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Linux kernel mirror (for testing)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
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linux
1================================================================================
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
21The following git tree provides the file system formatting tool (mkfs.f2fs),
22a 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
25For reporting bugs and sending patches, please use the following mailing list:
26>> linux-f2fs-devel@lists.sourceforge.net
27
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
101background_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.
109disable_roll_forward Disable the roll-forward recovery routine
110norecovery Disable the roll-forward recovery routine, mounted read-
111 only (i.e., -o ro,disable_roll_forward)
112discard/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.
115no_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.
127inline_xattr Enable the inline xattrs feature.
128noinline_xattr Disable the inline xattrs feature.
129inline_data Enable the inline data feature: New created small(<~3.4k)
130 files can be written into inode block.
131inline_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.
135noinline_dentry Disable the inline dentry feature.
136flush_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.
140nobarrier 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.
145fastboot 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.
148extent_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.
152noextent_cache Disable an extent cache based on rb-tree explicitly, see
153 the above extent_cache mount option.
154noinline_data Disable the inline data feature, inline data feature is
155 enabled by default.
156data_flush Enable data flushing before checkpoint in order to
157 persist data of regular and symlink.
158fault_injection=%d Enable fault injection in all supported types with
159 specified injection rate.
160fault_type=%d Support configuring fault injection type, should be
161 enabled with fault_injection option, fault type value
162 is shown below, it supports single or combined type.
163 Type_Name Type_Value
164 FAULT_KMALLOC 0x000000001
165 FAULT_KVMALLOC 0x000000002
166 FAULT_PAGE_ALLOC 0x000000004
167 FAULT_PAGE_GET 0x000000008
168 FAULT_ALLOC_BIO 0x000000010
169 FAULT_ALLOC_NID 0x000000020
170 FAULT_ORPHAN 0x000000040
171 FAULT_BLOCK 0x000000080
172 FAULT_DIR_DEPTH 0x000000100
173 FAULT_EVICT_INODE 0x000000200
174 FAULT_TRUNCATE 0x000000400
175 FAULT_READ_IO 0x000000800
176 FAULT_CHECKPOINT 0x000001000
177 FAULT_DISCARD 0x000002000
178 FAULT_WRITE_IO 0x000004000
179mode=%s Control block allocation mode which supports "adaptive"
180 and "lfs". In "lfs" mode, there should be no random
181 writes towards main area.
182io_bits=%u Set the bit size of write IO requests. It should be set
183 with "mode=lfs".
184usrquota Enable plain user disk quota accounting.
185grpquota Enable plain group disk quota accounting.
186prjquota Enable plain project quota accounting.
187usrjquota=<file> Appoint specified file and type during mount, so that quota
188grpjquota=<file> information can be properly updated during recovery flow,
189prjjquota=<file> <quota file>: must be in root directory;
190jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
191offusrjquota Turn off user journelled quota.
192offgrpjquota Turn off group journelled quota.
193offprjjquota Turn off project journelled quota.
194quota Enable plain user disk quota accounting.
195noquota Disable all plain disk quota option.
196whint_mode=%s Control which write hints are passed down to block
197 layer. This supports "off", "user-based", and
198 "fs-based". In "off" mode (default), f2fs does not pass
199 down hints. In "user-based" mode, f2fs tries to pass
200 down hints given by users. And in "fs-based" mode, f2fs
201 passes down hints with its policy.
202alloc_mode=%s Adjust block allocation policy, which supports "reuse"
203 and "default".
204fsync_mode=%s Control the policy of fsync. Currently supports "posix",
205 "strict", and "nobarrier". In "posix" mode, which is
206 default, fsync will follow POSIX semantics and does a
207 light operation to improve the filesystem performance.
208 In "strict" mode, fsync will be heavy and behaves in line
209 with xfs, ext4 and btrfs, where xfstest generic/342 will
210 pass, but the performance will regress. "nobarrier" is
211 based on "posix", but doesn't issue flush command for
212 non-atomic files likewise "nobarrier" mount option.
