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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
109discard Issue discard/TRIM commands when a segment is cleaned.
110no_heap Disable heap-style segment allocation which finds free
111 segments for data from the beginning of main area, while
112 for node from the end of main area.
113nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
114 by default if CONFIG_F2FS_FS_XATTR is selected.
115noacl Disable POSIX Access Control List. Note: acl is enabled
116 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
117active_logs=%u Support configuring the number of active logs. In the
118 current design, f2fs supports only 2, 4, and 6 logs.
119 Default number is 6.
120disable_ext_identify Disable the extension list configured by mkfs, so f2fs
121 does not aware of cold files such as media files.
Jaegeuk Kim66e960c2013-11-01 11:20:05 +0900122inline_xattr Enable the inline xattrs feature.
Huajun Lie4024e82013-11-10 23:13:21 +0800123inline_data Enable the inline data feature: New created small(<~3.4k)
124 files can be written into inode block.
Chao Yud37a8682014-09-24 18:20:23 +0800125inline_dentry Enable the inline dir feature: data in new created
126 directory entries can be written into inode block. The
127 space of inode block which is used to store inline
128 dentries is limited to ~3.4k.
Jaegeuk Kim6b4afdd2014-04-02 15:34:36 +0900129flush_merge Merge concurrent cache_flush commands as much as possible
130 to eliminate redundant command issues. If the underlying
131 device handles the cache_flush command relatively slowly,
132 recommend to enable this option.
Jaegeuk Kim0f7b2ab2014-07-23 09:57:31 -0700133nobarrier This option can be used if underlying storage guarantees
134 its cached data should be written to the novolatile area.
135 If this option is set, no cache_flush commands are issued
136 but f2fs still guarantees the write ordering of all the
137 data writes.
Jaegeuk Kimd5053a342014-10-30 22:47:03 -0700138fastboot This option is used when a system wants to reduce mount
139 time as much as possible, even though normal performance
140 can be sacrificed.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900141
142================================================================================
143DEBUGFS ENTRIES
144================================================================================
145
146/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
147f2fs. Each file shows the whole f2fs information.
148
149/sys/kernel/debug/f2fs/status includes:
150 - major file system information managed by f2fs currently
151 - average SIT information about whole segments
152 - current memory footprint consumed by f2fs.
153
154================================================================================
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900155SYSFS ENTRIES
156================================================================================
157
158Information about mounted f2f2 file systems can be found in
159/sys/fs/f2fs. Each mounted filesystem will have a directory in
160/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
161The files in each per-device directory are shown in table below.
162
163Files in /sys/fs/f2fs/<devname>
164(see also Documentation/ABI/testing/sysfs-fs-f2fs)
165..............................................................................
166 File Content
167
168 gc_max_sleep_time This tuning parameter controls the maximum sleep
169 time for the garbage collection thread. Time is
170 in milliseconds.
171
172 gc_min_sleep_time This tuning parameter controls the minimum sleep
173 time for the garbage collection thread. Time is
174 in milliseconds.
175
176 gc_no_gc_sleep_time This tuning parameter controls the default sleep
177 time for the garbage collection thread. Time is
178 in milliseconds.
179
Namjae Jeond2dc0952013-08-04 23:10:15 +0900180 gc_idle This parameter controls the selection of victim
181 policy for garbage collection. Setting gc_idle = 0
182 (default) will disable this option. Setting
183 gc_idle = 1 will select the Cost Benefit approach
184 & setting gc_idle = 2 will select the greedy aproach.
185
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900186 reclaim_segments This parameter controls the number of prefree
187 segments to be reclaimed. If the number of prefree
Jaegeuk Kim58c41032014-03-19 14:17:21 +0900188 segments is larger than the number of segments
189 in the proportion to the percentage over total
190 volume size, f2fs tries to conduct checkpoint to
191 reclaim the prefree segments to free segments.
192 By default, 5% over total # of segments.
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900193
Jaegeuk Kimba0697e2013-12-19 17:44:41 +0900194 max_small_discards This parameter controls the number of discard
195 commands that consist small blocks less than 2MB.
196 The candidates to be discarded are cached until
197 checkpoint is triggered, and issued during the
198 checkpoint. By default, it is disabled with 0.
