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1 ================================================================================
2 WHAT IS Flash-Friendly File System (F2FS)?
3 ================================================================================
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.
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.
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.
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
25 For reporting bugs and sending patches, please use the following mailing list:
26 >> linux-f2fs-devel@lists.sourceforge.net
28 ================================================================================
29 BACKGROUND AND DESIGN ISSUES
30 ================================================================================
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.
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.
54 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
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.
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.
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.
74 ================================================================================
75 KEY FEATURES
76 ================================================================================
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
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.
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
97 ================================================================================
98 MOUNT OPTIONS
99 ================================================================================
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 truned off.
106 Default value for this option is on. So garbage
107 collection is on by default.
108 disable_roll_forward Disable the roll-forward recovery routine
109 discard Issue discard/TRIM commands when a segment is cleaned.
110 no_heap Disable heap-style segment allocation which finds free
111 segments for data from the beginning of main area, while
112 for node from the end of main area.
113 nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
114 by default if CONFIG_F2FS_FS_XATTR is selected.
115 noacl Disable POSIX Access Control List. Note: acl is enabled
116 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
117 active_logs=%u Support configuring the number of active logs. In the
118 current design, f2fs supports only 2, 4, and 6 logs.
119 Default number is 6.
120 disable_ext_identify Disable the extension list configured by mkfs, so f2fs
121 does not aware of cold files such as media files.
122 inline_xattr Enable the inline xattrs feature.
123 inline_data Enable the inline data feature: New created small(<~3.4k)
124 files can be written into inode block.
125 flush_merge Merge concurrent cache_flush commands as much as possible
126 to eliminate redundant command issues. If the underlying
127 device handles the cache_flush command relatively slowly,
128 recommend to enable this option.
129 nobarrier This option can be used if underlying storage guarantees
130 its cached data should be written to the novolatile area.
131 If this option is set, no cache_flush commands are issued
132 but f2fs still guarantees the write ordering of all the
133 data writes.
135 ================================================================================
136 DEBUGFS ENTRIES
137 ================================================================================
139 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
140 f2fs. Each file shows the whole f2fs information.
142 /sys/kernel/debug/f2fs/status includes:
143 - major file system information managed by f2fs currently
144 - average SIT information about whole segments
145 - current memory footprint consumed by f2fs.
147 ================================================================================
148 SYSFS ENTRIES
149 ================================================================================
151 Information about mounted f2f2 file systems can be found in
152 /sys/fs/f2fs. Each mounted filesystem will have a directory in
153 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
154 The files in each per-device directory are shown in table below.
156 Files in /sys/fs/f2fs/<devname>
157 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
158 ..............................................................................
159 File Content
161 gc_max_sleep_time This tuning parameter controls the maximum sleep
162 time for the garbage collection thread. Time is
163 in milliseconds.
165 gc_min_sleep_time This tuning parameter controls the minimum sleep
166 time for the garbage collection thread. Time is
167 in milliseconds.
169 gc_no_gc_sleep_time This tuning parameter controls the default sleep
170 time for the garbage collection thread. Time is
171 in milliseconds.
173 gc_idle This parameter controls the selection of victim
174 policy for garbage collection. Setting gc_idle = 0
175 (default) will disable this option. Setting
176 gc_idle = 1 will select the Cost Benefit approach
177 & setting gc_idle = 2 will select the greedy aproach.
179 reclaim_segments This parameter controls the number of prefree
180 segments to be reclaimed. If the number of prefree
181 segments is larger than the number of segments
182 in the proportion to the percentage over total
183 volume size, f2fs tries to conduct checkpoint to
184 reclaim the prefree segments to free segments.
185 By default, 5% over total # of segments.
187 max_small_discards This parameter controls the number of discard
188 commands that consist small blocks less than 2MB.
189 The candidates to be discarded are cached until
190 checkpoint is triggered, and issued during the
191 checkpoint. By default, it is disabled with 0.
193 ipu_policy This parameter controls the policy of in-place
194 updates in f2fs. There are five policies:
195 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
196 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
197 0x10: F2FS_IPU_FSYNC.
199 min_ipu_util This parameter controls the threshold to trigger
200 in-place-updates. The number indicates percentage
201 of the filesystem utilization, and used by
202 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
204 min_fsync_blocks This parameter controls the threshold to trigger
205 in-place-updates when F2FS_IPU_FSYNC mode is set.
206 The number indicates the number of dirty pages
207 when fsync needs to flush on its call path. If
208 the number is less than this value, it triggers
209 in-place-updates.
211 max_victim_search This parameter controls the number of trials to
212 find a victim segment when conducting SSR and
213 cleaning operations. The default value is 4096
214 which covers 8GB block address range.
216 dir_level This parameter controls the directory level to
217 support large directory. If a directory has a
218 number of files, it can reduce the file lookup
219 latency by increasing this dir_level value.
220 Otherwise, it needs to decrease this value to
221 reduce the space overhead. The default value is 0.
223 ram_thresh This parameter controls the memory footprint used
224 by free nids and cached nat entries. By default,
225 10 is set, which indicates 10 MB / 1 GB RAM.
