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Jaegeuk Kim98e4da82012-11-02 17:05:42 +09001================================================================================
2WHAT IS Flash-Friendly File System (F2FS)?
3================================================================================
4
5NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
6been equipped on a variety systems ranging from mobile to server systems. Since
7they are known to have different characteristics from the conventional rotating
8disks, a file system, an upper layer to the storage device, should adapt to the
9changes from the sketch in the design level.
10
11F2FS is a file system exploiting NAND flash memory-based storage devices, which
12is based on Log-structured File System (LFS). The design has been focused on
13addressing the fundamental issues in LFS, which are snowball effect of wandering
14tree and high cleaning overhead.
15
16Since a NAND flash memory-based storage device shows different characteristic
17according to its internal geometry or flash memory management scheme, namely FTL,
18F2FS and its tools support various parameters not only for configuring on-disk
19layout, but also for selecting allocation and cleaning algorithms.
20
Changman Leed51a7fb2013-07-04 17:12:47 +090021The following git tree provides the file system formatting tool (mkfs.f2fs),
22a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
Jaegeuk Kim5bb446a2012-11-27 14:36:14 +090023>> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
24
25For reporting bugs and sending patches, please use the following mailing list:
26>> linux-f2fs-devel@lists.sourceforge.net
Jaegeuk Kim98e4da82012-11-02 17:05:42 +090027
28================================================================================
29BACKGROUND AND DESIGN ISSUES
30================================================================================
31
32Log-structured File System (LFS)
33--------------------------------
34"A log-structured file system writes all modifications to disk sequentially in
35a log-like structure, thereby speeding up both file writing and crash recovery.
36The log is the only structure on disk; it contains indexing information so that
37files can be read back from the log efficiently. In order to maintain large free
38areas on disk for fast writing, we divide the log into segments and use a
39segment cleaner to compress the live information from heavily fragmented
40segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
41implementation of a log-structured file system", ACM Trans. Computer Systems
4210, 1, 26–52.
43
44Wandering Tree Problem
45----------------------
46In LFS, when a file data is updated and written to the end of log, its direct
47pointer block is updated due to the changed location. Then the indirect pointer
48block is also updated due to the direct pointer block update. In this manner,
49the upper index structures such as inode, inode map, and checkpoint block are
50also updated recursively. This problem is called as wandering tree problem [1],
51and in order to enhance the performance, it should eliminate or relax the update
52propagation as much as possible.
53
54[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
55
56Cleaning Overhead
57-----------------
58Since LFS is based on out-of-place writes, it produces so many obsolete blocks
59scattered across the whole storage. In order to serve new empty log space, it
60needs to reclaim these obsolete blocks seamlessly to users. This job is called
61as a cleaning process.
62
63The process consists of three operations as follows.
641. A victim segment is selected through referencing segment usage table.
652. It loads parent index structures of all the data in the victim identified by
66 segment summary blocks.
673. It checks the cross-reference between the data and its parent index structure.
684. It moves valid data selectively.
69
70This cleaning job may cause unexpected long delays, so the most important goal
71is to hide the latencies to users. And also definitely, it should reduce the
72amount of valid data to be moved, and move them quickly as well.
73
74================================================================================
75KEY FEATURES
76================================================================================
77
78Flash Awareness
79---------------
80- Enlarge the random write area for better performance, but provide the high
81 spatial locality
82- Align FS data structures to the operational units in FTL as best efforts
83
84Wandering 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.
89
90Cleaning Overhead
91-----------------
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
96
97================================================================================
98MOUNT OPTIONS
99================================================================================
100
Namjae Jeon696c0182013-06-16 09:48:48 +0900101background_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 truned off.
106 Default value for this option is on. So garbage
107 collection is on by default.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900108disable_roll_forward Disable the roll-forward recovery routine
Jaegeuk Kim2d834bf2015-01-23 18:33:46 -0800109norecovery Disable the roll-forward recovery routine, mounted read-
110 only (i.e., -o ro,disable_roll_forward)
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900111discard Issue discard/TRIM commands when a segment is cleaned.
112no_heap Disable heap-style segment allocation which finds free
113 segments for data from the beginning of main area, while
114 for node from the end of main area.
115nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
116 by default if CONFIG_F2FS_FS_XATTR is selected.
