to the disk.
414 features This shows current features enabled on F2FS.
416 current_reserved_blocks This shows # of blocks currently reserved.
418 unusable If checkpoint=disable, this shows the number of
419 blocks that are unusable.
420 If checkpoint=enable it shows the number of blocks
421 that would be unusable if checkpoint=disable were
424 encoding This shows the encoding used for casefolding.
425 If casefolding is not enabled, returns (none)
427 ================================================================================
429 ================================================================================
431 1. Download userland tools and compile them.
433 2. Skip, if f2fs was compiled statically inside kernel.
434 Otherwise, insert the f2fs.ko module.
437 3. Create a directory trying to mount
440 4. Format the block device, and then mount as f2fs
441 # mkfs.f2fs -l label /dev/block_device
442 # mount -t f2fs /dev/block_device /mnt/f2fs
446 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
447 which builds a basic on-disk layout.
449 The options consist of:
450 -l [label] : Give a volume label, up to 512 unicode name.
451 -a [0 or 1] : Split start location of each area for heap-based allocation.
452 1 is set by default, which performs this.
453 -o [int] : Set overprovision ratio in percent over volume size.
455 -s [int] : Set the number of segments per section.
457 -z [int] : Set the number of sections per zone.
459 -e [str] : Set basic extension list. e.g. "mp3,gif,mov"
460 -t [0 or 1] : Disable discard command or not.
461 1 is set by default, which conducts discard.
465 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
466 partition, which examines whether the filesystem metadata and user-made data
467 are cross-referenced correctly or not.
468 Note that, initial version of the tool does not fix any inconsistency.
470 The options consist of:
471 -d debug level [default:0]
475 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
476 file. Each file is dump_ssa and dump_sit.
478 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
479 It shows on-disk inode information recognized by a given inode number, and is
480 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
481 ./dump_sit respectively.
483 The options consist of:
484 -d debug level [default:0]
486 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
487 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
490 # dump.f2fs -i [ino] /dev/sdx
491 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
492 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
494 ================================================================================
496 ================================================================================
501 F2FS divides the whole volume into a number of segments, each of which is fixed
502 to 2MB in size. A section is composed of consecutive segments, and a zone
503 consists of a set of sections. By default, section and zone sizes are set to one
504 segment size identically, but users can easily modify the sizes by mkfs.
506 F2FS splits the entire volume into six areas, and all the areas except superblock
507 consists of multiple segments as described below.
509 align with the zone size <-|
510 |-> align with the segment size
511 _________________________________________________________________________
512 | | | Segment | Node | Segment | |
513 | Superblock | Checkpoint | Info. | Address | Summary | Main |
514 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
515 |____________|_____2______|______N______|______N______|______N_____|__N___|
519 ._________________________________________.
520 |_Segment_|_..._|_Segment_|_..._|_Segment_|
529 : It is located at the beginning of the partition, and there exist two copies
530 to avoid file system crash. It contains basic partition information and some
531 default parameters of f2fs.
534 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
535 inode lists, and summary entries of current active segments.
537 - Segment Information Table (SIT)
538 : It contains segment information such as valid block count and bitmap for the
539 validity of all the blocks.
541 - Node Address Table (NAT)
542 : It is composed of a block address table for all the node blocks stored in
545 - Segment Summary Area (SSA)
546 : It contains summary entries which contains the owner information of all the
547 data and node blocks stored in Main area.
550 : It contains file and directory data including their indices.
552 In order to avoid misalignment between file system and flash-based storage, F2FS
553 aligns the start block address of CP with the segment size. Also, it aligns the
554 start block address of Main area with the zone size by reserving some segments
557 Reference the following survey for additional technical details.
558 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
560 File System Metadata Structure
561 ------------------------------
563 F2FS adopts the checkpointing scheme to maintain file system consistency. At
564 mount time, F2FS first tries to find the last valid checkpoint data by scanning
565 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
566 One of them always indicates the last valid data, which is called as shadow copy
567 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
569 For file system consistency, each CP points to which NAT and SIT copies are
570 valid, as shown as below.
572 +--------+----------+---------+
574 +--------+----------+---------+
578 +-------+-------+--------+--------+--------+--------+
579 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
580 +-------+-------+--------+--------+--------+--------+
583 `----------------------------------------'
588 The key data structure to manage the data locations is a "node". Similar to
589 traditional file structures, F2FS has three types of node: inode, direct node,
590 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
591 indices, two direct node pointers, two indirect node pointers, and one double
592 indirect node pointer as described below. One direct node block contains 1018
593 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
594 one inode block (i.e., a file) covers:
596 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
603 | `- direct node (1018)
605 `- double indirect node (1)
606 `- indirect node (1018)
607 `- direct node (1018)
610 Note that, all the node blocks are mapped by NAT which means the location of
611 each node is translated by the NAT table. In the consideration of the wandering
612 tree problem, F2FS is able to cut off the propagation of node updates caused by
618 A directory entry occupies 11 bytes, which consists of the following attributes.
620 - hash hash value of the file name
622 - len the length of file name
623 - type file type such as directory, symlink, etc
625 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
626 used to represent whether each dentry is valid or not. A dentry block occupies
627 4KB with the following composition.
629 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
630 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
633 +--------------------------------+
634 |dentry block 1 | dentry block 2 |
635 +--------------------------------+
638 . [Dentry Block Structure: 4KB] .