213test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt
214 context. The fake fscrypt context is used by xfstests.
215checkpoint=%s Set to "disable" to turn off checkpointing. Set to "enable"
216 to reenable checkpointing. Is enabled by default. While
217 disabled, any unmounting or unexpected shutdowns will cause
218 the filesystem contents to appear as they did when the
219 filesystem was mounted with that option.
220
221================================================================================
222DEBUGFS ENTRIES
223================================================================================
224
225/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
226f2fs. Each file shows the whole f2fs information.
227
228/sys/kernel/debug/f2fs/status includes:
229 - major file system information managed by f2fs currently
230 - average SIT information about whole segments
231 - current memory footprint consumed by f2fs.
232
233================================================================================
234SYSFS ENTRIES
235================================================================================
236
237Information about mounted f2fs file systems can be found in
238/sys/fs/f2fs. Each mounted filesystem will have a directory in
239/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
240The files in each per-device directory are shown in table below.
241
242Files in /sys/fs/f2fs/<devname>
243(see also Documentation/ABI/testing/sysfs-fs-f2fs)
244..............................................................................
245 File Content
246
247 gc_max_sleep_time This tuning parameter controls the maximum sleep
248 time for the garbage collection thread. Time is
249 in milliseconds.
250
251 gc_min_sleep_time This tuning parameter controls the minimum sleep
252 time for the garbage collection thread. Time is
253 in milliseconds.
254
255 gc_no_gc_sleep_time This tuning parameter controls the default sleep
256 time for the garbage collection thread. Time is
257 in milliseconds.
258
259 gc_idle This parameter controls the selection of victim
260 policy for garbage collection. Setting gc_idle = 0
261 (default) will disable this option. Setting
262 gc_idle = 1 will select the Cost Benefit approach
263 & setting gc_idle = 2 will select the greedy approach.
264
265 gc_urgent This parameter controls triggering background GCs
266 urgently or not. Setting gc_urgent = 0 [default]
267 makes back to default behavior, while if it is set
268 to 1, background thread starts to do GC by given
269 gc_urgent_sleep_time interval.
270
271 gc_urgent_sleep_time This parameter controls sleep time for gc_urgent.
272 500 ms is set by default. See above gc_urgent.
273
274 reclaim_segments This parameter controls the number of prefree
275 segments to be reclaimed. If the number of prefree
276 segments is larger than the number of segments
277 in the proportion to the percentage over total
278 volume size, f2fs tries to conduct checkpoint to
279 reclaim the prefree segments to free segments.
280 By default, 5% over total # of segments.
281
282 max_small_discards This parameter controls the number of discard
283 commands that consist small blocks less than 2MB.
284 The candidates to be discarded are cached until
285 checkpoint is triggered, and issued during the
286 checkpoint. By default, it is disabled with 0.
287
288 trim_sections This parameter controls the number of sections
289 to be trimmed out in batch mode when FITRIM
290 conducts. 32 sections is set by default.
291
292 ipu_policy This parameter controls the policy of in-place
293 updates in f2fs. There are five policies:
294 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
295 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
296 0x10: F2FS_IPU_FSYNC.
297
298 min_ipu_util This parameter controls the threshold to trigger
299 in-place-updates. The number indicates percentage
300 of the filesystem utilization, and used by
301 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
302
303 min_fsync_blocks This parameter controls the threshold to trigger
304 in-place-updates when F2FS_IPU_FSYNC mode is set.
305 The number indicates the number of dirty pages
306 when fsync needs to flush on its call path. If
307 the number is less than this value, it triggers
308 in-place-updates.
309
310 max_victim_search This parameter controls the number of trials to
311 find a victim segment when conducting SSR and
312 cleaning operations. The default value is 4096
313 which covers 8GB block address range.
314
315 dir_level This parameter controls the directory level to
316 support large directory. If a directory has a
317 number of files, it can reduce the file lookup
318 latency by increasing this dir_level value.
319 Otherwise, it needs to decrease this value to
320 reduce the space overhead. The default value is 0.
