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 Diable 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 fault_injection=%d Enable fault injection in all supported types with
161 specified injection rate.
162 fault_type=%d Support configuring fault injection type, should be
163 enabled with fault_injection option, fault type value
164 is shown below, it supports single or combined type.
166 FAULT_KMALLOC 0x000000001
167 FAULT_KVMALLOC 0x000000002
168 FAULT_PAGE_ALLOC 0x000000004
169 FAULT_PAGE_GET 0x000000008
170 FAULT_ALLOC_BIO 0x000000010
171 FAULT_ALLOC_NID 0x000000020
172 FAULT_ORPHAN 0x000000040
173 FAULT_BLOCK 0x000000080
174 FAULT_DIR_DEPTH 0x000000100
175 FAULT_EVICT_INODE 0x000000200
176 FAULT_TRUNCATE 0x000000400
177 FAULT_READ_IO 0x000000800
178 FAULT_CHECKPOINT 0x000001000
179 FAULT_DISCARD 0x000002000
180 FAULT_WRITE_IO 0x000004000
181 mode=%s Control block allocation mode which supports "adaptive"
182 and "lfs". In "lfs" mode, there should be no random
183 writes towards main area.
184 io_bits=%u Set the bit size of write IO requests. It should be set
186 usrquota Enable plain user disk quota accounting.
187 grpquota Enable plain group disk quota accounting.
188 prjquota Enable plain project quota accounting.
189 usrjquota=<file> Appoint specified file and type during mount, so that quota
190 grpjquota=<file> information can be properly updated during recovery flow,
191 prjjquota=<file> <quota file>: must be in root directory;
192 jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
193 offusrjquota Turn off user journelled quota.
194 offgrpjquota Turn off group journelled quota.
195 offprjjquota Turn off project journelled quota.
196 quota Enable plain user disk quota accounting.
197 noquota Disable all plain disk quota option.
198 whint_mode=%s Control which write hints are passed down to block
199 layer. This supports "off", "user-based", and
200 "fs-based". In "off" mode (default), f2fs does not pass
201 down hints. In "user-based" mode, f2fs tries to pass
202 down hints given by users. And in "fs-based" mode, f2fs
203 passes down hints with its policy.
204 alloc_mode=%s Adjust block allocation policy, which supports "reuse"
206 fsync_mode=%s Control the policy of fsync. Currently supports "posix",
207 "strict", and "nobarrier". In "posix" mode, which is
208 default, fsync will follow POSIX semantics and does a
209 light operation to improve the filesystem performance.
210 In "strict" mode, fsync will be heavy and behaves in line
211 with xfs, ext4 and btrfs, where xfstest generic/342 will
212 pass, but the performance will regress. "nobarrier" is
213 based on "posix", but doesn't issue flush command for
214 non-atomic files likewise "nobarrier" mount option.
215 test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt
216 context. The fake fscrypt context is used by xfstests.
217 checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable"
218 to reenable checkpointing. Is enabled by default. While
219 disabled, any unmounting or unexpected shutdowns will cause
220 the filesystem contents to appear as they did when the
221 filesystem was mounted with that option.
222 While mounting with checkpoint=disabled, the filesystem must
223 run garbage collection to ensure that all available space can
224 be used. If this takes too much time, the mount may return
225 EAGAIN. You may optionally add a value to indicate how much
226 of the disk you would be willing to temporarily give up to
227 avoid additional garbage collection. This can be given as a
228 number of blocks, or as a percent. For instance, mounting
229 with checkpoint=disable:100% would always succeed, but it may
230 hide up to all remaining free space. The actual space that
231 would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable
232 This space is reclaimed once checkpoint=enable.
234 ================================================================================
236 ================================================================================
238 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
239 f2fs. Each file shows the whole f2fs information.
241 /sys/kernel/debug/f2fs/status includes:
242 - major file system information managed by f2fs currently
243 - average SIT information about whole segments
244 - current memory footprint consumed by f2fs.
246 ================================================================================
248 ================================================================================
250 Information about mounted f2f2 file systems can be found in
251 /sys/fs/f2fs. Each mounted filesystem will have a directory in
252 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
253 The files in each per-device directory are shown in table below.
255 Files in /sys/fs/f2fs/<devname>
256 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
257 ..............................................................................
260 gc_urgent_sleep_time This parameter controls sleep time for gc_urgent.
261 500 ms is set by default. See above gc_urgent.
263 gc_min_sleep_time This tuning parameter controls the minimum sleep
264 time for the garbage collection thread. Time is
267 gc_max_sleep_time This tuning parameter controls the maximum sleep
268 time for the garbage collection thread. Time is
271 gc_no_gc_sleep_time This tuning parameter controls the default sleep
272 time for the garbage collection thread. Time is
275 gc_idle This parameter controls the selection of victim
276 policy for garbage collection. Setting gc_idle = 0
277 (default) will disable this option. Setting
278 gc_idle = 1 will select the Cost Benefit approach
279 & setting gc_idle = 2 will select the greedy approach.
