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Based on kernel version 4.13.3. Page generated on 2017-09-23 13:55 EST.

1	================================================================================
2	WHAT IS Flash-Friendly File System (F2FS)?
3	================================================================================
4	
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.
10	
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.
15	
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.
20	
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
24	
25	For reporting bugs and sending patches, please use the following mailing list:
26	>> linux-f2fs-devel@lists.sourceforge.net
27	
28	================================================================================
29	BACKGROUND AND DESIGN ISSUES
30	================================================================================
31	
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
42	10, 1, 26–52.
43	
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.
53	
54	[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
55	
56	Cleaning Overhead
57	-----------------
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.
62	
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.
69	
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.
73	
74	================================================================================
75	KEY FEATURES
76	================================================================================
77	
78	Flash 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	
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.
89	
90	Cleaning 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	================================================================================
98	MOUNT OPTIONS
99	================================================================================
100	
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
114			       segment is cleaned.
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.
124	                       Default number is 6.
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_data            Enable the inline data feature: New created small(<~3.4k)
130	                       files can be written into inode block.
131	inline_dentry          Enable the inline dir feature: data in new created
132	                       directory entries can be written into inode block. The
133	                       space of inode block which is used to store inline
134	                       dentries is limited to ~3.4k.
135	noinline_dentry        Disable the inline dentry feature.
136	flush_merge	       Merge concurrent cache_flush commands as much as possible
137	                       to eliminate redundant command issues. If the underlying
138			       device handles the cache_flush command relatively slowly,
139			       recommend to enable this option.
140	nobarrier              This option can be used if underlying storage guarantees
141	                       its cached data should be written to the novolatile area.
142			       If this option is set, no cache_flush commands are issued
143			       but f2fs still guarantees the write ordering of all the
144			       data writes.
145	fastboot               This option is used when a system wants to reduce mount
146	                       time as much as possible, even though normal performance
147			       can be sacrificed.
148	extent_cache           Enable an extent cache based on rb-tree, it can cache
149	                       as many as extent which map between contiguous logical
150	                       address and physical address per inode, resulting in
151	                       increasing the cache hit ratio. Set by default.
152	noextent_cache         Disable an extent cache based on rb-tree explicitly, see
153	                       the above extent_cache mount option.
154	noinline_data          Disable the inline data feature, inline data feature is
155	                       enabled by default.
156	data_flush             Enable data flushing before checkpoint in order to
157	                       persist data of regular and symlink.
158	fault_injection=%d     Enable fault injection in all supported types with
159	                       specified injection rate.
160	mode=%s                Control block allocation mode which supports "adaptive"
161	                       and "lfs". In "lfs" mode, there should be no random
162	                       writes towards main area.
163	io_bits=%u             Set the bit size of write IO requests. It should be set
164	                       with "mode=lfs".
165	usrquota               Enable plain user disk quota accounting.
166	grpquota               Enable plain group disk quota accounting.
167	
168	================================================================================
169	DEBUGFS ENTRIES
170	================================================================================
171	
172	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
173	f2fs. Each file shows the whole f2fs information.
174	
175	/sys/kernel/debug/f2fs/status includes:
176	 - major file system information managed by f2fs currently
177	 - average SIT information about whole segments
178	 - current memory footprint consumed by f2fs.
179	
180	================================================================================
181	SYSFS ENTRIES
182	================================================================================
183	
184	Information about mounted f2fs file systems can be found in
185	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
186	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
187	The files in each per-device directory are shown in table below.
188	
189	Files in /sys/fs/f2fs/<devname>
190	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
191	..............................................................................
192	 File                         Content
193	
194	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
195	                              time for the garbage collection thread. Time is
196	                              in milliseconds.
197	
198	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
199	                              time for the garbage collection thread. Time is
200	                              in milliseconds.
201	
202	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
203	                              time for the garbage collection thread. Time is
204	                              in milliseconds.
205	
206	 gc_idle                      This parameter controls the selection of victim
207	                              policy for garbage collection. Setting gc_idle = 0
208	                              (default) will disable this option. Setting
209	                              gc_idle = 1 will select the Cost Benefit approach
210	                              & setting gc_idle = 2 will select the greedy approach.