199
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900200 ipu_policy This parameter controls the policy of in-place
201 updates in f2fs. There are five policies:
Jaegeuk Kim9b5f1362014-09-16 18:30:54 -0700202 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
203 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
204 0x10: F2FS_IPU_FSYNC.
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900205
206 min_ipu_util This parameter controls the threshold to trigger
207 in-place-updates. The number indicates percentage
208 of the filesystem utilization, and used by
209 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
210
Jaegeuk Kimc1ce1b02014-09-10 16:53:02 -0700211 min_fsync_blocks This parameter controls the threshold to trigger
212 in-place-updates when F2FS_IPU_FSYNC mode is set.
213 The number indicates the number of dirty pages
214 when fsync needs to flush on its call path. If
215 the number is less than this value, it triggers
216 in-place-updates.
217
Jaegeuk Kim3bac3802014-01-09 21:00:06 +0900218 max_victim_search This parameter controls the number of trials to
219 find a victim segment when conducting SSR and
220 cleaning operations. The default value is 4096
221 which covers 8GB block address range.
222
Jaegeuk Kimab9fa662014-02-27 20:09:05 +0900223 dir_level This parameter controls the directory level to
224 support large directory. If a directory has a
225 number of files, it can reduce the file lookup
226 latency by increasing this dir_level value.
227 Otherwise, it needs to decrease this value to
228 reduce the space overhead. The default value is 0.
229
Jaegeuk Kimcdfc41c2014-03-19 13:31:37 +0900230 ram_thresh This parameter controls the memory footprint used
231 by free nids and cached nat entries. By default,
232 10 is set, which indicates 10 MB / 1 GB RAM.
233
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900234================================================================================
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900235USAGE
236================================================================================
237
2381. Download userland tools and compile them.
239
2402. Skip, if f2fs was compiled statically inside kernel.
241 Otherwise, insert the f2fs.ko module.
242 # insmod f2fs.ko
243
2443. Create a directory trying to mount
245 # mkdir /mnt/f2fs
246
2474. Format the block device, and then mount as f2fs
248 # mkfs.f2fs -l label /dev/block_device
249 # mount -t f2fs /dev/block_device /mnt/f2fs
250
Changman Leed51a7fb2013-07-04 17:12:47 +0900251mkfs.f2fs
252---------
253The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
254which builds a basic on-disk layout.
255
256The options consist of:
Changman Lee1571f842013-04-03 15:26:49 +0900257-l [label] : Give a volume label, up to 512 unicode name.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900258-a [0 or 1] : Split start location of each area for heap-based allocation.
259 1 is set by default, which performs this.
260-o [int] : Set overprovision ratio in percent over volume size.
261 5 is set by default.
262-s [int] : Set the number of segments per section.
263 1 is set by default.
264-z [int] : Set the number of sections per zone.
265 1 is set by default.
266-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
Changman Lee1571f842013-04-03 15:26:49 +0900267-t [0 or 1] : Disable discard command or not.
268 1 is set by default, which conducts discard.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900269
Changman Leed51a7fb2013-07-04 17:12:47 +0900270fsck.f2fs
271---------
272The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
273partition, which examines whether the filesystem metadata and user-made data
274are cross-referenced correctly or not.
275Note that, initial version of the tool does not fix any inconsistency.
276
277The options consist of:
278 -d debug level [default:0]
279
280dump.f2fs
281---------
282The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
283file. Each file is dump_ssa and dump_sit.
284
285The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
286It shows on-disk inode information reconized by a given inode number, and is
287able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
288./dump_sit respectively.
289
290The options consist of:
291 -d debug level [default:0]
292 -i inode no (hex)
293 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
294 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
295
296Examples:
297# dump.f2fs -i [ino] /dev/sdx
298# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
299# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
300
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900301================================================================================
302DESIGN
303================================================================================
304
305On-disk Layout
306--------------
307
308F2FS divides the whole volume into a number of segments, each of which is fixed
309to 2MB in size. A section is composed of consecutive segments, and a zone
310consists of a set of sections. By default, section and zone sizes are set to one
311segment size identically, but users can easily modify the sizes by mkfs.
312
313F2FS splits the entire volume into six areas, and all the areas except superblock
314consists of multiple segments as described below.
