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Based on kernel version 4.9. Page generated on 2016-12-21 14:34 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	inline_data            Enable the inline data feature: New created small(<~3.4k)
129	                       files can be written into inode block.
130	inline_dentry          Enable the inline dir feature: data in new created
131	                       directory entries can be written into inode block. The
132	                       space of inode block which is used to store inline
133	                       dentries is limited to ~3.4k.
134	noinline_dentry        Diable the inline dentry feature.
135	flush_merge	       Merge concurrent cache_flush commands as much as possible
136	                       to eliminate redundant command issues. If the underlying
137			       device handles the cache_flush command relatively slowly,
138			       recommend to enable this option.
139	nobarrier              This option can be used if underlying storage guarantees
140	                       its cached data should be written to the novolatile area.
141			       If this option is set, no cache_flush commands are issued
142			       but f2fs still guarantees the write ordering of all the
143			       data writes.
144	fastboot               This option is used when a system wants to reduce mount
145	                       time as much as possible, even though normal performance
146			       can be sacrificed.
147	extent_cache           Enable an extent cache based on rb-tree, it can cache
148	                       as many as extent which map between contiguous logical
149	                       address and physical address per inode, resulting in
150	                       increasing the cache hit ratio. Set by default.
151	noextent_cache         Disable an extent cache based on rb-tree explicitly, see
152	                       the above extent_cache mount option.
153	noinline_data          Disable the inline data feature, inline data feature is
154	                       enabled by default.
155	data_flush             Enable data flushing before checkpoint in order to
156	                       persist data of regular and symlink.
157	mode=%s                Control block allocation mode which supports "adaptive"
158	                       and "lfs". In "lfs" mode, there should be no random
159	                       writes towards main area.
160	
161	================================================================================
162	DEBUGFS ENTRIES
163	================================================================================
164	
165	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
166	f2fs. Each file shows the whole f2fs information.
167	
168	/sys/kernel/debug/f2fs/status includes:
169	 - major file system information managed by f2fs currently
170	 - average SIT information about whole segments
171	 - current memory footprint consumed by f2fs.
172	
173	================================================================================
174	SYSFS ENTRIES
175	================================================================================
176	
177	Information about mounted f2f2 file systems can be found in
178	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
179	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
180	The files in each per-device directory are shown in table below.
181	
182	Files in /sys/fs/f2fs/<devname>
183	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
184	..............................................................................
185	 File                         Content
186	
187	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
188	                              time for the garbage collection thread. Time is
189	                              in milliseconds.
190	
191	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
192	                              time for the garbage collection thread. Time is
193	                              in milliseconds.
194	
195	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
196	                              time for the garbage collection thread. Time is
197	                              in milliseconds.
198	
199	 gc_idle                      This parameter controls the selection of victim
200	                              policy for garbage collection. Setting gc_idle = 0
201	                              (default) will disable this option. Setting
202	                              gc_idle = 1 will select the Cost Benefit approach
203	                              & setting gc_idle = 2 will select the greedy approach.
204	
205	 reclaim_segments             This parameter controls the number of prefree
206	                              segments to be reclaimed. If the number of prefree
207				      segments is larger than the number of segments
208				      in the proportion to the percentage over total
209				      volume size, f2fs tries to conduct checkpoint to
210				      reclaim the prefree segments to free segments.
211				      By default, 5% over total # of segments.
212	
213	 max_small_discards	      This parameter controls the number of discard
214				      commands that consist small blocks less than 2MB.
215				      The candidates to be discarded are cached until
216				      checkpoint is triggered, and issued during the
217				      checkpoint. By default, it is disabled with 0.
218	
219	 trim_sections                This parameter controls the number of sections
220	                              to be trimmed out in batch mode when FITRIM
221	                              conducts. 32 sections is set by default.
222	
223	 ipu_policy                   This parameter controls the policy of in-place
224	                              updates in f2fs. There are five policies:
225	                               0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
226	                               0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
227	                               0x10: F2FS_IPU_FSYNC.
