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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)
Chao Yu64058be2016-07-03 22:05:14 +0800112discard/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.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900115no_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.
118nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
119 by default if CONFIG_F2FS_FS_XATTR is selected.
120noacl Disable POSIX Access Control List. Note: acl is enabled
121 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
122active_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.
125disable_ext_identify Disable the extension list configured by mkfs, so f2fs
126 does not aware of cold files such as media files.
Jaegeuk Kim66e960c2013-11-01 11:20:05 +0900127inline_xattr Enable the inline xattrs feature.
Huajun Lie4024e82013-11-10 23:13:21 +0800128inline_data Enable the inline data feature: New created small(<~3.4k)
129 files can be written into inode block.
Chao Yud37a8682014-09-24 18:20:23 +0800130inline_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.
Jaegeuk Kim6b4afdd2014-04-02 15:34:36 +0900134flush_merge Merge concurrent cache_flush commands as much as possible
135 to eliminate redundant command issues. If the underlying
136 device handles the cache_flush command relatively slowly,
137 recommend to enable this option.
Jaegeuk Kim0f7b2ab2014-07-23 09:57:31 -0700138nobarrier This option can be used if underlying storage guarantees
139 its cached data should be written to the novolatile area.
140 If this option is set, no cache_flush commands are issued
141 but f2fs still guarantees the write ordering of all the
142 data writes.
Jaegeuk Kimd5053a342014-10-30 22:47:03 -0700143fastboot This option is used when a system wants to reduce mount
144 time as much as possible, even though normal performance
145 can be sacrificed.
Chao Yu89672152015-02-05 17:55:51 +0800146extent_cache Enable an extent cache based on rb-tree, it can cache
147 as many as extent which map between contiguous logical
148 address and physical address per inode, resulting in
Jaegeuk Kim7daaea22015-06-25 17:43:04 -0700149 increasing the cache hit ratio. Set by default.
Masanari Iida4bb99982015-11-16 20:46:28 +0900150noextent_cache Disable an extent cache based on rb-tree explicitly, see
Jaegeuk Kim7daaea22015-06-25 17:43:04 -0700151 the above extent_cache mount option.
Wanpeng Li75342792015-03-24 10:20:27 +0800152noinline_data Disable the inline data feature, inline data feature is
153 enabled by default.
Chao Yu343f40f2015-12-16 13:12:16 +0800154data_flush Enable data flushing before checkpoint in order to
155 persist data of regular and symlink.
Jaegeuk Kim36abef42016-06-03 19:29:38 -0700156mode=%s Control block allocation mode which supports "adaptive"
157 and "lfs". In "lfs" mode, there should be no random
158 writes towards main area.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900159
160================================================================================
161DEBUGFS ENTRIES
162================================================================================
163
164/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
165f2fs. Each file shows the whole f2fs information.
166
167/sys/kernel/debug/f2fs/status includes:
168 - major file system information managed by f2fs currently
169 - average SIT information about whole segments
170 - current memory footprint consumed by f2fs.
171
172================================================================================
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900173SYSFS ENTRIES
174================================================================================
175
176Information about mounted f2f2 file systems can be found in
177/sys/fs/f2fs. Each mounted filesystem will have a directory in
178/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
179The files in each per-device directory are shown in table below.
180
181Files in /sys/fs/f2fs/<devname>
182(see also Documentation/ABI/testing/sysfs-fs-f2fs)
183..............................................................................
184 File Content
185
186 gc_max_sleep_time This tuning parameter controls the maximum sleep
187 time for the garbage collection thread. Time is
188 in milliseconds.
189
190 gc_min_sleep_time This tuning parameter controls the minimum sleep
191 time for the garbage collection thread. Time is
192 in milliseconds.
193
194 gc_no_gc_sleep_time This tuning parameter controls the default sleep
195 time for the garbage collection thread. Time is
196 in milliseconds.
197
Namjae Jeond2dc0952013-08-04 23:10:15 +0900198 gc_idle This parameter controls the selection of victim
199 policy for garbage collection. Setting gc_idle = 0
200 (default) will disable this option. Setting
201 gc_idle = 1 will select the Cost Benefit approach
Masanari Iida4bb99982015-11-16 20:46:28 +0900202 & setting gc_idle = 2 will select the greedy approach.
Namjae Jeond2dc0952013-08-04 23:10:15 +0900203
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900204 reclaim_segments This parameter controls the number of prefree
205 segments to be reclaimed. If the number of prefree
Jaegeuk Kim58c41032014-03-19 14:17:21 +0900206 segments is larger than the number of segments
207 in the proportion to the percentage over total
208 volume size, f2fs tries to conduct checkpoint to
209 reclaim the prefree segments to free segments.
