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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
Masanari Iida4bb99982015-11-16 20:46:28 +0900105 will be turned off. If background_gc=sync, it will turn
Jaegeuk Kim6aefd932015-10-05 11:02:54 -0700106 on synchronous garbage collection running in background.
Namjae Jeon696c0182013-06-16 09:48:48 +0900107 Default value for this option is on. So garbage
108 collection is on by default.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900109disable_roll_forward Disable the roll-forward recovery routine
Jaegeuk Kim2d834bf2015-01-23 18:33:46 -0800110norecovery Disable the roll-forward recovery routine, mounted read-
111 only (i.e., -o ro,disable_roll_forward)
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900112discard Issue discard/TRIM commands when a segment is cleaned.
113no_heap Disable heap-style segment allocation which finds free
114 segments for data from the beginning of main area, while
115 for node from the end of main area.
116nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
117 by default if CONFIG_F2FS_FS_XATTR is selected.
118noacl Disable POSIX Access Control List. Note: acl is enabled
119 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
120active_logs=%u Support configuring the number of active logs. In the
121 current design, f2fs supports only 2, 4, and 6 logs.
122 Default number is 6.
123disable_ext_identify Disable the extension list configured by mkfs, so f2fs
124 does not aware of cold files such as media files.
Jaegeuk Kim66e960c2013-11-01 11:20:05 +0900125inline_xattr Enable the inline xattrs feature.
Huajun Lie4024e82013-11-10 23:13:21 +0800126inline_data Enable the inline data feature: New created small(<~3.4k)
127 files can be written into inode block.
Chao Yud37a8682014-09-24 18:20:23 +0800128inline_dentry Enable the inline dir feature: data in new created
129 directory entries can be written into inode block. The
130 space of inode block which is used to store inline
131 dentries is limited to ~3.4k.
Jaegeuk Kim6b4afdd2014-04-02 15:34:36 +0900132flush_merge Merge concurrent cache_flush commands as much as possible
133 to eliminate redundant command issues. If the underlying
134 device handles the cache_flush command relatively slowly,
135 recommend to enable this option.
Jaegeuk Kim0f7b2ab2014-07-23 09:57:31 -0700136nobarrier This option can be used if underlying storage guarantees
137 its cached data should be written to the novolatile area.
138 If this option is set, no cache_flush commands are issued
139 but f2fs still guarantees the write ordering of all the
140 data writes.
Jaegeuk Kimd5053a342014-10-30 22:47:03 -0700141fastboot This option is used when a system wants to reduce mount
142 time as much as possible, even though normal performance
143 can be sacrificed.
Chao Yu89672152015-02-05 17:55:51 +0800144extent_cache Enable an extent cache based on rb-tree, it can cache
145 as many as extent which map between contiguous logical
146 address and physical address per inode, resulting in
Jaegeuk Kim7daaea22015-06-25 17:43:04 -0700147 increasing the cache hit ratio. Set by default.
Masanari Iida4bb99982015-11-16 20:46:28 +0900148noextent_cache Disable an extent cache based on rb-tree explicitly, see
Jaegeuk Kim7daaea22015-06-25 17:43:04 -0700149 the above extent_cache mount option.
Wanpeng Li75342792015-03-24 10:20:27 +0800150noinline_data Disable the inline data feature, inline data feature is
151 enabled by default.
Chao Yu343f40f2015-12-16 13:12:16 +0800152data_flush Enable data flushing before checkpoint in order to
153 persist data of regular and symlink.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900154
155================================================================================
156DEBUGFS ENTRIES
157================================================================================
158
159/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
160f2fs. Each file shows the whole f2fs information.
161
162/sys/kernel/debug/f2fs/status includes:
163 - major file system information managed by f2fs currently
164 - average SIT information about whole segments
165 - current memory footprint consumed by f2fs.
166
167================================================================================
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900168SYSFS ENTRIES
169================================================================================
170
171Information about mounted f2f2 file systems can be found in
172/sys/fs/f2fs. Each mounted filesystem will have a directory in
173/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
174The files in each per-device directory are shown in table below.
175
176Files in /sys/fs/f2fs/<devname>
177(see also Documentation/ABI/testing/sysfs-fs-f2fs)
178..............................................................................
179 File Content
180
181 gc_max_sleep_time This tuning parameter controls the maximum sleep
182 time for the garbage collection thread. Time is
183 in milliseconds.
184
185 gc_min_sleep_time This tuning parameter controls the minimum sleep
186 time for the garbage collection thread. Time is
187 in milliseconds.
188
189 gc_no_gc_sleep_time This tuning parameter controls the default sleep
190 time for the garbage collection thread. Time is
191 in milliseconds.