211	
212	 reclaim_segments             This parameter controls the number of prefree
213	                              segments to be reclaimed. If the number of prefree
214				      segments is larger than the number of segments
215				      in the proportion to the percentage over total
216				      volume size, f2fs tries to conduct checkpoint to
217				      reclaim the prefree segments to free segments.
218				      By default, 5% over total # of segments.
219	
220	 max_small_discards	      This parameter controls the number of discard
221				      commands that consist small blocks less than 2MB.
222				      The candidates to be discarded are cached until
223				      checkpoint is triggered, and issued during the
224				      checkpoint. By default, it is disabled with 0.
225	
226	 trim_sections                This parameter controls the number of sections
227	                              to be trimmed out in batch mode when FITRIM
228	                              conducts. 32 sections is set by default.
229	
230	 ipu_policy                   This parameter controls the policy of in-place
231	                              updates in f2fs. There are five policies:
232	                               0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
233	                               0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
234	                               0x10: F2FS_IPU_FSYNC.
235	
236	 min_ipu_util                 This parameter controls the threshold to trigger
237	                              in-place-updates. The number indicates percentage
238	                              of the filesystem utilization, and used by
239	                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
240	
241	 min_fsync_blocks             This parameter controls the threshold to trigger
242	                              in-place-updates when F2FS_IPU_FSYNC mode is set.
243				      The number indicates the number of dirty pages
244				      when fsync needs to flush on its call path. If
245				      the number is less than this value, it triggers
246				      in-place-updates.
247	
248	 max_victim_search	      This parameter controls the number of trials to
249				      find a victim segment when conducting SSR and
250				      cleaning operations. The default value is 4096
251				      which covers 8GB block address range.
252	
253	 dir_level                    This parameter controls the directory level to
254				      support large directory. If a directory has a
255				      number of files, it can reduce the file lookup
256				      latency by increasing this dir_level value.
257				      Otherwise, it needs to decrease this value to
258				      reduce the space overhead. The default value is 0.
259	
260	 ram_thresh                   This parameter controls the memory footprint used
261				      by free nids and cached nat entries. By default,
262				      10 is set, which indicates 10 MB / 1 GB RAM.
263	
264	================================================================================
265	USAGE
266	================================================================================
267	
268	1. Download userland tools and compile them.
269	
270	2. Skip, if f2fs was compiled statically inside kernel.
271	   Otherwise, insert the f2fs.ko module.
272	 # insmod f2fs.ko
273	
274	3. Create a directory trying to mount
275	 # mkdir /mnt/f2fs
276	
277	4. Format the block device, and then mount as f2fs
278	 # mkfs.f2fs -l label /dev/block_device
279	 # mount -t f2fs /dev/block_device /mnt/f2fs
280	
281	mkfs.f2fs
282	---------
283	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
284	which builds a basic on-disk layout.
285	
286	The options consist of:
287	-l [label]   : Give a volume label, up to 512 unicode name.
288	-a [0 or 1]  : Split start location of each area for heap-based allocation.
289	               1 is set by default, which performs this.
290	-o [int]     : Set overprovision ratio in percent over volume size.
291	               5 is set by default.
292	-s [int]     : Set the number of segments per section.
293	               1 is set by default.
294	-z [int]     : Set the number of sections per zone.
295	               1 is set by default.
296	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
297	-t [0 or 1]  : Disable discard command or not.
298	               1 is set by default, which conducts discard.
299	
300	fsck.f2fs
301	---------
302	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
303	partition, which examines whether the filesystem metadata and user-made data
304	are cross-referenced correctly or not.
305	Note that, initial version of the tool does not fix any inconsistency.
306	
307	The options consist of:
308	  -d debug level [default:0]
309	
310	dump.f2fs
311	---------
312	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
313	file. Each file is dump_ssa and dump_sit.
314	
315	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
316	It shows on-disk inode information recognized by a given inode number, and is
317	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
318	./dump_sit respectively.
319	
320	The options consist of:
321	  -d debug level [default:0]
322	  -i inode no (hex)
323	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
324	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
325	
326	Examples:
327	# dump.f2fs -i [ino] /dev/sdx
328	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
329	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
330	
331	================================================================================
332	DESIGN
333	================================================================================
334	
335	On-disk Layout
336	--------------
337	
338	F2FS divides the whole volume into a number of segments, each of which is fixed
339	to 2MB in size. A section is composed of consecutive segments, and a zone
340	consists of a set of sections. By default, section and zone sizes are set to one
341	segment size identically, but users can easily modify the sizes by mkfs.