210 By default, 5% over total # of segments.
Jaegeuk Kimea91e9b2013-10-24 15:49:07 +0900211
Jaegeuk Kimba0697e2013-12-19 17:44:41 +0900212 max_small_discards This parameter controls the number of discard
213 commands that consist small blocks less than 2MB.
214 The candidates to be discarded are cached until
215 checkpoint is triggered, and issued during the
216 checkpoint. By default, it is disabled with 0.
217
Jaegeuk Kimbba681c2015-01-26 17:41:23 -0800218 trim_sections This parameter controls the number of sections
219 to be trimmed out in batch mode when FITRIM
220 conducts. 32 sections is set by default.
221
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900222 ipu_policy This parameter controls the policy of in-place
223 updates in f2fs. There are five policies:
Jaegeuk Kim9b5f1362014-09-16 18:30:54 -0700224 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
225 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
226 0x10: F2FS_IPU_FSYNC.
Jaegeuk Kim216fbd62013-11-07 13:13:42 +0900227
228 min_ipu_util This parameter controls the threshold to trigger
229 in-place-updates. The number indicates percentage
230 of the filesystem utilization, and used by
231 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
232
Jaegeuk Kimc1ce1b02014-09-10 16:53:02 -0700233 min_fsync_blocks This parameter controls the threshold to trigger
234 in-place-updates when F2FS_IPU_FSYNC mode is set.
235 The number indicates the number of dirty pages
236 when fsync needs to flush on its call path. If
237 the number is less than this value, it triggers
238 in-place-updates.
239
Jaegeuk Kim3bac3802014-01-09 21:00:06 +0900240 max_victim_search This parameter controls the number of trials to
241 find a victim segment when conducting SSR and
242 cleaning operations. The default value is 4096
243 which covers 8GB block address range.
244
Jaegeuk Kimab9fa662014-02-27 20:09:05 +0900245 dir_level This parameter controls the directory level to
246 support large directory. If a directory has a
247 number of files, it can reduce the file lookup
248 latency by increasing this dir_level value.
249 Otherwise, it needs to decrease this value to
250 reduce the space overhead. The default value is 0.
251
Jaegeuk Kimcdfc41c2014-03-19 13:31:37 +0900252 ram_thresh This parameter controls the memory footprint used
253 by free nids and cached nat entries. By default,
254 10 is set, which indicates 10 MB / 1 GB RAM.
255
Namjae Jeonb59d0ba2013-08-04 23:09:40 +0900256================================================================================
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900257USAGE
258================================================================================
259
2601. Download userland tools and compile them.
261
2622. Skip, if f2fs was compiled statically inside kernel.
263 Otherwise, insert the f2fs.ko module.
264 # insmod f2fs.ko
265
2663. Create a directory trying to mount
267 # mkdir /mnt/f2fs
268
2694. Format the block device, and then mount as f2fs
270 # mkfs.f2fs -l label /dev/block_device
271 # mount -t f2fs /dev/block_device /mnt/f2fs
272
Changman Leed51a7fb2013-07-04 17:12:47 +0900273mkfs.f2fs
274---------
275The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
276which builds a basic on-disk layout.
277
278The options consist of:
Changman Lee1571f842013-04-03 15:26:49 +0900279-l [label] : Give a volume label, up to 512 unicode name.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900280-a [0 or 1] : Split start location of each area for heap-based allocation.
281 1 is set by default, which performs this.
282-o [int] : Set overprovision ratio in percent over volume size.
283 5 is set by default.
284-s [int] : Set the number of segments per section.
285 1 is set by default.
286-z [int] : Set the number of sections per zone.
287 1 is set by default.
288-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
Changman Lee1571f842013-04-03 15:26:49 +0900289-t [0 or 1] : Disable discard command or not.
290 1 is set by default, which conducts discard.
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900291
Changman Leed51a7fb2013-07-04 17:12:47 +0900292fsck.f2fs
293---------
294The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
295partition, which examines whether the filesystem metadata and user-made data
296are cross-referenced correctly or not.
297Note that, initial version of the tool does not fix any inconsistency.
298
299The options consist of:
300 -d debug level [default:0]
301
302dump.f2fs
303---------
304The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
305file. Each file is dump_ssa and dump_sit.
306
307The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
Masanari Iida4bb99982015-11-16 20:46:28 +0900308It shows on-disk inode information recognized by a given inode number, and is
Changman Leed51a7fb2013-07-04 17:12:47 +0900309able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
310./dump_sit respectively.