192
Namjae Jeond2dc0952013-08-04 23:10:15 +0900193 gc_idle This parameter controls the selection of victim
194 policy for garbage collection. Setting gc_idle = 0
195 (default) will disable this option. Setting
196 gc_idle = 1 will select the Cost Benefit approach
Masanari Iida4bb99982015-11-16 20:46:28 +0900197 & setting gc_idle = 2 will select the greedy approach.
Namjae Jeond2dc0952013-08-04 23:10:15 +0900198
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900199 reclaim_segments This parameter controls the number of prefree
200 segments to be reclaimed. If the number of prefree
Jaegeuk Kim58c41032014-03-19 14:17:21 +0900201 segments is larger than the number of segments
202 in the proportion to the percentage over total
203 volume size, f2fs tries to conduct checkpoint to
204 reclaim the prefree segments to free segments.
205 By default, 5% over total # of segments.
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900206
Jaegeuk Kimba0697e2013-12-19 17:44:41 +0900207 max_small_discards This parameter controls the number of discard
208 commands that consist small blocks less than 2MB.
209 The candidates to be discarded are cached until
210 checkpoint is triggered, and issued during the
211 checkpoint. By default, it is disabled with 0.
212
Jaegeuk Kimbba681c2015-01-26 17:41:23 -0800213 trim_sections This parameter controls the number of sections
214 to be trimmed out in batch mode when FITRIM
215 conducts. 32 sections is set by default.
216
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900217 ipu_policy This parameter controls the policy of in-place
218 updates in f2fs. There are five policies:
Jaegeuk Kim9b5f1362014-09-16 18:30:54 -0700219 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
220 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
221 0x10: F2FS_IPU_FSYNC.
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900222
223 min_ipu_util This parameter controls the threshold to trigger
224 in-place-updates. The number indicates percentage
225 of the filesystem utilization, and used by
226 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
227
Jaegeuk Kimc1ce1b02014-09-10 16:53:02 -0700228 min_fsync_blocks This parameter controls the threshold to trigger
229 in-place-updates when F2FS_IPU_FSYNC mode is set.
230 The number indicates the number of dirty pages
231 when fsync needs to flush on its call path. If
232 the number is less than this value, it triggers
233 in-place-updates.
234
Jaegeuk Kim3bac3802014-01-09 21:00:06 +0900235 max_victim_search This parameter controls the number of trials to
236 find a victim segment when conducting SSR and
237 cleaning operations. The default value is 4096
238 which covers 8GB block address range.
239
Jaegeuk Kimab9fa662014-02-27 20:09:05 +0900240 dir_level This parameter controls the directory level to
241 support large directory. If a directory has a
242 number of files, it can reduce the file lookup
243 latency by increasing this dir_level value.
244 Otherwise, it needs to decrease this value to
245 reduce the space overhead. The default value is 0.
246
Jaegeuk Kimcdfc41c2014-03-19 13:31:37 +0900247 ram_thresh This parameter controls the memory footprint used
248 by free nids and cached nat entries. By default,
249 10 is set, which indicates 10 MB / 1 GB RAM.
250
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900251================================================================================
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900252USAGE
253================================================================================
254
2551. Download userland tools and compile them.
256
2572. Skip, if f2fs was compiled statically inside kernel.
258 Otherwise, insert the f2fs.ko module.
259 # insmod f2fs.ko
260
2613. Create a directory trying to mount
262 # mkdir /mnt/f2fs
263
2644. Format the block device, and then mount as f2fs
265 # mkfs.f2fs -l label /dev/block_device
266 # mount -t f2fs /dev/block_device /mnt/f2fs
267
Changman Leed51a7fb2013-07-04 17:12:47 +0900268mkfs.f2fs
269---------
270The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
271which builds a basic on-disk layout.
272
273The options consist of:
Changman Lee1571f842013-04-03 15:26:49 +0900274-l [label] : Give a volume label, up to 512 unicode name.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900275-a [0 or 1] : Split start location of each area for heap-based allocation.
276 1 is set by default, which performs this.
277-o [int] : Set overprovision ratio in percent over volume size.
278 5 is set by default.
279-s [int] : Set the number of segments per section.
280 1 is set by default.
281-z [int] : Set the number of sections per zone.
282 1 is set by default.
283-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
Changman Lee1571f842013-04-03 15:26:49 +0900284-t [0 or 1] : Disable discard command or not.
285 1 is set by default, which conducts discard.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900286
Changman Leed51a7fb2013-07-04 17:12:47 +0900287fsck.f2fs
288---------
289The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
290partition, which examines whether the filesystem metadata and user-made data
291are cross-referenced correctly or not.
292Note that, initial version of the tool does not fix any inconsistency.