321
322 ram_thresh This parameter controls the memory footprint used
323 by free nids and cached nat entries. By default,
324 10 is set, which indicates 10 MB / 1 GB RAM.
325
326================================================================================
327USAGE
328================================================================================
329
3301. Download userland tools and compile them.
331
3322. Skip, if f2fs was compiled statically inside kernel.
333 Otherwise, insert the f2fs.ko module.
334 # insmod f2fs.ko
335
3363. Create a directory trying to mount
337 # mkdir /mnt/f2fs
338
3394. Format the block device, and then mount as f2fs
340 # mkfs.f2fs -l label /dev/block_device
341 # mount -t f2fs /dev/block_device /mnt/f2fs
342
343mkfs.f2fs
344---------
345The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
346which builds a basic on-disk layout.
347
348The options consist of:
349-l [label] : Give a volume label, up to 512 unicode name.
350-a [0 or 1] : Split start location of each area for heap-based allocation.
351 1 is set by default, which performs this.
352-o [int] : Set overprovision ratio in percent over volume size.
353 5 is set by default.
354-s [int] : Set the number of segments per section.
355 1 is set by default.
356-z [int] : Set the number of sections per zone.
357 1 is set by default.
358-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
359-t [0 or 1] : Disable discard command or not.
360 1 is set by default, which conducts discard.
361
362fsck.f2fs
363---------
364The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
365partition, which examines whether the filesystem metadata and user-made data
366are cross-referenced correctly or not.
367Note that, initial version of the tool does not fix any inconsistency.
368
369The options consist of:
370 -d debug level [default:0]
371
372dump.f2fs
373---------
374The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
375file. Each file is dump_ssa and dump_sit.
376
377The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
378It shows on-disk inode information recognized by a given inode number, and is
379able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
380./dump_sit respectively.
381
382The options consist of:
383 -d debug level [default:0]
384 -i inode no (hex)
385 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
386 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
387
388Examples:
389# dump.f2fs -i [ino] /dev/sdx
390# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
391# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
392
393================================================================================
394DESIGN
395================================================================================
396
397On-disk Layout
398--------------
399
400F2FS divides the whole volume into a number of segments, each of which is fixed
401to 2MB in size. A section is composed of consecutive segments, and a zone
402consists of a set of sections. By default, section and zone sizes are set to one
403segment size identically, but users can easily modify the sizes by mkfs.
404
405F2FS splits the entire volume into six areas, and all the areas except superblock
406consists of multiple segments as described below.
407
408 align with the zone size <-|
409 |-> align with the segment size
410 _________________________________________________________________________
411 | | | Segment | Node | Segment | |
412 | Superblock | Checkpoint | Info. | Address | Summary | Main |
413 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
414 |____________|_____2______|______N______|______N______|______N_____|__N___|
415 . .
416 . .
417 . .
418 ._________________________________________.
419 |_Segment_|_..._|_Segment_|_..._|_Segment_|
420 . .
421 ._________._________
422 |_section_|__...__|_
423 . .
424 .________.
425 |__zone__|
426
427- Superblock (SB)
428 : It is located at the beginning of the partition, and there exist two copies
429 to avoid file system crash. It contains basic partition information and some
430 default parameters of f2fs.
431
432- Checkpoint (CP)
433 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
434 inode lists, and summary entries of current active segments.
435
436- Segment Information Table (SIT)
437 : It contains segment information such as valid block count and bitmap for the
438 validity of all the blocks.
439
440- Node Address Table (NAT)
441 : It is composed of a block address table for all the node blocks stored in
442 Main area.
443
444- Segment Summary Area (SSA)
445 : It contains summary entries which contains the owner information of all the
446 data and node blocks stored in Main area.
447
448- Main Area
449 : It contains file and directory data including their indices.
450
451In order to avoid misalignment between file system and flash-based storage, F2FS
452aligns the start block address of CP with the segment size. Also, it aligns the
453start block address of Main area with the zone size by reserving some segments
454in SSA area.