311
312The options consist of:
313 -d debug level [default:0]
314 -i inode no (hex)
315 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
316 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
317
318Examples:
319# dump.f2fs -i [ino] /dev/sdx
320# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
321# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
322
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900323================================================================================
324DESIGN
325================================================================================
326
327On-disk Layout
328--------------
329
330F2FS divides the whole volume into a number of segments, each of which is fixed
331to 2MB in size. A section is composed of consecutive segments, and a zone
332consists of a set of sections. By default, section and zone sizes are set to one
333segment size identically, but users can easily modify the sizes by mkfs.
334
335F2FS splits the entire volume into six areas, and all the areas except superblock
336consists of multiple segments as described below.
337
338 align with the zone size <-|
339 |-> align with the segment size
340 _________________________________________________________________________
Huajun Li9268cc32012-12-31 13:59:04 +0800341 | | | Segment | Node | Segment | |
342 | Superblock | Checkpoint | Info. | Address | Summary | Main |
343 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900344 |____________|_____2______|______N______|______N______|______N_____|__N___|
345 . .
346 . .
347 . .
348 ._________________________________________.
349 |_Segment_|_..._|_Segment_|_..._|_Segment_|
350 . .
351 ._________._________
352 |_section_|__...__|_
353 . .
354 .________.
355 |__zone__|
356
357- Superblock (SB)
358 : It is located at the beginning of the partition, and there exist two copies
359 to avoid file system crash. It contains basic partition information and some
360 default parameters of f2fs.
361
362- Checkpoint (CP)
363 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
364 inode lists, and summary entries of current active segments.
365
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900366- Segment Information Table (SIT)
367 : It contains segment information such as valid block count and bitmap for the
368 validity of all the blocks.
369
Huajun Li9268cc32012-12-31 13:59:04 +0800370- Node Address Table (NAT)
371 : It is composed of a block address table for all the node blocks stored in
372 Main area.
373
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900374- Segment Summary Area (SSA)
375 : It contains summary entries which contains the owner information of all the
376 data and node blocks stored in Main area.
377
378- Main Area
379 : It contains file and directory data including their indices.
380
381In order to avoid misalignment between file system and flash-based storage, F2FS
382aligns the start block address of CP with the segment size. Also, it aligns the
383start block address of Main area with the zone size by reserving some segments
384in SSA area.
385
386Reference the following survey for additional technical details.
387https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
388
389File System Metadata Structure
390------------------------------
391
392F2FS adopts the checkpointing scheme to maintain file system consistency. At
393mount time, F2FS first tries to find the last valid checkpoint data by scanning
394CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
395One of them always indicates the last valid data, which is called as shadow copy
396mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
397
398For file system consistency, each CP points to which NAT and SIT copies are
399valid, as shown as below.
400
401 +--------+----------+---------+
Huajun Li9268cc32012-12-31 13:59:04 +0800402 | CP | SIT | NAT |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900403 +--------+----------+---------+
404 . . . .
405 . . . .
406 . . . .
407 +-------+-------+--------+--------+--------+--------+
Huajun Li9268cc32012-12-31 13:59:04 +0800408 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900409 +-------+-------+--------+--------+--------+--------+
410 | ^ ^
411 | | |
412 `----------------------------------------'
413
414Index Structure
415---------------
416
417The key data structure to manage the data locations is a "node". Similar to
418traditional file structures, F2FS has three types of node: inode, direct node,
Huajun Lid08ab082012-12-05 16:45:32 +0800419indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900420indices, two direct node pointers, two indirect node pointers, and one double
421indirect node pointer as described below. One direct node block contains 1018
422data blocks, and one indirect node block contains also 1018 node blocks. Thus,
423one inode block (i.e., a file) covers:
424
425 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
426
427 Inode block (4KB)
428 |- data (923)
429 |- direct node (2)
430 | `- data (1018)
431 |- indirect node (2)
432 | `- direct node (1018)
433 | `- data (1018)
434 `- double indirect node (1)
435 `- indirect node (1018)
436 `- direct node (1018)
437 `- data (1018)
438
439Note that, all the node blocks are mapped by NAT which means the location of
440each node is translated by the NAT table. In the consideration of the wandering
441tree problem, F2FS is able to cut off the propagation of node updates caused by
442leaf data writes.
443
444Directory Structure
445-------------------
446
447A directory entry occupies 11 bytes, which consists of the following attributes.
448
449- hash hash value of the file name
450- ino inode number
451- len the length of file name
452- type file type such as directory, symlink, etc
453
454A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
455used to represent whether each dentry is valid or not. A dentry block occupies
4564KB with the following composition.