281 gc_urgent This parameter controls triggering background GCs
282 urgently or not. Setting gc_urgent = 0 [default]
283 makes back to default behavior, while if it is set
284 to 1, background thread starts to do GC by given
285 gc_urgent_sleep_time interval.
287 reclaim_segments This parameter controls the number of prefree
288 segments to be reclaimed. If the number of prefree
289 segments is larger than the number of segments
290 in the proportion to the percentage over total
291 volume size, f2fs tries to conduct checkpoint to
292 reclaim the prefree segments to free segments.
293 By default, 5% over total # of segments.
295 max_small_discards This parameter controls the number of discard
296 commands that consist small blocks less than 2MB.
297 The candidates to be discarded are cached until
298 checkpoint is triggered, and issued during the
299 checkpoint. By default, it is disabled with 0.
301 discard_granularity This parameter controls the granularity of discard
302 command size. It will issue discard commands iif
303 the size is larger than given granularity. Its
304 unit size is 4KB, and 4 (=16KB) is set by default.
305 The maximum value is 128 (=512KB).
307 reserved_blocks This parameter indicates the number of blocks that
308 f2fs reserves internally for root.
310 batched_trim_sections This parameter controls the number of sections
311 to be trimmed out in batch mode when FITRIM
312 conducts. 32 sections is set by default.
314 ipu_policy This parameter controls the policy of in-place
315 updates in f2fs. There are five policies:
316 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
317 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
318 0x10: F2FS_IPU_FSYNC.
320 min_ipu_util This parameter controls the threshold to trigger
321 in-place-updates. The number indicates percentage
322 of the filesystem utilization, and used by
323 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
325 min_fsync_blocks This parameter controls the threshold to trigger
326 in-place-updates when F2FS_IPU_FSYNC mode is set.
327 The number indicates the number of dirty pages
328 when fsync needs to flush on its call path. If
329 the number is less than this value, it triggers
332 min_seq_blocks This parameter controls the threshold to serialize
333 write IOs issued by multiple threads in parallel.
335 min_hot_blocks This parameter controls the threshold to allocate
336 a hot data log for pending data blocks to write.
338 min_ssr_sections This parameter adds the threshold when deciding
339 SSR block allocation. If this is large, SSR mode
340 will be enabled early.
342 ram_thresh This parameter controls the memory footprint used
343 by free nids and cached nat entries. By default,
344 10 is set, which indicates 10 MB / 1 GB RAM.
346 ra_nid_pages When building free nids, F2FS reads NAT blocks
347 ahead for speed up. Default is 0.
349 dirty_nats_ratio Given dirty ratio of cached nat entries, F2FS
350 determines flushing them in background.
352 max_victim_search This parameter controls the number of trials to
353 find a victim segment when conducting SSR and
354 cleaning operations. The default value is 4096
355 which covers 8GB block address range.
357 migration_granularity For large-sized sections, F2FS can stop GC given
358 this granularity instead of reclaiming entire
361 dir_level This parameter controls the directory level to
362 support large directory. If a directory has a
363 number of files, it can reduce the file lookup
364 latency by increasing this dir_level value.
365 Otherwise, it needs to decrease this value to
366 reduce the space overhead. The default value is 0.
368 cp_interval F2FS tries to do checkpoint periodically, 60 secs
371 idle_interval F2FS detects system is idle, if there's no F2FS
372 operations during given interval, 5 secs by
375 discard_idle_interval F2FS detects the discard thread is idle, given
376 time interval. Default is 5 secs.
378 gc_idle_interval F2FS detects the GC thread is idle, given time
379 interval. Default is 5 secs.
381 umount_discard_timeout When unmounting the disk, F2FS waits for finishing
382 queued discard commands which can take huge time.
383 This gives time out for it, 5 secs by default.
385 iostat_enable This controls to enable/disable iostat in F2FS.
387 readdir_ra This enables/disabled readahead of inode blocks
388 in readdir, and default is enabled.
390 gc_pin_file_thresh This indicates how many GC can be failed for the
391 pinned file. If it exceeds this, F2FS doesn't
392 guarantee its pinning state. 2048 trials is set
395 extension_list This enables to change extension_list for hot/cold
398 inject_rate This controls injection rate of arbitrary faults.
400 inject_type This controls injection type of arbitrary faults.
402 dirty_segments This shows # of dirty segments.
404 lifetime_write_kbytes This shows # of data written to the disk.
406 features This shows current features enabled on F2FS.