117noacl Disable POSIX Access Control List. Note: acl is enabled
118 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
119active_logs=%u Support configuring the number of active logs. In the
120 current design, f2fs supports only 2, 4, and 6 logs.
121 Default number is 6.
122disable_ext_identify Disable the extension list configured by mkfs, so f2fs
123 does not aware of cold files such as media files.
Jaegeuk Kim66e960c2013-11-01 11:20:05 +0900124inline_xattr Enable the inline xattrs feature.
Huajun Lie4024e82013-11-10 23:13:21 +0800125inline_data Enable the inline data feature: New created small(<~3.4k)
126 files can be written into inode block.
Chao Yud37a8682014-09-24 18:20:23 +0800127inline_dentry Enable the inline dir feature: data in new created
128 directory entries can be written into inode block. The
129 space of inode block which is used to store inline
130 dentries is limited to ~3.4k.
Jaegeuk Kim6b4afdd2014-04-02 15:34:36 +0900131flush_merge Merge concurrent cache_flush commands as much as possible
132 to eliminate redundant command issues. If the underlying
133 device handles the cache_flush command relatively slowly,
134 recommend to enable this option.
Jaegeuk Kim0f7b2ab2014-07-23 09:57:31 -0700135nobarrier This option can be used if underlying storage guarantees
136 its cached data should be written to the novolatile area.
137 If this option is set, no cache_flush commands are issued
138 but f2fs still guarantees the write ordering of all the
139 data writes.
Jaegeuk Kimd5053a342014-10-30 22:47:03 -0700140fastboot This option is used when a system wants to reduce mount
141 time as much as possible, even though normal performance
142 can be sacrificed.
Chao Yu89672152015-02-05 17:55:51 +0800143extent_cache Enable an extent cache based on rb-tree, it can cache
144 as many as extent which map between contiguous logical
145 address and physical address per inode, resulting in
146 increasing the cache hit ratio.
Wanpeng Li75342792015-03-24 10:20:27 +0800147noinline_data Disable the inline data feature, inline data feature is
148 enabled by default.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900149
150================================================================================
151DEBUGFS ENTRIES
152================================================================================
153
154/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
155f2fs. Each file shows the whole f2fs information.
156
157/sys/kernel/debug/f2fs/status includes:
158 - major file system information managed by f2fs currently
159 - average SIT information about whole segments
160 - current memory footprint consumed by f2fs.
161
162================================================================================
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900163SYSFS ENTRIES
164================================================================================
165
166Information about mounted f2f2 file systems can be found in
167/sys/fs/f2fs. Each mounted filesystem will have a directory in
168/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
169The files in each per-device directory are shown in table below.
170
171Files in /sys/fs/f2fs/<devname>
172(see also Documentation/ABI/testing/sysfs-fs-f2fs)
173..............................................................................
174 File Content
175
176 gc_max_sleep_time This tuning parameter controls the maximum sleep
177 time for the garbage collection thread. Time is
178 in milliseconds.
179
180 gc_min_sleep_time This tuning parameter controls the minimum sleep
181 time for the garbage collection thread. Time is
182 in milliseconds.
183
184 gc_no_gc_sleep_time This tuning parameter controls the default sleep
185 time for the garbage collection thread. Time is
186 in milliseconds.
187
Namjae Jeond2dc0952013-08-04 23:10:15 +0900188 gc_idle This parameter controls the selection of victim
189 policy for garbage collection. Setting gc_idle = 0
190 (default) will disable this option. Setting
191 gc_idle = 1 will select the Cost Benefit approach
192 & setting gc_idle = 2 will select the greedy aproach.
193
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900194 reclaim_segments This parameter controls the number of prefree
195 segments to be reclaimed. If the number of prefree
Jaegeuk Kim58c41032014-03-19 14:17:21 +0900196 segments is larger than the number of segments
197 in the proportion to the percentage over total
198 volume size, f2fs tries to conduct checkpoint to
199 reclaim the prefree segments to free segments.
200 By default, 5% over total # of segments.
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900201
Jaegeuk Kimba0697e2013-12-19 17:44:41 +0900202 max_small_discards This parameter controls the number of discard
203 commands that consist small blocks less than 2MB.
204 The candidates to be discarded are cached until
205 checkpoint is triggered, and issued during the
206 checkpoint. By default, it is disabled with 0.