455
456Reference the following survey for additional technical details.
457https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
458
459File System Metadata Structure
460------------------------------
461
462F2FS adopts the checkpointing scheme to maintain file system consistency. At
463mount time, F2FS first tries to find the last valid checkpoint data by scanning
464CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
465One of them always indicates the last valid data, which is called as shadow copy
466mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
467
468For file system consistency, each CP points to which NAT and SIT copies are
469valid, as shown as below.
470
471 +--------+----------+---------+
472 | CP | SIT | NAT |
473 +--------+----------+---------+
474 . . . .
475 . . . .
476 . . . .
477 +-------+-------+--------+--------+--------+--------+
478 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
479 +-------+-------+--------+--------+--------+--------+
480 | ^ ^
481 | | |
482 `----------------------------------------'
483
484Index Structure
485---------------
486
487The key data structure to manage the data locations is a "node". Similar to
488traditional file structures, F2FS has three types of node: inode, direct node,
489indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
490indices, two direct node pointers, two indirect node pointers, and one double
491indirect node pointer as described below. One direct node block contains 1018
492data blocks, and one indirect node block contains also 1018 node blocks. Thus,
493one inode block (i.e., a file) covers:
494
495 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
496
497 Inode block (4KB)
498 |- data (923)
499 |- direct node (2)
500 | `- data (1018)
501 |- indirect node (2)
502 | `- direct node (1018)
503 | `- data (1018)
504 `- double indirect node (1)
505 `- indirect node (1018)
506 `- direct node (1018)
507 `- data (1018)
508
509Note that, all the node blocks are mapped by NAT which means the location of
510each node is translated by the NAT table. In the consideration of the wandering
511tree problem, F2FS is able to cut off the propagation of node updates caused by
512leaf data writes.
513
514Directory Structure
515-------------------
516
517A directory entry occupies 11 bytes, which consists of the following attributes.
518
519- hash hash value of the file name
520- ino inode number
521- len the length of file name
522- type file type such as directory, symlink, etc
523
524A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
525used to represent whether each dentry is valid or not. A dentry block occupies
5264KB with the following composition.
527
528 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
529 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
530
531 [Bucket]
532 +--------------------------------+
533 |dentry block 1 | dentry block 2 |
534 +--------------------------------+
535 . .
536 . .
537 . [Dentry Block Structure: 4KB] .
538 +--------+----------+----------+------------+
539 | bitmap | reserved | dentries | file names |
540 +--------+----------+----------+------------+
541 [Dentry Block: 4KB] . .
542 . .
543 . .
544 +------+------+-----+------+
545 | hash | ino | len | type |
546 +------+------+-----+------+
547 [Dentry Structure: 11 bytes]
548
549F2FS implements multi-level hash tables for directory structure. Each level has
550a hash table with dedicated number of hash buckets as shown below. Note that
551"A(2B)" means a bucket includes 2 data blocks.
552
553----------------------
554A : bucket
555B : block
556N : MAX_DIR_HASH_DEPTH
557----------------------
558
559level #0 | A(2B)
560 |
561level #1 | A(2B) - A(2B)
562 |
563level #2 | A(2B) - A(2B) - A(2B) - A(2B)
564 . | . . . .
565level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
566 . | . . . .
567level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
568
569The number of blocks and buckets are determined by,
570
571 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
572 # of blocks in level #n = |
573 `- 4, Otherwise
574
575 ,- 2^(n + dir_level),
576 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
577 # of buckets in level #n = |
578 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
579 Otherwise
580
581When F2FS finds a file name in a directory, at first a hash value of the file
582name is calculated. Then, F2FS scans the hash table in level #0 to find the
583dentry consisting of the file name and its inode number. If not found, F2FS
584scans the next hash table in level #1. In this way, F2FS scans hash tables in
585each levels incrementally from 1 to N. In each levels F2FS needs to scan only
586one bucket determined by the following equation, which shows O(log(# of files))
587complexity.