315
316 align with the zone size <-|
317 |-> align with the segment size
318 _________________________________________________________________________
Huajun Li9268cc32012-12-31 13:59:04 +0800319 | | | Segment | Node | Segment | |
320 | Superblock | Checkpoint | Info. | Address | Summary | Main |
321 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900322 |____________|_____2______|______N______|______N______|______N_____|__N___|
323 . .
324 . .
325 . .
326 ._________________________________________.
327 |_Segment_|_..._|_Segment_|_..._|_Segment_|
328 . .
329 ._________._________
330 |_section_|__...__|_
331 . .
332 .________.
333 |__zone__|
334
335- Superblock (SB)
336 : It is located at the beginning of the partition, and there exist two copies
337 to avoid file system crash. It contains basic partition information and some
338 default parameters of f2fs.
339
340- Checkpoint (CP)
341 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
342 inode lists, and summary entries of current active segments.
343
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900344- Segment Information Table (SIT)
345 : It contains segment information such as valid block count and bitmap for the
346 validity of all the blocks.
347
Huajun Li9268cc32012-12-31 13:59:04 +0800348- Node Address Table (NAT)
349 : It is composed of a block address table for all the node blocks stored in
350 Main area.
351
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900352- Segment Summary Area (SSA)
353 : It contains summary entries which contains the owner information of all the
354 data and node blocks stored in Main area.
355
356- Main Area
357 : It contains file and directory data including their indices.
358
359In order to avoid misalignment between file system and flash-based storage, F2FS
360aligns the start block address of CP with the segment size. Also, it aligns the
361start block address of Main area with the zone size by reserving some segments
362in SSA area.
363
364Reference the following survey for additional technical details.
365https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
366
367File System Metadata Structure
368------------------------------
369
370F2FS adopts the checkpointing scheme to maintain file system consistency. At
371mount time, F2FS first tries to find the last valid checkpoint data by scanning
372CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
373One of them always indicates the last valid data, which is called as shadow copy
374mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
375
376For file system consistency, each CP points to which NAT and SIT copies are
377valid, as shown as below.
378
379 +--------+----------+---------+
Huajun Li9268cc32012-12-31 13:59:04 +0800380 | CP | SIT | NAT |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900381 +--------+----------+---------+
382 . . . .
383 . . . .
384 . . . .
385 +-------+-------+--------+--------+--------+--------+
Huajun Li9268cc32012-12-31 13:59:04 +0800386 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900387 +-------+-------+--------+--------+--------+--------+
388 | ^ ^
389 | | |
390 `----------------------------------------'
391
392Index Structure
393---------------
394
395The key data structure to manage the data locations is a "node". Similar to
396traditional file structures, F2FS has three types of node: inode, direct node,
Huajun Lid08ab082012-12-05 16:45:32 +0800397indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900398indices, two direct node pointers, two indirect node pointers, and one double
399indirect node pointer as described below. One direct node block contains 1018
400data blocks, and one indirect node block contains also 1018 node blocks. Thus,
401one inode block (i.e., a file) covers:
402
403 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
404
405 Inode block (4KB)
406 |- data (923)
407 |- direct node (2)
408 | `- data (1018)
409 |- indirect node (2)
410 | `- direct node (1018)
411 | `- data (1018)
412 `- double indirect node (1)
413 `- indirect node (1018)
414 `- direct node (1018)
415 `- data (1018)
416
417Note that, all the node blocks are mapped by NAT which means the location of
418each node is translated by the NAT table. In the consideration of the wandering
419tree problem, F2FS is able to cut off the propagation of node updates caused by
420leaf data writes.
421
422Directory Structure
423-------------------
424
425A directory entry occupies 11 bytes, which consists of the following attributes.
426
427- hash hash value of the file name
428- ino inode number
429- len the length of file name
430- type file type such as directory, symlink, etc
431
432A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
433used to represent whether each dentry is valid or not. A dentry block occupies
4344KB with the following composition.
435
436 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
437 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
438
439 [Bucket]
440 +--------------------------------+
441 |dentry block 1 | dentry block 2 |
442 +--------------------------------+
443 . .
444 . .
445 . [Dentry Block Structure: 4KB] .