342	
343	F2FS splits the entire volume into six areas, and all the areas except superblock
344	consists of multiple segments as described below.
345	
346	                                            align with the zone size <-|
347	                 |-> align with the segment size
348	     _________________________________________________________________________
349	    |            |            |   Segment   |    Node     |   Segment  |      |
350	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
351	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
352	    |____________|_____2______|______N______|______N______|______N_____|__N___|
353	                                                                       .      .
354	                                                             .                .
355	                                                 .                            .
356	                                    ._________________________________________.
357	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
358	                                    .           .
359	                                    ._________._________
360	                                    |_section_|__...__|_
361	                                    .            .
362			                    .________.
363		                            |__zone__|
364	
365	- Superblock (SB)
366	 : It is located at the beginning of the partition, and there exist two copies
367	   to avoid file system crash. It contains basic partition information and some
368	   default parameters of f2fs.
369	
370	- Checkpoint (CP)
371	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
372	   inode lists, and summary entries of current active segments.
373	
374	- Segment Information Table (SIT)
375	 : It contains segment information such as valid block count and bitmap for the
376	   validity of all the blocks.
377	
378	- Node Address Table (NAT)
379	 : It is composed of a block address table for all the node blocks stored in
380	   Main area.
381	
382	- Segment Summary Area (SSA)
383	 : It contains summary entries which contains the owner information of all the
384	   data and node blocks stored in Main area.
385	
386	- Main Area
387	 : It contains file and directory data including their indices.
388	
389	In order to avoid misalignment between file system and flash-based storage, F2FS
390	aligns the start block address of CP with the segment size. Also, it aligns the
391	start block address of Main area with the zone size by reserving some segments
392	in SSA area.
393	
394	Reference the following survey for additional technical details.
395	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
396	
397	File System Metadata Structure
398	------------------------------
399	
400	F2FS adopts the checkpointing scheme to maintain file system consistency. At
401	mount time, F2FS first tries to find the last valid checkpoint data by scanning
402	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
403	One of them always indicates the last valid data, which is called as shadow copy
404	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
405	
406	For file system consistency, each CP points to which NAT and SIT copies are
407	valid, as shown as below.
408	
409	  +--------+----------+---------+
410	  |   CP   |    SIT   |   NAT   |
411	  +--------+----------+---------+
412	  .         .          .          .
413	  .            .              .              .
414	  .               .                 .                 .
415	  +-------+-------+--------+--------+--------+--------+
416	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
417	  +-------+-------+--------+--------+--------+--------+
418	     |             ^                          ^
419	     |             |                          |
420	     `----------------------------------------'
421	
422	Index Structure
423	---------------
424	
425	The key data structure to manage the data locations is a "node". Similar to
426	traditional file structures, F2FS has three types of node: inode, direct node,
427	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
428	indices, two direct node pointers, two indirect node pointers, and one double
429	indirect node pointer as described below. One direct node block contains 1018
430	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
431	one inode block (i.e., a file) covers:
432	
433	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
434	
435	   Inode block (4KB)
436	     |- data (923)
437	     |- direct node (2)
438	     |          `- data (1018)
439	     |- indirect node (2)
440	     |            `- direct node (1018)
441	     |                       `- data (1018)
442	     `- double indirect node (1)
443	                         `- indirect node (1018)
444				              `- direct node (1018)
445		                                         `- data (1018)
446	
447	Note that, all the node blocks are mapped by NAT which means the location of
448	each node is translated by the NAT table. In the consideration of the wandering
449	tree problem, F2FS is able to cut off the propagation of node updates caused by
450	leaf data writes.
451	
452	Directory Structure
453	-------------------
454	
455	A directory entry occupies 11 bytes, which consists of the following attributes.
456	
457	- hash		hash value of the file name
458	- ino		inode number
459	- len		the length of file name
460	- type		file type such as directory, symlink, etc
461	
462	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
463	used to represent whether each dentry is valid or not. A dentry block occupies
464	4KB with the following composition.
465	
466	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
467		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
468	
469	                         [Bucket]
470	             +--------------------------------+
471	             |dentry block 1 | dentry block 2 |
472	             +--------------------------------+
473	             .               .