293
294The options consist of:
295 -d debug level [default:0]
296
297dump.f2fs
298---------
299The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
300file. Each file is dump_ssa and dump_sit.
301
302The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
Masanari Iida4bb99982015-11-16 20:46:28 +0900303It shows on-disk inode information recognized by a given inode number, and is
Changman Leed51a7fb2013-07-04 17:12:47 +0900304able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
305./dump_sit respectively.
306
307The options consist of:
308 -d debug level [default:0]
309 -i inode no (hex)
310 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
311 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
312
313Examples:
314# dump.f2fs -i [ino] /dev/sdx
315# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
316# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
317
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900318================================================================================
319DESIGN
320================================================================================
321
322On-disk Layout
323--------------
324
325F2FS divides the whole volume into a number of segments, each of which is fixed
326to 2MB in size. A section is composed of consecutive segments, and a zone
327consists of a set of sections. By default, section and zone sizes are set to one
328segment size identically, but users can easily modify the sizes by mkfs.
329
330F2FS splits the entire volume into six areas, and all the areas except superblock
331consists of multiple segments as described below.
332
333 align with the zone size <-|
334 |-> align with the segment size
335 _________________________________________________________________________
Huajun Li9268cc32012-12-31 13:59:04 +0800336 | | | Segment | Node | Segment | |
337 | Superblock | Checkpoint | Info. | Address | Summary | Main |
338 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900339 |____________|_____2______|______N______|______N______|______N_____|__N___|
340 . .
341 . .
342 . .
343 ._________________________________________.
344 |_Segment_|_..._|_Segment_|_..._|_Segment_|
345 . .
346 ._________._________
347 |_section_|__...__|_
348 . .
349 .________.
350 |__zone__|
351
352- Superblock (SB)
353 : It is located at the beginning of the partition, and there exist two copies
354 to avoid file system crash. It contains basic partition information and some
355 default parameters of f2fs.
356
357- Checkpoint (CP)
358 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
359 inode lists, and summary entries of current active segments.
360
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900361- Segment Information Table (SIT)
362 : It contains segment information such as valid block count and bitmap for the
363 validity of all the blocks.
364
Huajun Li9268cc32012-12-31 13:59:04 +0800365- Node Address Table (NAT)
366 : It is composed of a block address table for all the node blocks stored in
367 Main area.
368
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900369- Segment Summary Area (SSA)
370 : It contains summary entries which contains the owner information of all the
371 data and node blocks stored in Main area.
372
373- Main Area
374 : It contains file and directory data including their indices.
375
376In order to avoid misalignment between file system and flash-based storage, F2FS
377aligns the start block address of CP with the segment size. Also, it aligns the
378start block address of Main area with the zone size by reserving some segments
379in SSA area.
380
381Reference the following survey for additional technical details.
382https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
383
384File System Metadata Structure
385------------------------------
386
387F2FS adopts the checkpointing scheme to maintain file system consistency. At
388mount time, F2FS first tries to find the last valid checkpoint data by scanning
389CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
390One of them always indicates the last valid data, which is called as shadow copy
391mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
392
393For file system consistency, each CP points to which NAT and SIT copies are
394valid, as shown as below.
395
396 +--------+----------+---------+
Huajun Li9268cc32012-12-31 13:59:04 +0800397 | CP | SIT | NAT |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900398 +--------+----------+---------+
399 . . . .
400 . . . .
401 . . . .
402 +-------+-------+--------+--------+--------+--------+
Huajun Li9268cc32012-12-31 13:59:04 +0800403 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900404 +-------+-------+--------+--------+--------+--------+
405 | ^ ^
406 | | |
407 `----------------------------------------'
408
409Index Structure
410---------------
411
412The key data structure to manage the data locations is a "node". Similar to
413traditional file structures, F2FS has three types of node: inode, direct node,
Huajun Lid08ab082012-12-05 16:45:32 +0800414indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900415indices, two direct node pointers, two indirect node pointers, and one double
416indirect node pointer as described below. One direct node block contains 1018
417data blocks, and one indirect node block contains also 1018 node blocks. Thus,
418one inode block (i.e., a file) covers:
419
420 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
421
422 Inode block (4KB)
423 |- data (923)
424 |- direct node (2)
425 | `- data (1018)
426 |- indirect node (2)
427 | `- direct node (1018)
428 | `- data (1018)
429 `- double indirect node (1)
430 `- indirect node (1018)
431 `- direct node (1018)
432 `- data (1018)
433
434Note that, all the node blocks are mapped by NAT which means the location of
435each node is translated by the NAT table. In the consideration of the wandering
436tree problem, F2FS is able to cut off the propagation of node updates caused by
437leaf data writes.
438
439Directory Structure
440-------------------
441
442A directory entry occupies 11 bytes, which consists of the following attributes.