446 +--------+----------+----------+------------+
447 | bitmap | reserved | dentries | file names |
448 +--------+----------+----------+------------+
449 [Dentry Block: 4KB] . .
450 . .
451 . .
452 +------+------+-----+------+
453 | hash | ino | len | type |
454 +------+------+-----+------+
455 [Dentry Structure: 11 bytes]
456
457F2FS implements multi-level hash tables for directory structure. Each level has
458a hash table with dedicated number of hash buckets as shown below. Note that
459"A(2B)" means a bucket includes 2 data blocks.
460
461----------------------
462A : bucket
463B : block
464N : MAX_DIR_HASH_DEPTH
465----------------------
466
467level #0 | A(2B)
468 |
469level #1 | A(2B) - A(2B)
470 |
471level #2 | A(2B) - A(2B) - A(2B) - A(2B)
472 . | . . . .
473level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
474 . | . . . .
475level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
476
477The number of blocks and buckets are determined by,
478
479 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
480 # of blocks in level #n = |
481 `- 4, Otherwise
482
Chao Yubfec07d2014-05-28 08:56:09 +0800483 ,- 2^(n + dir_level),
484 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900485 # of buckets in level #n = |
Chao Yubfec07d2014-05-28 08:56:09 +0800486 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
487 Otherwise
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900488
489When F2FS finds a file name in a directory, at first a hash value of the file
490name is calculated. Then, F2FS scans the hash table in level #0 to find the
491dentry consisting of the file name and its inode number. If not found, F2FS
492scans the next hash table in level #1. In this way, F2FS scans hash tables in
493each levels incrementally from 1 to N. In each levels F2FS needs to scan only
494one bucket determined by the following equation, which shows O(log(# of files))
495complexity.
496
497 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
498
499In the case of file creation, F2FS finds empty consecutive slots that cover the
500file name. F2FS searches the empty slots in the hash tables of whole levels from
5011 to N in the same way as the lookup operation.
502
503The following figure shows an example of two cases holding children.
504 --------------> Dir <--------------
505 | |
506 child child
507
508 child - child [hole] - child
509
510 child - child - child [hole] - [hole] - child
511
512 Case 1: Case 2:
513 Number of children = 6, Number of children = 3,
514 File size = 7 File size = 7
515
516Default Block Allocation
517------------------------
518
519At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
520and Hot/Warm/Cold data.
521
522- Hot node contains direct node blocks of directories.
523- Warm node contains direct node blocks except hot node blocks.
524- Cold node contains indirect node blocks
525- Hot data contains dentry blocks
526- Warm data contains data blocks except hot and cold data blocks
527- Cold data contains multimedia data or migrated data blocks
528
529LFS has two schemes for free space management: threaded log and copy-and-compac-
530tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
531for devices showing very good sequential write performance, since free segments
532are served all the time for writing new data. However, it suffers from cleaning
533overhead under high utilization. Contrarily, the threaded log scheme suffers
534from random writes, but no cleaning process is needed. F2FS adopts a hybrid
535scheme where the copy-and-compaction scheme is adopted by default, but the
536policy is dynamically changed to the threaded log scheme according to the file
537system status.
538
539In order to align F2FS with underlying flash-based storage, F2FS allocates a
540segment in a unit of section. F2FS expects that the section size would be the
541same as the unit size of garbage collection in FTL. Furthermore, with respect
542to the mapping granularity in FTL, F2FS allocates each section of the active
543logs from different zones as much as possible, since FTL can write the data in
544the active logs into one allocation unit according to its mapping granularity.
545
546Cleaning process
547----------------
548
549F2FS does cleaning both on demand and in the background. On-demand cleaning is
550triggered when there are not enough free segments to serve VFS calls. Background
551cleaner is operated by a kernel thread, and triggers the cleaning job when the
552system is idle.
553
554F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
555In the greedy algorithm, F2FS selects a victim segment having the smallest number
556of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
557according to the segment age and the number of valid blocks in order to address
558log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
559algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
560algorithm.
561
562In order to identify whether the data in the victim segment are valid or not,
563F2FS manages a bitmap. Each bit represents the validity of a block, and the
564bitmap is composed of a bit stream covering whole blocks in main area.