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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, 26â52.
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, ânodeâ, that represents inodes as well as various pointer blocks
87 - Introduce Node Address Table (NAT) containing the locations of all the ânodeâ
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.
126 ================================================================================
127 DEBUGFS ENTRIES
128 ================================================================================
130 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
131 f2fs. Each file shows the whole f2fs information.
133 /sys/kernel/debug/f2fs/status includes:
134 - major file system information managed by f2fs currently
135 - average SIT information about whole segments
136 - current memory footprint consumed by f2fs.
138 ================================================================================
139 SYSFS ENTRIES
140 ================================================================================
142 Information about mounted f2f2 file systems can be found in
143 /sys/fs/f2fs. Each mounted filesystem will have a directory in
144 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
145 The files in each per-device directory are shown in table below.
147 Files in /sys/fs/f2fs/<devname>
148 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
149 ..............................................................................
150 File Content
152 gc_max_sleep_time This tuning parameter controls the maximum sleep
153 time for the garbage collection thread. Time is
154 in milliseconds.
156 gc_min_sleep_time This tuning parameter controls the minimum sleep
157 time for the garbage collection thread. Time is
158 in milliseconds.
160 gc_no_gc_sleep_time This tuning parameter controls the default sleep
161 time for the garbage collection thread. Time is
162 in milliseconds.
164 gc_idle This parameter controls the selection of victim
165 policy for garbage collection. Setting gc_idle = 0
166 (default) will disable this option. Setting
167 gc_idle = 1 will select the Cost Benefit approach
168 & setting gc_idle = 2 will select the greedy aproach.
170 reclaim_segments This parameter controls the number of prefree
171 segments to be reclaimed. If the number of prefree
172 segments is larger than this number, f2fs tries to
173 conduct checkpoint to reclaim the prefree segments
174 to free segments. By default, 100 segments, 200MB.
176 max_small_discards This parameter controls the number of discard
177 commands that consist small blocks less than 2MB.
178 The candidates to be discarded are cached until
179 checkpoint is triggered, and issued during the
180 checkpoint. By default, it is disabled with 0.
182 ipu_policy This parameter controls the policy of in-place
183 updates in f2fs. There are five policies:
184 0: F2FS_IPU_FORCE, 1: F2FS_IPU_SSR,
185 2: F2FS_IPU_UTIL, 3: F2FS_IPU_SSR_UTIL,
186 4: F2FS_IPU_DISABLE.
188 min_ipu_util This parameter controls the threshold to trigger
189 in-place-updates. The number indicates percentage
190 of the filesystem utilization, and used by
191 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
193 max_victim_search This parameter controls the number of trials to
194 find a victim segment when conducting SSR and
195 cleaning operations. The default value is 4096
196 which covers 8GB block address range.
198 ================================================================================
199 USAGE
200 ================================================================================
202 1. Download userland tools and compile them.
204 2. Skip, if f2fs was compiled statically inside kernel.
205 Otherwise, insert the f2fs.ko module.
206 # insmod f2fs.ko
208 3. Create a directory trying to mount
209 # mkdir /mnt/f2fs
211 4. Format the block device, and then mount as f2fs
212 # mkfs.f2fs -l label /dev/block_device
213 # mount -t f2fs /dev/block_device /mnt/f2fs
215 mkfs.f2fs
216 ---------
217 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
218 which builds a basic on-disk layout.
220 The options consist of:
221 -l [label] : Give a volume label, up to 512 unicode name.
222 -a [0 or 1] : Split start location of each area for heap-based allocation.
223 1 is set by default, which performs this.
224 -o [int] : Set overprovision ratio in percent over volume size.
225 5 is set by default.
226 -s [int] : Set the number of segments per section.
227 1 is set by default.
228 -z [int] : Set the number of sections per zone.
229 1 is set by default.
230 -e [str] : Set basic extension list. e.g. "mp3,gif,mov"
231 -t [0 or 1] : Disable discard command or not.
232 1 is set by default, which conducts discard.
234 fsck.f2fs
235 ---------
236 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
237 partition, which examines whether the filesystem metadata and user-made data
238 are cross-referenced correctly or not.
239 Note that, initial version of the tool does not fix any inconsistency.
241 The options consist of:
242 -d debug level [default:0]
244 dump.f2fs
245 ---------
246 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
247 file. Each file is dump_ssa and dump_sit.
249 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
250 It shows on-disk inode information reconized by a given inode number, and is
251 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
252 ./dump_sit respectively.
254 The options consist of:
255 -d debug level [default:0]
256 -i inode no (hex)
257 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
258 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
260 Examples:
261 # dump.f2fs -i [ino] /dev/sdx
262 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
263 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
265 ================================================================================
266 DESIGN
267 ================================================================================
269 On-disk Layout
270 --------------
272 F2FS divides the whole volume into a number of segments, each of which is fixed
273 to 2MB in size. A section is composed of consecutive segments, and a zone
274 consists of a set of sections. By default, section and zone sizes are set to one
275 segment size identically, but users can easily modify the sizes by mkfs.
277 F2FS splits the entire volume into six areas, and all the areas except superblock
278 consists of multiple segments as described below.
280 align with the zone size <-|
281 |-> align with the segment size
282 _________________________________________________________________________
283 | | | Segment | Node | Segment | |
284 | Superblock | Checkpoint | Info. | Address | Summary | Main |
285 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
286 |____________|_____2______|______N______|______N______|______N_____|__N___|
287 . .
288 . .
289 . .
290 ._________________________________________.
291 |_Segment_|_..._|_Segment_|_..._|_Segment_|
292 . .
293 ._________._________
294 |_section_|__...__|_
295 . .