227 ================================================================================
228 USAGE
229 ================================================================================
231 1. Download userland tools and compile them.
233 2. Skip, if f2fs was compiled statically inside kernel.
234 Otherwise, insert the f2fs.ko module.
235 # insmod f2fs.ko
237 3. Create a directory trying to mount
238 # mkdir /mnt/f2fs
240 4. Format the block device, and then mount as f2fs
241 # mkfs.f2fs -l label /dev/block_device
242 # mount -t f2fs /dev/block_device /mnt/f2fs
244 mkfs.f2fs
245 ---------
246 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
247 which builds a basic on-disk layout.
249 The options consist of:
250 -l [label] : Give a volume label, up to 512 unicode name.
251 -a [0 or 1] : Split start location of each area for heap-based allocation.
252 1 is set by default, which performs this.
253 -o [int] : Set overprovision ratio in percent over volume size.
254 5 is set by default.
255 -s [int] : Set the number of segments per section.
256 1 is set by default.
257 -z [int] : Set the number of sections per zone.
258 1 is set by default.
259 -e [str] : Set basic extension list. e.g. "mp3,gif,mov"
260 -t [0 or 1] : Disable discard command or not.
261 1 is set by default, which conducts discard.
263 fsck.f2fs
264 ---------
265 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
266 partition, which examines whether the filesystem metadata and user-made data
267 are cross-referenced correctly or not.
268 Note that, initial version of the tool does not fix any inconsistency.
270 The options consist of:
271 -d debug level [default:0]
273 dump.f2fs
274 ---------
275 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
276 file. Each file is dump_ssa and dump_sit.
278 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
279 It shows on-disk inode information reconized by a given inode number, and is
280 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
281 ./dump_sit respectively.
283 The options consist of:
284 -d debug level [default:0]
285 -i inode no (hex)
286 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
287 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
289 Examples:
290 # dump.f2fs -i [ino] /dev/sdx
291 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
292 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
294 ================================================================================
295 DESIGN
296 ================================================================================
298 On-disk Layout
299 --------------
301 F2FS divides the whole volume into a number of segments, each of which is fixed
302 to 2MB in size. A section is composed of consecutive segments, and a zone
303 consists of a set of sections. By default, section and zone sizes are set to one
304 segment size identically, but users can easily modify the sizes by mkfs.
306 F2FS splits the entire volume into six areas, and all the areas except superblock
307 consists of multiple segments as described below.
309 align with the zone size <-|
310 |-> align with the segment size
311 _________________________________________________________________________
312 | | | Segment | Node | Segment | |
313 | Superblock | Checkpoint | Info. | Address | Summary | Main |
314 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
315 |____________|_____2______|______N______|______N______|______N_____|__N___|
316 . .
317 . .
318 . .
319 ._________________________________________.
320 |_Segment_|_..._|_Segment_|_..._|_Segment_|
321 . .
322 ._________._________
323 |_section_|__...__|_
324 . .
325 .________.
326 |__zone__|
328 - Superblock (SB)
329 : It is located at the beginning of the partition, and there exist two copies
330 to avoid file system crash. It contains basic partition information and some
331 default parameters of f2fs.
333 - Checkpoint (CP)
334 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
335 inode lists, and summary entries of current active segments.
337 - Segment Information Table (SIT)
338 : It contains segment information such as valid block count and bitmap for the
339 validity of all the blocks.
341 - Node Address Table (NAT)
342 : It is composed of a block address table for all the node blocks stored in
343 Main area.
345 - Segment Summary Area (SSA)
346 : It contains summary entries which contains the owner information of all the
347 data and node blocks stored in Main area.
349 - Main Area
350 : It contains file and directory data including their indices.
352 In order to avoid misalignment between file system and flash-based storage, F2FS
353 aligns the start block address of CP with the segment size. Also, it aligns the
354 start block address of Main area with the zone size by reserving some segments
355 in SSA area.
357 Reference the following survey for additional technical details.
358 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
360 File System Metadata Structure
361 ------------------------------
363 F2FS adopts the checkpointing scheme to maintain file system consistency. At
364 mount time, F2FS first tries to find the last valid checkpoint data by scanning
365 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
366 One of them always indicates the last valid data, which is called as shadow copy
367 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
369 For file system consistency, each CP points to which NAT and SIT copies are
370 valid, as shown as below.
372 +--------+----------+---------+
373 | CP | SIT | NAT |
374 +--------+----------+---------+
375 . . . .
376 . . . .
377 . . . .