228	
229	 min_ipu_util                 This parameter controls the threshold to trigger
230	                              in-place-updates. The number indicates percentage
231	                              of the filesystem utilization, and used by
232	                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
233	
234	 min_fsync_blocks             This parameter controls the threshold to trigger
235	                              in-place-updates when F2FS_IPU_FSYNC mode is set.
236				      The number indicates the number of dirty pages
237				      when fsync needs to flush on its call path. If
238				      the number is less than this value, it triggers
239				      in-place-updates.
240	
241	 max_victim_search	      This parameter controls the number of trials to
242				      find a victim segment when conducting SSR and
243				      cleaning operations. The default value is 4096
244				      which covers 8GB block address range.
245	
246	 dir_level                    This parameter controls the directory level to
247				      support large directory. If a directory has a
248				      number of files, it can reduce the file lookup
249				      latency by increasing this dir_level value.
250				      Otherwise, it needs to decrease this value to
251				      reduce the space overhead. The default value is 0.
252	
253	 ram_thresh                   This parameter controls the memory footprint used
254				      by free nids and cached nat entries. By default,
255				      10 is set, which indicates 10 MB / 1 GB RAM.
256	
257	================================================================================
258	USAGE
259	================================================================================
260	
261	1. Download userland tools and compile them.
262	
263	2. Skip, if f2fs was compiled statically inside kernel.
264	   Otherwise, insert the f2fs.ko module.
265	 # insmod f2fs.ko
266	
267	3. Create a directory trying to mount
268	 # mkdir /mnt/f2fs
269	
270	4. Format the block device, and then mount as f2fs
271	 # mkfs.f2fs -l label /dev/block_device
272	 # mount -t f2fs /dev/block_device /mnt/f2fs
273	
274	mkfs.f2fs
275	---------
276	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
277	which builds a basic on-disk layout.
278	
279	The options consist of:
280	-l [label]   : Give a volume label, up to 512 unicode name.
281	-a [0 or 1]  : Split start location of each area for heap-based allocation.
282	               1 is set by default, which performs this.
283	-o [int]     : Set overprovision ratio in percent over volume size.
284	               5 is set by default.
285	-s [int]     : Set the number of segments per section.
286	               1 is set by default.
287	-z [int]     : Set the number of sections per zone.
288	               1 is set by default.
289	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
290	-t [0 or 1]  : Disable discard command or not.
291	               1 is set by default, which conducts discard.
292	
293	fsck.f2fs
294	---------
295	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
296	partition, which examines whether the filesystem metadata and user-made data
297	are cross-referenced correctly or not.
298	Note that, initial version of the tool does not fix any inconsistency.
299	
300	The options consist of:
301	  -d debug level [default:0]
302	
303	dump.f2fs
304	---------
305	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
306	file. Each file is dump_ssa and dump_sit.
307	
308	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
309	It shows on-disk inode information recognized by a given inode number, and is
310	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
311	./dump_sit respectively.
312	
313	The options consist of:
314	  -d debug level [default:0]
315	  -i inode no (hex)
316	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
317	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
318	
319	Examples:
320	# dump.f2fs -i [ino] /dev/sdx
321	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
322	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
323	
324	================================================================================
325	DESIGN
326	================================================================================
327	
328	On-disk Layout
329	--------------
330	
331	F2FS divides the whole volume into a number of segments, each of which is fixed
332	to 2MB in size. A section is composed of consecutive segments, and a zone
333	consists of a set of sections. By default, section and zone sizes are set to one
334	segment size identically, but users can easily modify the sizes by mkfs.
335	
336	F2FS splits the entire volume into six areas, and all the areas except superblock
337	consists of multiple segments as described below.
338	
339	                                            align with the zone size <-|
340	                 |-> align with the segment size
341	     _________________________________________________________________________
342	    |            |            |   Segment   |    Node     |   Segment  |      |
343	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
344	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
345	    |____________|_____2______|______N______|______N______|______N_____|__N___|
346	                                                                       .      .
347	                                                             .                .