408 current_reserved_blocks This shows # of blocks currently reserved.
410 unusable If checkpoint=disable, this shows the number of
411 blocks that are unusable.
412 If checkpoint=enable it shows the number of blocks
413 that would be unusable if checkpoint=disable were
416 ================================================================================
418 ================================================================================
420 1. Download userland tools and compile them.
422 2. Skip, if f2fs was compiled statically inside kernel.
423 Otherwise, insert the f2fs.ko module.
426 3. Create a directory trying to mount
429 4. Format the block device, and then mount as f2fs
430 # mkfs.f2fs -l label /dev/block_device
431 # mount -t f2fs /dev/block_device /mnt/f2fs
435 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
436 which builds a basic on-disk layout.
438 The options consist of:
439 -l [label] : Give a volume label, up to 512 unicode name.
440 -a [0 or 1] : Split start location of each area for heap-based allocation.
441 1 is set by default, which performs this.
442 -o [int] : Set overprovision ratio in percent over volume size.
444 -s [int] : Set the number of segments per section.
446 -z [int] : Set the number of sections per zone.
448 -e [str] : Set basic extension list. e.g. "mp3,gif,mov"
449 -t [0 or 1] : Disable discard command or not.
450 1 is set by default, which conducts discard.
454 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
455 partition, which examines whether the filesystem metadata and user-made data
456 are cross-referenced correctly or not.
457 Note that, initial version of the tool does not fix any inconsistency.
459 The options consist of:
460 -d debug level [default:0]
464 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
465 file. Each file is dump_ssa and dump_sit.
467 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
468 It shows on-disk inode information recognized by a given inode number, and is
469 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
470 ./dump_sit respectively.
472 The options consist of:
473 -d debug level [default:0]
475 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
476 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
479 # dump.f2fs -i [ino] /dev/sdx
480 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
481 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
483 ================================================================================
485 ================================================================================
490 F2FS divides the whole volume into a number of segments, each of which is fixed
491 to 2MB in size. A section is composed of consecutive segments, and a zone
492 consists of a set of sections. By default, section and zone sizes are set to one
493 segment size identically, but users can easily modify the sizes by mkfs.
495 F2FS splits the entire volume into six areas, and all the areas except superblock
496 consists of multiple segments as described below.
498 align with the zone size <-|
499 |-> align with the segment size
500 _________________________________________________________________________
501 | | | Segment | Node | Segment | |
502 | Superblock | Checkpoint | Info. | Address | Summary | Main |
503 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
504 |____________|_____2______|______N______|______N______|______N_____|__N___|
508 ._________________________________________.
509 |_Segment_|_..._|_Segment_|_..._|_Segment_|
518 : It is located at the beginning of the partition, and there exist two copies
519 to avoid file system crash. It contains basic partition information and some
520 default parameters of f2fs.
523 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
524 inode lists, and summary entries of current active segments.
526 - Segment Information Table (SIT)
527 : It contains segment information such as valid block count and bitmap for the
528 validity of all the blocks.
530 - Node Address Table (NAT)
531 : It is composed of a block address table for all the node blocks stored in
534 - Segment Summary Area (SSA)
535 : It contains summary entries which contains the owner information of all the
536 data and node blocks stored in Main area.
539 : It contains file and directory data including their indices.
541 In order to avoid misalignment between file system and flash-based storage, F2FS
542 aligns the start block address of CP with the segment size. Also, it aligns the
543 start block address of Main area with the zone size by reserving some segments
546 Reference the following survey for additional technical details.
547 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
549 File System Metadata Structure
550 ------------------------------
552 F2FS adopts the checkpointing scheme to maintain file system consistency. At
553 mount time, F2FS first tries to find the last valid checkpoint data by scanning
554 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
555 One of them always indicates the last valid data, which is called as shadow copy
556 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
558 For file system consistency, each CP points to which NAT and SIT copies are
559 valid, as shown as below.
561 +--------+----------+---------+
563 +--------+----------+---------+
567 +-------+-------+--------+--------+--------+--------+
568 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
569 +-------+-------+--------+--------+--------+--------+
572 `----------------------------------------'
577 The key data structure to manage the data locations is a "node". Similar to
578 traditional file structures, F2FS has three types of node: inode, direct node,
579 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
580 indices, two direct node pointers, two indirect node pointers, and one double
581 indirect node pointer as described below. One direct node block contains 1018
582 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
583 one inode block (i.e., a file) covers:
585 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
592 | `- direct node (1018)
594 `- double indirect node (1)
595 `- indirect node (1018)
596 `- direct node (1018)
599 Note that, all the node blocks are mapped by NAT which means the location of
600 each node is translated by the NAT table. In the consideration of the wandering
601 tree problem, F2FS is able to cut off the propagation of node updates caused by
607 A directory entry occupies 11 bytes, which consists of the following attributes.
609 - hash hash value of the file name
611 - len the length of file name
612 - type file type such as directory, symlink, etc
614 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
615 used to represent whether each dentry is valid or not. A dentry block occupies
616 4KB with the following composition.