443
444- hash hash value of the file name
445- ino inode number
446- len the length of file name
447- type file type such as directory, symlink, etc
448
449A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
450used to represent whether each dentry is valid or not. A dentry block occupies
4514KB with the following composition.
452
453 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
454 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
455
456 [Bucket]
457 +--------------------------------+
458 |dentry block 1 | dentry block 2 |
459 +--------------------------------+
460 . .
461 . .
462 . [Dentry Block Structure: 4KB] .
463 +--------+----------+----------+------------+
464 | bitmap | reserved | dentries | file names |
465 +--------+----------+----------+------------+
466 [Dentry Block: 4KB] . .
467 . .
468 . .
469 +------+------+-----+------+
470 | hash | ino | len | type |
471 +------+------+-----+------+
472 [Dentry Structure: 11 bytes]
473
474F2FS implements multi-level hash tables for directory structure. Each level has
475a hash table with dedicated number of hash buckets as shown below. Note that
476"A(2B)" means a bucket includes 2 data blocks.
477
478----------------------
479A : bucket
480B : block
481N : MAX_DIR_HASH_DEPTH
482----------------------
483
484level #0 | A(2B)
485 |
486level #1 | A(2B) - A(2B)
487 |
488level #2 | A(2B) - A(2B) - A(2B) - A(2B)
489 . | . . . .
490level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
491 . | . . . .
492level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
493
494The number of blocks and buckets are determined by,
495
496 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
497 # of blocks in level #n = |
498 `- 4, Otherwise
499
Chao Yubfec07d2014-05-28 08:56:09 +0800500 ,- 2^(n + dir_level),
501 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900502 # of buckets in level #n = |
Chao Yubfec07d2014-05-28 08:56:09 +0800503 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
504 Otherwise
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900505
506When F2FS finds a file name in a directory, at first a hash value of the file
507name is calculated. Then, F2FS scans the hash table in level #0 to find the
508dentry consisting of the file name and its inode number. If not found, F2FS
509scans the next hash table in level #1. In this way, F2FS scans hash tables in
510each levels incrementally from 1 to N. In each levels F2FS needs to scan only
511one bucket determined by the following equation, which shows O(log(# of files))
512complexity.
513
514 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
515
516In the case of file creation, F2FS finds empty consecutive slots that cover the
517file name. F2FS searches the empty slots in the hash tables of whole levels from
5181 to N in the same way as the lookup operation.
519
520The following figure shows an example of two cases holding children.
521 --------------> Dir <--------------
522 | |
523 child child
524
525 child - child [hole] - child
526
527 child - child - child [hole] - [hole] - child
528
529 Case 1: Case 2:
530 Number of children = 6, Number of children = 3,
531 File size = 7 File size = 7
532
533Default Block Allocation
534------------------------
535
536At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
537and Hot/Warm/Cold data.
538
539- Hot node contains direct node blocks of directories.
540- Warm node contains direct node blocks except hot node blocks.
541- Cold node contains indirect node blocks
542- Hot data contains dentry blocks
543- Warm data contains data blocks except hot and cold data blocks
544- Cold data contains multimedia data or migrated data blocks
545
546LFS has two schemes for free space management: threaded log and copy-and-compac-
547tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
548for devices showing very good sequential write performance, since free segments
549are served all the time for writing new data. However, it suffers from cleaning
550overhead under high utilization. Contrarily, the threaded log scheme suffers
551from random writes, but no cleaning process is needed. F2FS adopts a hybrid
552scheme where the copy-and-compaction scheme is adopted by default, but the
553policy is dynamically changed to the threaded log scheme according to the file
554system status.
555
556In order to align F2FS with underlying flash-based storage, F2FS allocates a
557segment in a unit of section. F2FS expects that the section size would be the
558same as the unit size of garbage collection in FTL. Furthermore, with respect
559to the mapping granularity in FTL, F2FS allocates each section of the active
560logs from different zones as much as possible, since FTL can write the data in
561the active logs into one allocation unit according to its mapping granularity.
562
563Cleaning process
564----------------
565
566F2FS does cleaning both on demand and in the background. On-demand cleaning is
567triggered when there are not enough free segments to serve VFS calls. Background
568cleaner is operated by a kernel thread, and triggers the cleaning job when the
569system is idle.
570
571F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
572In the greedy algorithm, F2FS selects a victim segment having the smallest number
573of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
574according to the segment age and the number of valid blocks in order to address
575log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
576algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
577algorithm.
578
579In order to identify whether the data in the victim segment are valid or not,
580F2FS manages a bitmap. Each bit represents the validity of a block, and the
581bitmap is composed of a bit stream covering whole blocks in main area.