588
589 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
590
591In the case of file creation, F2FS finds empty consecutive slots that cover the
592file name. F2FS searches the empty slots in the hash tables of whole levels from
5931 to N in the same way as the lookup operation.
594
595The following figure shows an example of two cases holding children.
596 --------------> Dir <--------------
597 | |
598 child child
599
600 child - child [hole] - child
601
602 child - child - child [hole] - [hole] - child
603
604 Case 1: Case 2:
605 Number of children = 6, Number of children = 3,
606 File size = 7 File size = 7
607
608Default Block Allocation
609------------------------
610
611At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
612and Hot/Warm/Cold data.
613
614- Hot node contains direct node blocks of directories.
615- Warm node contains direct node blocks except hot node blocks.
616- Cold node contains indirect node blocks
617- Hot data contains dentry blocks
618- Warm data contains data blocks except hot and cold data blocks
619- Cold data contains multimedia data or migrated data blocks
620
621LFS has two schemes for free space management: threaded log and copy-and-compac-
622tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
623for devices showing very good sequential write performance, since free segments
624are served all the time for writing new data. However, it suffers from cleaning
625overhead under high utilization. Contrarily, the threaded log scheme suffers
626from random writes, but no cleaning process is needed. F2FS adopts a hybrid
627scheme where the copy-and-compaction scheme is adopted by default, but the
628policy is dynamically changed to the threaded log scheme according to the file
629system status.
630
631In order to align F2FS with underlying flash-based storage, F2FS allocates a
632segment in a unit of section. F2FS expects that the section size would be the
633same as the unit size of garbage collection in FTL. Furthermore, with respect
634to the mapping granularity in FTL, F2FS allocates each section of the active
635logs from different zones as much as possible, since FTL can write the data in
636the active logs into one allocation unit according to its mapping granularity.
637
638Cleaning process
639----------------
640
641F2FS does cleaning both on demand and in the background. On-demand cleaning is
642triggered when there are not enough free segments to serve VFS calls. Background
643cleaner is operated by a kernel thread, and triggers the cleaning job when the
644system is idle.
645
646F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
647In the greedy algorithm, F2FS selects a victim segment having the smallest number
648of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
649according to the segment age and the number of valid blocks in order to address
650log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
651algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
652algorithm.
653
654In order to identify whether the data in the victim segment are valid or not,
655F2FS manages a bitmap. Each bit represents the validity of a block, and the
656bitmap is composed of a bit stream covering whole blocks in main area.
657
658Write-hint Policy
659-----------------
660
6611) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET.
662
6632) whint_mode=user-based. F2FS tries to pass down hints given by
664users.
665
666User F2FS Block
667---- ---- -----
668 META WRITE_LIFE_NOT_SET
669 HOT_NODE "
670 WARM_NODE "
671 COLD_NODE "
672*ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
673*extension list " "
674
675-- buffered io
676WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
677WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
678WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
679WRITE_LIFE_NONE " "
680WRITE_LIFE_MEDIUM " "
681WRITE_LIFE_LONG " "
682
683-- direct io
684WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
685WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
686WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
687WRITE_LIFE_NONE " WRITE_LIFE_NONE
688WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
689WRITE_LIFE_LONG " WRITE_LIFE_LONG
690
6913) whint_mode=fs-based. F2FS passes down hints with its policy.
692
693User F2FS Block
694---- ---- -----
695 META WRITE_LIFE_MEDIUM;
696 HOT_NODE WRITE_LIFE_NOT_SET
697 WARM_NODE "
698 COLD_NODE WRITE_LIFE_NONE
699ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME
700extension list " "
701
702-- buffered io
703WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
704WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
705WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG
706WRITE_LIFE_NONE " "
707WRITE_LIFE_MEDIUM " "
708WRITE_LIFE_LONG " "
709
710-- direct io
711WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME
712WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT
713WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET
714WRITE_LIFE_NONE " WRITE_LIFE_NONE
715WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM
716WRITE_LIFE_LONG " WRITE_LIFE_LONG