348	                                                 .                            .
349	                                    ._________________________________________.
350	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
351	                                    .           .
352	                                    ._________._________
353	                                    |_section_|__...__|_
354	                                    .            .
355			                    .________.
356		                            |__zone__|
357	
358	- Superblock (SB)
359	 : It is located at the beginning of the partition, and there exist two copies
360	   to avoid file system crash. It contains basic partition information and some
361	   default parameters of f2fs.
362	
363	- Checkpoint (CP)
364	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
365	   inode lists, and summary entries of current active segments.
366	
367	- Segment Information Table (SIT)
368	 : It contains segment information such as valid block count and bitmap for the
369	   validity of all the blocks.
370	
371	- Node Address Table (NAT)
372	 : It is composed of a block address table for all the node blocks stored in
373	   Main area.
374	
375	- Segment Summary Area (SSA)
376	 : It contains summary entries which contains the owner information of all the
377	   data and node blocks stored in Main area.
378	
379	- Main Area
380	 : It contains file and directory data including their indices.
381	
382	In order to avoid misalignment between file system and flash-based storage, F2FS
383	aligns the start block address of CP with the segment size. Also, it aligns the
384	start block address of Main area with the zone size by reserving some segments
385	in SSA area.
386	
387	Reference the following survey for additional technical details.
388	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
389	
390	File System Metadata Structure
391	------------------------------
392	
393	F2FS adopts the checkpointing scheme to maintain file system consistency. At
394	mount time, F2FS first tries to find the last valid checkpoint data by scanning
395	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
396	One of them always indicates the last valid data, which is called as shadow copy
397	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
398	
399	For file system consistency, each CP points to which NAT and SIT copies are
400	valid, as shown as below.
401	
402	  +--------+----------+---------+
403	  |   CP   |    SIT   |   NAT   |
404	  +--------+----------+---------+
405	  .         .          .          .
406	  .            .              .              .
407	  .               .                 .                 .
408	  +-------+-------+--------+--------+--------+--------+
409	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
410	  +-------+-------+--------+--------+--------+--------+
411	     |             ^                          ^
412	     |             |                          |
413	     `----------------------------------------'
414	
415	Index Structure
416	---------------
417	
418	The key data structure to manage the data locations is a "node". Similar to
419	traditional file structures, F2FS has three types of node: inode, direct node,
420	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
421	indices, two direct node pointers, two indirect node pointers, and one double
422	indirect node pointer as described below. One direct node block contains 1018
423	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
424	one inode block (i.e., a file) covers:
425	
426	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
427	
428	   Inode block (4KB)
429	     |- data (923)
430	     |- direct node (2)
431	     |          `- data (1018)
432	     |- indirect node (2)
433	     |            `- direct node (1018)
434	     |                       `- data (1018)
435	     `- double indirect node (1)
436	                         `- indirect node (1018)
437				              `- direct node (1018)
438		                                         `- data (1018)
439	
440	Note that, all the node blocks are mapped by NAT which means the location of
441	each node is translated by the NAT table. In the consideration of the wandering
442	tree problem, F2FS is able to cut off the propagation of node updates caused by
443	leaf data writes.
444	
445	Directory Structure
446	-------------------
447	
448	A directory entry occupies 11 bytes, which consists of the following attributes.
449	
450	- hash		hash value of the file name
451	- ino		inode number
452	- len		the length of file name
453	- type		file type such as directory, symlink, etc
454	
455	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
456	used to represent whether each dentry is valid or not. A dentry block occupies
457	4KB with the following composition.
458	
459	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
460		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
461	
462	                         [Bucket]
463	             +--------------------------------+
464	             |dentry block 1 | dentry block 2 |
465	             +--------------------------------+
466	             .               .
467	       .                             .
468	  .       [Dentry Block Structure: 4KB]       .
469	  +--------+----------+----------+------------+
470	  | bitmap | reserved | dentries | file names |
471	  +--------+----------+----------+------------+
472	  [Dentry Block: 4KB] .   .
473			 .               .