639 +--------+----------+----------+------------+
640 | bitmap | reserved | dentries | file names |
641 +--------+----------+----------+------------+
642 [Dentry Block: 4KB] . .
645 +------+------+-----+------+
646 | hash | ino | len | type |
647 +------+------+-----+------+
648 [Dentry Structure: 11 bytes]
650 F2FS implements multi-level hash tables for directory structure. Each level has
651 a hash table with dedicated number of hash buckets as shown below. Note that
652 "A(2B)" means a bucket includes 2 data blocks.
654 ----------------------
657 N : MAX_DIR_HASH_DEPTH
658 ----------------------
662 level #1 | A(2B) - A(2B)
664 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
666 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
668 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
670 The number of blocks and buckets are determined by,
672 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
673 # of blocks in level #n = |
676 ,- 2^(n + dir_level),
677 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
678 # of buckets in level #n = |
679 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
682 When F2FS finds a file name in a directory, at first a hash value of the file
683 name is calculated. Then, F2FS scans the hash table in level #0 to find the
684 dentry consisting of the file name and its inode number. If not found, F2FS
685 scans the next hash table in level #1. In this way, F2FS scans hash tables in
686 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
687 one bucket determined by the following equation, which shows O(log(# of files))
690 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
692 In the case of file creation, F2FS finds empty consecutive slots that cover the
693 file name. F2FS searches the empty slots in the hash tables of whole levels from
694 1 to N in the same way as the lookup operation.
696 The following figure shows an example of two cases holding children.
697 --------------> Dir <--------------
701 child - child [hole] - child
703 child - child - child [hole] - [hole] - child
706 Number of children = 6, Number of children = 3,
707 File size = 7 File size = 7
709 Default Block Allocation
710 ------------------------
712 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
713 and Hot/Warm/Cold data.
715 - Hot node contains direct node blocks of directories.
716 - Warm node contains direct node blocks except hot node blocks.
717 - Cold node contains indirect node blocks
718 - Hot data contains dentry blocks
719 - Warm data contains data blocks except hot and cold data blocks
720 - Cold data contains multimedia data or migrated data blocks
722 LFS has two schemes for free space management: threaded log and copy-and-compac-
723 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
724 for devices showing very good sequential write performance, since free segments
725 are served all the time for writing new data. However, it suffers from cleaning
726 overhead under high utilization. Contrarily, the threaded log scheme suffers
727 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
728 scheme where the copy-and-compaction scheme is adopted by default, but the
729 policy is dynamically changed to the threaded log scheme according to the file
732 In order to align F2FS with underlying flash-based storage, F2FS allocates a
733 segment in a unit of section. F2FS expects that the section size would be the
734 same as the unit size of garbage collection in FTL. Furthermore, with respect
735 to the mapping granularity in FTL, F2FS allocates each section of the active
736 logs from different zones as much as possible, since FTL can write the data in
737 the active logs into one allocation unit according to its mapping granularity.
742 F2FS does cleaning both on demand and in the background. On-demand cleaning is
743 triggered when there are not enough free segments to serve VFS calls. Background
744 cleaner is operated by a kernel thread, and triggers the cleaning job when the
747 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
748 In the greedy algorithm, F2FS selects a victim segment having the smallest number
749 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
750 according to the segment age and the number of valid blocks in order to address
751 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
752 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
755 In order to identify whether the data in the victim segment are valid or not,
756 F2FS manages a bitmap. Each bit represents the validity of a block, and the
757 bitmap is composed of a bit stream covering whole blocks in main area.
762 1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
764 2) whint_mode=user-based. F2FS tries to pass down hints given by
769 META WRITE_LIFE_NOT_SET
773 *ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
777 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
778 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
779 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
781 WRITE_LIFE_MEDIUM " "
785 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
786 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
787 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
788 WRITE_LIFE_NONE " WRITE_LIFE_NONE
789 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
790 WRITE_LIFE_LONG " WRITE_LIFE_LONG
792 3) whint_mode=fs-based. F2FS passes down hints with its policy.
796 META WRITE_LIFE_MEDIUM;
797 HOT_NODE WRITE_LIFE_NOT_SET
799 COLD_NODE WRITE_LIFE_NONE
800 ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
804 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
805 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
806 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
808 WRITE_LIFE_MEDIUM " "
812 WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
813 WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
814 WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
815 WRITE_LIFE_NONE " WRITE_LIFE_NONE
816 WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
817 WRITE_LIFE_LONG " WRITE_LIFE_LONG
822 The default policy follows the below posix rule.
824 Allocating disk space
825 The default operation (i.e., mode is zero) of fallocate() allocates
826 the disk space within the range specified by offset and len. The
827 file size (as reported by stat(2)) will be changed if offset+len is
828 greater than the file size. Any subregion within the range specified
829 by offset and len that did not contain data before the call will be
830 initialized to zero. This default behavior closely resembles the
831 behavior of the posix_fallocate(3) library function, and is intended
832 as a method of optimally implementing that function.
834 However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
835 fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addressess having
836 zero or random data, which is useful to the below scenario where:
838 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
839 3. fallocate(fd, 0, 0, size)
840 4. address = fibmap(fd, offset)
842 6. write(blkdev, address)
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