207
Jaegeuk Kimbba681c2015-01-26 17:41:23 -0800208 trim_sections This parameter controls the number of sections
209 to be trimmed out in batch mode when FITRIM
210 conducts. 32 sections is set by default.
211
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900212 ipu_policy This parameter controls the policy of in-place
213 updates in f2fs. There are five policies:
Jaegeuk Kim9b5f1362014-09-16 18:30:54 -0700214 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
215 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
216 0x10: F2FS_IPU_FSYNC.
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900217
218 min_ipu_util This parameter controls the threshold to trigger
219 in-place-updates. The number indicates percentage
220 of the filesystem utilization, and used by
221 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
222
Jaegeuk Kimc1ce1b02014-09-10 16:53:02 -0700223 min_fsync_blocks This parameter controls the threshold to trigger
224 in-place-updates when F2FS_IPU_FSYNC mode is set.
225 The number indicates the number of dirty pages
226 when fsync needs to flush on its call path. If
227 the number is less than this value, it triggers
228 in-place-updates.
229
Jaegeuk Kim3bac3802014-01-09 21:00:06 +0900230 max_victim_search This parameter controls the number of trials to
231 find a victim segment when conducting SSR and
232 cleaning operations. The default value is 4096
233 which covers 8GB block address range.
234
Jaegeuk Kimab9fa662014-02-27 20:09:05 +0900235 dir_level This parameter controls the directory level to
236 support large directory. If a directory has a
237 number of files, it can reduce the file lookup
238 latency by increasing this dir_level value.
239 Otherwise, it needs to decrease this value to
240 reduce the space overhead. The default value is 0.
241
Jaegeuk Kimcdfc41c2014-03-19 13:31:37 +0900242 ram_thresh This parameter controls the memory footprint used
243 by free nids and cached nat entries. By default,
244 10 is set, which indicates 10 MB / 1 GB RAM.
245
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900246================================================================================
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900247USAGE
248================================================================================
249
2501. Download userland tools and compile them.
251
2522. Skip, if f2fs was compiled statically inside kernel.
253 Otherwise, insert the f2fs.ko module.
254 # insmod f2fs.ko
255
2563. Create a directory trying to mount
257 # mkdir /mnt/f2fs
258
2594. Format the block device, and then mount as f2fs
260 # mkfs.f2fs -l label /dev/block_device
261 # mount -t f2fs /dev/block_device /mnt/f2fs
262
Changman Leed51a7fb2013-07-04 17:12:47 +0900263mkfs.f2fs
264---------
265The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
266which builds a basic on-disk layout.
267
268The options consist of:
Changman Lee1571f842013-04-03 15:26:49 +0900269-l [label] : Give a volume label, up to 512 unicode name.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900270-a [0 or 1] : Split start location of each area for heap-based allocation.
271 1 is set by default, which performs this.
272-o [int] : Set overprovision ratio in percent over volume size.
273 5 is set by default.
274-s [int] : Set the number of segments per section.
275 1 is set by default.
276-z [int] : Set the number of sections per zone.
277 1 is set by default.
278-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
Changman Lee1571f842013-04-03 15:26:49 +0900279-t [0 or 1] : Disable discard command or not.
280 1 is set by default, which conducts discard.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900281
Changman Leed51a7fb2013-07-04 17:12:47 +0900282fsck.f2fs
283---------
284The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
285partition, which examines whether the filesystem metadata and user-made data
286are cross-referenced correctly or not.
287Note that, initial version of the tool does not fix any inconsistency.
288
289The options consist of:
290 -d debug level [default:0]
291
292dump.f2fs
293---------
294The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
295file. Each file is dump_ssa and dump_sit.
296
297The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
298It shows on-disk inode information reconized by a given inode number, and is
299able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
300./dump_sit respectively.
301
302The options consist of:
303 -d debug level [default:0]
304 -i inode no (hex)
305 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
306 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
307
308Examples:
309# dump.f2fs -i [ino] /dev/sdx
310# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
311# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
312
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900313================================================================================
314DESIGN
315================================================================================
316
317On-disk Layout
318--------------
319
320F2FS divides the whole volume into a number of segments, each of which is fixed
321to 2MB in size. A section is composed of consecutive segments, and a zone
322consists of a set of sections. By default, section and zone sizes are set to one
323segment size identically, but users can easily modify the sizes by mkfs.