474	       .                             .
475	  .       [Dentry Block Structure: 4KB]       .
476	  +--------+----------+----------+------------+
477	  | bitmap | reserved | dentries | file names |
478	  +--------+----------+----------+------------+
479	  [Dentry Block: 4KB] .   .
480			 .               .
481	            .                          .
482	            +------+------+-----+------+
483	            | hash | ino  | len | type |
484	            +------+------+-----+------+
485	            [Dentry Structure: 11 bytes]
486	
487	F2FS implements multi-level hash tables for directory structure. Each level has
488	a hash table with dedicated number of hash buckets as shown below. Note that
489	"A(2B)" means a bucket includes 2 data blocks.
490	
491	----------------------
492	A : bucket
493	B : block
494	N : MAX_DIR_HASH_DEPTH
495	----------------------
496	
497	level #0   | A(2B)
498	           |
499	level #1   | A(2B) - A(2B)
500	           |
501	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
502	     .     |   .       .       .       .
503	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
504	     .     |   .       .       .       .
505	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
506	
507	The number of blocks and buckets are determined by,
508	
509	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
510	  # of blocks in level #n = |
511	                            `- 4, Otherwise
512	
513	                             ,- 2^(n + dir_level),
514				     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
515	  # of buckets in level #n = |
516	                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
517				              Otherwise
518	
519	When F2FS finds a file name in a directory, at first a hash value of the file
520	name is calculated. Then, F2FS scans the hash table in level #0 to find the
521	dentry consisting of the file name and its inode number. If not found, F2FS
522	scans the next hash table in level #1. In this way, F2FS scans hash tables in
523	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
524	one bucket determined by the following equation, which shows O(log(# of files))
525	complexity.
526	
527	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
528	
529	In the case of file creation, F2FS finds empty consecutive slots that cover the
530	file name. F2FS searches the empty slots in the hash tables of whole levels from
531	1 to N in the same way as the lookup operation.
532	
533	The following figure shows an example of two cases holding children.
534	       --------------> Dir <--------------
535	       |                                 |
536	    child                             child
537	
538	    child - child                     [hole] - child
539	
540	    child - child - child             [hole] - [hole] - child
541	
542	   Case 1:                           Case 2:
543	   Number of children = 6,           Number of children = 3,
544	   File size = 7                     File size = 7
545	
546	Default Block Allocation
547	------------------------
548	
549	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
550	and Hot/Warm/Cold data.
551	
552	- Hot node	contains direct node blocks of directories.
553	- Warm node	contains direct node blocks except hot node blocks.
554	- Cold node	contains indirect node blocks
555	- Hot data	contains dentry blocks
556	- Warm data	contains data blocks except hot and cold data blocks
557	- Cold data	contains multimedia data or migrated data blocks
558	
559	LFS has two schemes for free space management: threaded log and copy-and-compac-
560	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
561	for devices showing very good sequential write performance, since free segments
562	are served all the time for writing new data. However, it suffers from cleaning
563	overhead under high utilization. Contrarily, the threaded log scheme suffers
564	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
565	scheme where the copy-and-compaction scheme is adopted by default, but the
566	policy is dynamically changed to the threaded log scheme according to the file
567	system status.
568	
569	In order to align F2FS with underlying flash-based storage, F2FS allocates a
570	segment in a unit of section. F2FS expects that the section size would be the
571	same as the unit size of garbage collection in FTL. Furthermore, with respect
572	to the mapping granularity in FTL, F2FS allocates each section of the active
573	logs from different zones as much as possible, since FTL can write the data in
574	the active logs into one allocation unit according to its mapping granularity.
575	
576	Cleaning process
577	----------------
578	
579	F2FS does cleaning both on demand and in the background. On-demand cleaning is
580	triggered when there are not enough free segments to serve VFS calls. Background
581	cleaner is operated by a kernel thread, and triggers the cleaning job when the
582	system is idle.
583	
584	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
585	In the greedy algorithm, F2FS selects a victim segment having the smallest number
586	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
587	according to the segment age and the number of valid blocks in order to address
588	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
589	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
590	algorithm.
591	
592	In order to identify whether the data in the victim segment are valid or not,
593	F2FS manages a bitmap. Each bit represents the validity of a block, and the
594	bitmap is composed of a bit stream covering whole blocks in main area.
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