457
458 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
459 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
460
461 [Bucket]
462 +--------------------------------+
463 |dentry block 1 | dentry block 2 |
464 +--------------------------------+
465 . .
466 . .
467 . [Dentry Block Structure: 4KB] .
468 +--------+----------+----------+------------+
469 | bitmap | reserved | dentries | file names |
470 +--------+----------+----------+------------+
471 [Dentry Block: 4KB] . .
472 . .
473 . .
474 +------+------+-----+------+
475 | hash | ino | len | type |
476 +------+------+-----+------+
477 [Dentry Structure: 11 bytes]
478
479F2FS implements multi-level hash tables for directory structure. Each level has
480a hash table with dedicated number of hash buckets as shown below. Note that
481"A(2B)" means a bucket includes 2 data blocks.
482
483----------------------
484A : bucket
485B : block
486N : MAX_DIR_HASH_DEPTH
487----------------------
488
489level #0 | A(2B)
490 |
491level #1 | A(2B) - A(2B)
492 |
493level #2 | A(2B) - A(2B) - A(2B) - A(2B)
494 . | . . . .
495level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
496 . | . . . .
497level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
498
499The number of blocks and buckets are determined by,
500
501 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
502 # of blocks in level #n = |
503 `- 4, Otherwise
504
Chao Yubfec07d2014-05-28 08:56:09 +0800505 ,- 2^(n + dir_level),
506 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900507 # of buckets in level #n = |
Chao Yubfec07d2014-05-28 08:56:09 +0800508 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
509 Otherwise
Jaegeuk Kim98e4da82012-11-02 17:05:42 +0900510
511When F2FS finds a file name in a directory, at first a hash value of the file
512name is calculated. Then, F2FS scans the hash table in level #0 to find the
513dentry consisting of the file name and its inode number. If not found, F2FS
514scans the next hash table in level #1. In this way, F2FS scans hash tables in
515each levels incrementally from 1 to N. In each levels F2FS needs to scan only
516one bucket determined by the following equation, which shows O(log(# of files))
517complexity.
518
519 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
520
521In the case of file creation, F2FS finds empty consecutive slots that cover the
522file name. F2FS searches the empty slots in the hash tables of whole levels from
5231 to N in the same way as the lookup operation.
524
525The following figure shows an example of two cases holding children.
526 --------------> Dir <--------------
527 | |
528 child child
529
530 child - child [hole] - child
531
532 child - child - child [hole] - [hole] - child
533
534 Case 1: Case 2:
535 Number of children = 6, Number of children = 3,
536 File size = 7 File size = 7
537
538Default Block Allocation
539------------------------
540
541At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
542and Hot/Warm/Cold data.
543
544- Hot node contains direct node blocks of directories.
545- Warm node contains direct node blocks except hot node blocks.
546- Cold node contains indirect node blocks
547- Hot data contains dentry blocks
548- Warm data contains data blocks except hot and cold data blocks
549- Cold data contains multimedia data or migrated data blocks
550
551LFS has two schemes for free space management: threaded log and copy-and-compac-
552tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
553for devices showing very good sequential write performance, since free segments
554are served all the time for writing new data. However, it suffers from cleaning
555overhead under high utilization. Contrarily, the threaded log scheme suffers
556from random writes, but no cleaning process is needed. F2FS adopts a hybrid
557scheme where the copy-and-compaction scheme is adopted by default, but the
558policy is dynamically changed to the threaded log scheme according to the file
559system status.
560
561In order to align F2FS with underlying flash-based storage, F2FS allocates a
562segment in a unit of section. F2FS expects that the section size would be the
563same as the unit size of garbage collection in FTL. Furthermore, with respect
564to the mapping granularity in FTL, F2FS allocates each section of the active
565logs from different zones as much as possible, since FTL can write the data in
566the active logs into one allocation unit according to its mapping granularity.
567
568Cleaning process
569----------------
570
571F2FS does cleaning both on demand and in the background. On-demand cleaning is
572triggered when there are not enough free segments to serve VFS calls. Background
573cleaner is operated by a kernel thread, and triggers the cleaning job when the
574system is idle.
575
576F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
577In the greedy algorithm, F2FS selects a victim segment having the smallest number
578of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
579according to the segment age and the number of valid blocks in order to address
580log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
581algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
582algorithm.
583
584In order to identify whether the data in the victim segment are valid or not,
585F2FS manages a bitmap. Each bit represents the validity of a block, and the
586bitmap is composed of a bit stream covering whole blocks in main area.