324
325F2FS splits the entire volume into six areas, and all the areas except superblock
326consists of multiple segments as described below.
327
328 align with the zone size <-|
329 |-> align with the segment size
330 _________________________________________________________________________
Huajun Li9268cc32012-12-31 13:59:04 +0800331 | | | Segment | Node | Segment | |
332 | Superblock | Checkpoint | Info. | Address | Summary | Main |
333 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900334 |____________|_____2______|______N______|______N______|______N_____|__N___|
335 . .
336 . .
337 . .
338 ._________________________________________.
339 |_Segment_|_..._|_Segment_|_..._|_Segment_|
340 . .
341 ._________._________
342 |_section_|__...__|_
343 . .
344 .________.
345 |__zone__|
346
347- Superblock (SB)
348 : It is located at the beginning of the partition, and there exist two copies
349 to avoid file system crash. It contains basic partition information and some
350 default parameters of f2fs.
351
352- Checkpoint (CP)
353 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
354 inode lists, and summary entries of current active segments.
355
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900356- Segment Information Table (SIT)
357 : It contains segment information such as valid block count and bitmap for the
358 validity of all the blocks.
359
Huajun Li9268cc32012-12-31 13:59:04 +0800360- Node Address Table (NAT)
361 : It is composed of a block address table for all the node blocks stored in
362 Main area.
363
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900364- Segment Summary Area (SSA)
365 : It contains summary entries which contains the owner information of all the
366 data and node blocks stored in Main area.
367
368- Main Area
369 : It contains file and directory data including their indices.
370
371In order to avoid misalignment between file system and flash-based storage, F2FS
372aligns the start block address of CP with the segment size. Also, it aligns the
373start block address of Main area with the zone size by reserving some segments
374in SSA area.
375
376Reference the following survey for additional technical details.
377https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
378
379File System Metadata Structure
380------------------------------
381
382F2FS adopts the checkpointing scheme to maintain file system consistency. At
383mount time, F2FS first tries to find the last valid checkpoint data by scanning
384CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
385One of them always indicates the last valid data, which is called as shadow copy
386mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
387
388For file system consistency, each CP points to which NAT and SIT copies are
389valid, as shown as below.
390
391 +--------+----------+---------+
Huajun Li9268cc32012-12-31 13:59:04 +0800392 | CP | SIT | NAT |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900393 +--------+----------+---------+
394 . . . .
395 . . . .
396 . . . .
397 +-------+-------+--------+--------+--------+--------+
Huajun Li9268cc32012-12-31 13:59:04 +0800398 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900399 +-------+-------+--------+--------+--------+--------+
400 | ^ ^
401 | | |
402 `----------------------------------------'
403
404Index Structure
405---------------
406
407The key data structure to manage the data locations is a "node". Similar to
408traditional file structures, F2FS has three types of node: inode, direct node,
Huajun Lid08ab082012-12-05 16:45:32 +0800409indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900410indices, two direct node pointers, two indirect node pointers, and one double
411indirect node pointer as described below. One direct node block contains 1018
412data blocks, and one indirect node block contains also 1018 node blocks. Thus,
413one inode block (i.e., a file) covers:
414
415 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
416
417 Inode block (4KB)
418 |- data (923)
419 |- direct node (2)
420 | `- data (1018)
421 |- indirect node (2)
422 | `- direct node (1018)
423 | `- data (1018)
424 `- double indirect node (1)
425 `- indirect node (1018)
426 `- direct node (1018)
427 `- data (1018)
428
429Note that, all the node blocks are mapped by NAT which means the location of
430each node is translated by the NAT table. In the consideration of the wandering
431tree problem, F2FS is able to cut off the propagation of node updates caused by
432leaf data writes.
433
434Directory Structure
435-------------------
436
437A directory entry occupies 11 bytes, which consists of the following attributes.
438
439- hash hash value of the file name
440- ino inode number
441- len the length of file name
442- type file type such as directory, symlink, etc
443
444A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
445used to represent whether each dentry is valid or not. A dentry block occupies
4464KB with the following composition.
447
448 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
449 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
450
451 [Bucket]
452 +--------------------------------+
453 |dentry block 1 | dentry block 2 |
454 +--------------------------------+
455 . .