474	            .                          .
475	            +------+------+-----+------+
476	            | hash | ino  | len | type |
477	            +------+------+-----+------+
478	            [Dentry Structure: 11 bytes]
479	
480	F2FS implements multi-level hash tables for directory structure. Each level has
481	a hash table with dedicated number of hash buckets as shown below. Note that
482	"A(2B)" means a bucket includes 2 data blocks.
483	
484	----------------------
485	A : bucket
486	B : block
487	N : MAX_DIR_HASH_DEPTH
488	----------------------
489	
490	level #0   | A(2B)
491	           |
492	level #1   | A(2B) - A(2B)
493	           |
494	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
495	     .     |   .       .       .       .
496	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
497	     .     |   .       .       .       .
498	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
499	
500	The number of blocks and buckets are determined by,
501	
502	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
503	  # of blocks in level #n = |
504	                            `- 4, Otherwise
505	
506	                             ,- 2^(n + dir_level),
507				     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
508	  # of buckets in level #n = |
509	                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
510				              Otherwise
511	
512	When F2FS finds a file name in a directory, at first a hash value of the file
513	name is calculated. Then, F2FS scans the hash table in level #0 to find the
514	dentry consisting of the file name and its inode number. If not found, F2FS
515	scans the next hash table in level #1. In this way, F2FS scans hash tables in
516	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
517	one bucket determined by the following equation, which shows O(log(# of files))
518	complexity.
519	
520	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
521	
522	In the case of file creation, F2FS finds empty consecutive slots that cover the
523	file name. F2FS searches the empty slots in the hash tables of whole levels from
524	1 to N in the same way as the lookup operation.
525	
526	The following figure shows an example of two cases holding children.
527	       --------------> Dir <--------------
528	       |                                 |
529	    child                             child
530	
531	    child - child                     [hole] - child
532	
533	    child - child - child             [hole] - [hole] - child
534	
535	   Case 1:                           Case 2:
536	   Number of children = 6,           Number of children = 3,
537	   File size = 7                     File size = 7
538	
539	Default Block Allocation
540	------------------------
541	
542	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
543	and Hot/Warm/Cold data.
544	
545	- Hot node	contains direct node blocks of directories.
546	- Warm node	contains direct node blocks except hot node blocks.
547	- Cold node	contains indirect node blocks
548	- Hot data	contains dentry blocks
549	- Warm data	contains data blocks except hot and cold data blocks
550	- Cold data	contains multimedia data or migrated data blocks
551	
552	LFS has two schemes for free space management: threaded log and copy-and-compac-
553	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
554	for devices showing very good sequential write performance, since free segments
555	are served all the time for writing new data. However, it suffers from cleaning
556	overhead under high utilization. Contrarily, the threaded log scheme suffers
557	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
558	scheme where the copy-and-compaction scheme is adopted by default, but the
559	policy is dynamically changed to the threaded log scheme according to the file
560	system status.
561	
562	In order to align F2FS with underlying flash-based storage, F2FS allocates a
563	segment in a unit of section. F2FS expects that the section size would be the
564	same as the unit size of garbage collection in FTL. Furthermore, with respect
565	to the mapping granularity in FTL, F2FS allocates each section of the active
566	logs from different zones as much as possible, since FTL can write the data in
567	the active logs into one allocation unit according to its mapping granularity.
568	
569	Cleaning process
570	----------------
571	
572	F2FS does cleaning both on demand and in the background. On-demand cleaning is
573	triggered when there are not enough free segments to serve VFS calls. Background
574	cleaner is operated by a kernel thread, and triggers the cleaning job when the
575	system is idle.
576	
577	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
578	In the greedy algorithm, F2FS selects a victim segment having the smallest number
579	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
580	according to the segment age and the number of valid blocks in order to address
581	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
582	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
583	algorithm.
584	
585	In order to identify whether the data in the victim segment are valid or not,
586	F2FS manages a bitmap. Each bit represents the validity of a block, and the
587	bitmap is composed of a bit stream covering whole blocks in main area.
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