378 +-------+-------+--------+--------+--------+--------+
379 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
380 +-------+-------+--------+--------+--------+--------+
381 | ^ ^
382 | | |
383 `----------------------------------------'
385 Index Structure
386 ---------------
388 The key data structure to manage the data locations is a "node". Similar to
389 traditional file structures, F2FS has three types of node: inode, direct node,
390 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
391 indices, two direct node pointers, two indirect node pointers, and one double
392 indirect node pointer as described below. One direct node block contains 1018
393 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
394 one inode block (i.e., a file) covers:
396 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
398 Inode block (4KB)
399 |- data (923)
400 |- direct node (2)
401 | `- data (1018)
402 |- indirect node (2)
403 | `- direct node (1018)
404 | `- data (1018)
405 `- double indirect node (1)
406 `- indirect node (1018)
407 `- direct node (1018)
408 `- data (1018)
410 Note that, all the node blocks are mapped by NAT which means the location of
411 each node is translated by the NAT table. In the consideration of the wandering
412 tree problem, F2FS is able to cut off the propagation of node updates caused by
413 leaf data writes.
415 Directory Structure
416 -------------------
418 A directory entry occupies 11 bytes, which consists of the following attributes.
420 - hash hash value of the file name
421 - ino inode number
422 - len the length of file name
423 - type file type such as directory, symlink, etc
425 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
426 used to represent whether each dentry is valid or not. A dentry block occupies
427 4KB with the following composition.
429 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
430 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
432 [Bucket]
433 +--------------------------------+
434 |dentry block 1 | dentry block 2 |
435 +--------------------------------+
436 . .
437 . .
438 . [Dentry Block Structure: 4KB] .
439 +--------+----------+----------+------------+
440 | bitmap | reserved | dentries | file names |
441 +--------+----------+----------+------------+
442 [Dentry Block: 4KB] . .
443 . .
444 . .
445 +------+------+-----+------+
446 | hash | ino | len | type |
447 +------+------+-----+------+
448 [Dentry Structure: 11 bytes]
450 F2FS implements multi-level hash tables for directory structure. Each level has
451 a hash table with dedicated number of hash buckets as shown below. Note that
452 "A(2B)" means a bucket includes 2 data blocks.
454 ----------------------
455 A : bucket
456 B : block
457 N : MAX_DIR_HASH_DEPTH
458 ----------------------
460 level #0 | A(2B)
461 |
462 level #1 | A(2B) - A(2B)
463 |
464 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
465 . | . . . .
466 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
467 . | . . . .
468 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
470 The number of blocks and buckets are determined by,
472 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
473 # of blocks in level #n = |
474 `- 4, Otherwise
476 ,- 2^(n + dir_level),
477 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
478 # of buckets in level #n = |
479 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
480 Otherwise
482 When F2FS finds a file name in a directory, at first a hash value of the file
483 name is calculated. Then, F2FS scans the hash table in level #0 to find the
484 dentry consisting of the file name and its inode number. If not found, F2FS
485 scans the next hash table in level #1. In this way, F2FS scans hash tables in
486 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
487 one bucket determined by the following equation, which shows O(log(# of files))
488 complexity.
490 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
492 In the case of file creation, F2FS finds empty consecutive slots that cover the
493 file name. F2FS searches the empty slots in the hash tables of whole levels from
494 1 to N in the same way as the lookup operation.
496 The following figure shows an example of two cases holding children.
497 --------------> Dir <--------------
498 | |
499 child child
501 child - child [hole] - child
503 child - child - child [hole] - [hole] - child
505 Case 1: Case 2:
506 Number of children = 6, Number of children = 3,
507 File size = 7 File size = 7
509 Default Block Allocation
510 ------------------------
512 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
513 and Hot/Warm/Cold data.
515 - Hot node contains direct node blocks of directories.
516 - Warm node contains direct node blocks except hot node blocks.
517 - Cold node contains indirect node blocks
518 - Hot data contains dentry blocks
519 - Warm data contains data blocks except hot and cold data blocks
520 - Cold data contains multimedia data or migrated data blocks
522 LFS has two schemes for free space management: threaded log and copy-and-compac-
523 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
524 for devices showing very good sequential write performance, since free segments
525 are served all the time for writing new data. However, it suffers from cleaning
526 overhead under high utilization. Contrarily, the threaded log scheme suffers
527 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
528 scheme where the copy-and-compaction scheme is adopted by default, but the
529 policy is dynamically changed to the threaded log scheme according to the file
530 system status.
532 In order to align F2FS with underlying flash-based storage, F2FS allocates a
533 segment in a unit of section. F2FS expects that the section size would be the
534 same as the unit size of garbage collection in FTL. Furthermore, with respect
535 to the mapping granularity in FTL, F2FS allocates each section of the active
536 logs from different zones as much as possible, since FTL can write the data in
537 the active logs into one allocation unit according to its mapping granularity.
539 Cleaning process
540 ----------------
542 F2FS does cleaning both on demand and in the background. On-demand cleaning is
543 triggered when there are not enough free segments to serve VFS calls. Background
544 cleaner is operated by a kernel thread, and triggers the cleaning job when the
545 system is idle.
547 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
548 In the greedy algorithm, F2FS selects a victim segment having the smallest number
549 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
550 according to the segment age and the number of valid blocks in order to address
551 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
552 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
553 algorithm.
555 In order to identify whether the data in the victim segment are valid or not,
556 F2FS manages a bitmap. Each bit represents the validity of a block, and the
557 bitmap is composed of a bit stream covering whole blocks in main area.