456 . .
457 . [Dentry Block Structure: 4KB] .
458 +--------+----------+----------+------------+
459 | bitmap | reserved | dentries | file names |
460 +--------+----------+----------+------------+
461 [Dentry Block: 4KB] . .
462 . .
463 . .
464 +------+------+-----+------+
465 | hash | ino | len | type |
466 +------+------+-----+------+
467 [Dentry Structure: 11 bytes]
468
469F2FS implements multi-level hash tables for directory structure. Each level has
470a hash table with dedicated number of hash buckets as shown below. Note that
471"A(2B)" means a bucket includes 2 data blocks.
472
473----------------------
474A : bucket
475B : block
476N : MAX_DIR_HASH_DEPTH
477----------------------
478
479level #0 | A(2B)
480 |
481level #1 | A(2B) - A(2B)
482 |
483level #2 | A(2B) - A(2B) - A(2B) - A(2B)
484 . | . . . .
485level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
486 . | . . . .
487level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
488
489The number of blocks and buckets are determined by,
490
491 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
492 # of blocks in level #n = |
493 `- 4, Otherwise
494
Chao Yubfec07d2014-05-28 08:56:09 +0800495 ,- 2^(n + dir_level),
496 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900497 # of buckets in level #n = |
Chao Yubfec07d2014-05-28 08:56:09 +0800498 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
499 Otherwise
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900500
501When F2FS finds a file name in a directory, at first a hash value of the file
502name is calculated. Then, F2FS scans the hash table in level #0 to find the
503dentry consisting of the file name and its inode number. If not found, F2FS
504scans the next hash table in level #1. In this way, F2FS scans hash tables in
505each levels incrementally from 1 to N. In each levels F2FS needs to scan only
506one bucket determined by the following equation, which shows O(log(# of files))
507complexity.
508
509 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
510
511In the case of file creation, F2FS finds empty consecutive slots that cover the
512file name. F2FS searches the empty slots in the hash tables of whole levels from
5131 to N in the same way as the lookup operation.
514
515The following figure shows an example of two cases holding children.
516 --------------> Dir <--------------
517 | |
518 child child
519
520 child - child [hole] - child
521
522 child - child - child [hole] - [hole] - child
523
524 Case 1: Case 2:
525 Number of children = 6, Number of children = 3,
526 File size = 7 File size = 7
527
528Default Block Allocation
529------------------------
530
531At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
532and Hot/Warm/Cold data.
533
534- Hot node contains direct node blocks of directories.
535- Warm node contains direct node blocks except hot node blocks.
536- Cold node contains indirect node blocks
537- Hot data contains dentry blocks
538- Warm data contains data blocks except hot and cold data blocks
539- Cold data contains multimedia data or migrated data blocks
540
541LFS has two schemes for free space management: threaded log and copy-and-compac-
542tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
543for devices showing very good sequential write performance, since free segments
544are served all the time for writing new data. However, it suffers from cleaning
545overhead under high utilization. Contrarily, the threaded log scheme suffers
546from random writes, but no cleaning process is needed. F2FS adopts a hybrid
547scheme where the copy-and-compaction scheme is adopted by default, but the
548policy is dynamically changed to the threaded log scheme according to the file
549system status.
550
551In order to align F2FS with underlying flash-based storage, F2FS allocates a
552segment in a unit of section. F2FS expects that the section size would be the
553same as the unit size of garbage collection in FTL. Furthermore, with respect
554to the mapping granularity in FTL, F2FS allocates each section of the active
555logs from different zones as much as possible, since FTL can write the data in
556the active logs into one allocation unit according to its mapping granularity.
557
558Cleaning process
559----------------
560
561F2FS does cleaning both on demand and in the background. On-demand cleaning is
562triggered when there are not enough free segments to serve VFS calls. Background
563cleaner is operated by a kernel thread, and triggers the cleaning job when the
564system is idle.
565
566F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
567In the greedy algorithm, F2FS selects a victim segment having the smallest number
568of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
569according to the segment age and the number of valid blocks in order to address
570log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
571algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
572algorithm.
573
574In order to identify whether the data in the victim segment are valid or not,
575F2FS manages a bitmap. Each bit represents the validity of a block, and the
576bitmap is composed of a bit stream covering whole blocks in main area.