618 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
619 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
622 +--------------------------------+
623 |dentry block 1 | dentry block 2 |
624 +--------------------------------+
627 . [Dentry Block Structure: 4KB] .
628 +--------+----------+----------+------------+
629 | bitmap | reserved | dentries | file names |
630 +--------+----------+----------+------------+
631 [Dentry Block: 4KB] . .
634 +------+------+-----+------+
635 | hash | ino | len | type |
636 +------+------+-----+------+
637 [Dentry Structure: 11 bytes]
639 F2FS implements multi-level hash tables for directory structure. Each level has
640 a hash table with dedicated number of hash buckets as shown below. Note that
641 "A(2B)" means a bucket includes 2 data blocks.
643 ----------------------
646 N : MAX_DIR_HASH_DEPTH
647 ----------------------
651 level #1 | A(2B) - A(2B)
653 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
655 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
657 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
659 The number of blocks and buckets are determined by,
661 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
662 # of blocks in level #n = |
665 ,- 2^(n + dir_level),
666 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
667 # of buckets in level #n = |
668 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
671 When F2FS finds a file name in a directory, at first a hash value of the file
672 name is calculated. Then, F2FS scans the hash table in level #0 to find the
673 dentry consisting of the file name and its inode number. If not found, F2FS
674 scans the next hash table in level #1. In this way, F2FS scans hash tables in
675 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
676 one bucket determined by the following equation, which shows O(log(# of files))
679 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
681 In the case of file creation, F2FS finds empty consecutive slots that cover the
682 file name. F2FS searches the empty slots in the hash tables of whole levels from
683 1 to N in the same way as the lookup operation.
685 The following figure shows an example of two cases holding children.
686 --------------> Dir <--------------
690 child - child [hole] - child
692 child - child - child [hole] - [hole] - child
695 Number of children = 6, Number of children = 3,
696 File size = 7 File size = 7
698 Default Block Allocation
699 ------------------------
701 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
702 and Hot/Warm/Cold data.
704 - Hot node contains direct node blocks of directories.
705 - Warm node contains direct node blocks except hot node blocks.
706 - Cold node contains indirect node blocks
707 - Hot data contains dentry blocks
708 - Warm data contains data blocks except hot and cold data blocks
709 - Cold data contains multimedia data or migrated data blocks
711 LFS has two schemes for free space management: threaded log and copy-and-compac-
712 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
713 for devices showing very good sequential write performance, since free segments
714 are served all the time for writing new data. However, it suffers from cleaning
715 overhead under high utilization. Contrarily, the threaded log scheme suffers
716 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
717 scheme where the copy-and-compaction scheme is adopted by default, but the
718 policy is dynamically changed to the threaded log scheme according to the file
721 In order to align F2FS with underlying flash-based storage, F2FS allocates a
722 segment in a unit of section. F2FS expects that the section size would be the
723 same as the unit size of garbage collection in FTL. Furthermore, with respect
724 to the mapping granularity in FTL, F2FS allocates each section of the active
725 logs from different zones as much as possible, since FTL can write the data in
726 the active logs into one allocation unit according to its mapping granularity.
731 F2FS does cleaning both on demand and in the background. On-demand cleaning is
732 triggered when there are not enough free segments to serve VFS calls. Background
733 cleaner is operated by a kernel thread, and triggers the cleaning job when the
736 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
737 In the greedy algorithm, F2FS selects a victim segment having the smallest number
738 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
739 according to the segment age and the number of valid blocks in order to address
740 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
741 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
744 In order to identify whether the data in the victim segment are valid or not,
745 F2FS manages a bitmap. Each bit represents the validity of a block, and the
746 bitmap is composed of a bit stream covering whole blocks in main area.
751 The default policy follows the below posix rule.
753 Allocating disk space
754 The default operation (i.e., mode is zero) of fallocate() allocates
755 the disk space within the range specified by offset and len. The
756 file size (as reported by stat(2)) will be changed if offset+len is
757 greater than the file size. Any subregion within the range specified
758 by offset and len that did not contain data before the call will be
759 initialized to zero. This default behavior closely resembles the
760 behavior of the posix_fallocate(3) library function, and is intended
761 as a method of optimally implementing that function.
763 However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to
764 fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addressess having
765 zero or random data, which is useful to the below scenario where:
767 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE)
768 3. fallocate(fd, 0, 0, size)
769 4. address = fibmap(fd, offset)
771 6. write(blkdev, address)