296 .________.
297 |__zone__|
299 - Superblock (SB)
300 : It is located at the beginning of the partition, and there exist two copies
301 to avoid file system crash. It contains basic partition information and some
302 default parameters of f2fs.
304 - Checkpoint (CP)
305 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
306 inode lists, and summary entries of current active segments.
308 - Segment Information Table (SIT)
309 : It contains segment information such as valid block count and bitmap for the
310 validity of all the blocks.
312 - Node Address Table (NAT)
313 : It is composed of a block address table for all the node blocks stored in
314 Main area.
316 - Segment Summary Area (SSA)
317 : It contains summary entries which contains the owner information of all the
318 data and node blocks stored in Main area.
320 - Main Area
321 : It contains file and directory data including their indices.
323 In order to avoid misalignment between file system and flash-based storage, F2FS
324 aligns the start block address of CP with the segment size. Also, it aligns the
325 start block address of Main area with the zone size by reserving some segments
326 in SSA area.
328 Reference the following survey for additional technical details.
329 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
331 File System Metadata Structure
332 ------------------------------
334 F2FS adopts the checkpointing scheme to maintain file system consistency. At
335 mount time, F2FS first tries to find the last valid checkpoint data by scanning
336 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
337 One of them always indicates the last valid data, which is called as shadow copy
338 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
340 For file system consistency, each CP points to which NAT and SIT copies are
341 valid, as shown as below.
343 +--------+----------+---------+
344 | CP | SIT | NAT |
345 +--------+----------+---------+
346 . . . .
347 . . . .
348 . . . .
349 +-------+-------+--------+--------+--------+--------+
350 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
351 +-------+-------+--------+--------+--------+--------+
352 | ^ ^
353 | | |
354 `----------------------------------------'
356 Index Structure
357 ---------------
359 The key data structure to manage the data locations is a "node". Similar to
360 traditional file structures, F2FS has three types of node: inode, direct node,
361 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
362 indices, two direct node pointers, two indirect node pointers, and one double
363 indirect node pointer as described below. One direct node block contains 1018
364 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
365 one inode block (i.e., a file) covers:
367 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
369 Inode block (4KB)
370 |- data (923)
371 |- direct node (2)
372 | `- data (1018)
373 |- indirect node (2)
374 | `- direct node (1018)
375 | `- data (1018)
376 `- double indirect node (1)
377 `- indirect node (1018)
378 `- direct node (1018)
379 `- data (1018)
381 Note that, all the node blocks are mapped by NAT which means the location of
382 each node is translated by the NAT table. In the consideration of the wandering
383 tree problem, F2FS is able to cut off the propagation of node updates caused by
384 leaf data writes.
386 Directory Structure
387 -------------------
389 A directory entry occupies 11 bytes, which consists of the following attributes.
391 - hash hash value of the file name
392 - ino inode number
393 - len the length of file name
394 - type file type such as directory, symlink, etc
396 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
397 used to represent whether each dentry is valid or not. A dentry block occupies
398 4KB with the following composition.
400 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
401 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
403 [Bucket]
404 +--------------------------------+
405 |dentry block 1 | dentry block 2 |
406 +--------------------------------+
407 . .
408 . .
409 . [Dentry Block Structure: 4KB] .
410 +--------+----------+----------+------------+
411 | bitmap | reserved | dentries | file names |
412 +--------+----------+----------+------------+
413 [Dentry Block: 4KB] . .
414 . .
415 . .
416 +------+------+-----+------+
417 | hash | ino | len | type |
418 +------+------+-----+------+
419 [Dentry Structure: 11 bytes]
421 F2FS implements multi-level hash tables for directory structure. Each level has
422 a hash table with dedicated number of hash buckets as shown below. Note that
423 "A(2B)" means a bucket includes 2 data blocks.
425 ----------------------
426 A : bucket
427 B : block
428 N : MAX_DIR_HASH_DEPTH
429 ----------------------
431 level #0 | A(2B)
432 |
433 level #1 | A(2B) - A(2B)
434 |
435 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
436 . | . . . .
437 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
438 . | . . . .
439 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
441 The number of blocks and buckets are determined by,
443 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
444 # of blocks in level #n = |
445 `- 4, Otherwise
447 ,- 2^n, if n < MAX_DIR_HASH_DEPTH / 2,
448 # of buckets in level #n = |
449 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise
451 When F2FS finds a file name in a directory, at first a hash value of the file
452 name is calculated. Then, F2FS scans the hash table in level #0 to find the
453 dentry consisting of the file name and its inode number. If not found, F2FS
454 scans the next hash table in level #1. In this way, F2FS scans hash tables in
455 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
456 one bucket determined by the following equation, which shows O(log(# of files))
457 complexity.
459 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
461 In the case of file creation, F2FS finds empty consecutive slots that cover the
462 file name. F2FS searches the empty slots in the hash tables of whole levels from
463 1 to N in the same way as the lookup operation.
465 The following figure shows an example of two cases holding children.
466 --------------> Dir <--------------
467 | |
468 child child
470 child - child [hole] - child
472 child - child - child [hole] - [hole] - child
474 Case 1: Case 2:
475 Number of children = 6, Number of children = 3,
476 File size = 7 File size = 7
478 Default Block Allocation
479 ------------------------
481 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
482 and Hot/Warm/Cold data.
484 - Hot node contains direct node blocks of directories.
485 - Warm node contains direct node blocks except hot node blocks.
486 - Cold node contains indirect node blocks
487 - Hot data contains dentry blocks
488 - Warm data contains data blocks except hot and cold data blocks
489 - Cold data contains multimedia data or migrated data blocks
491 LFS has two schemes for free space management: threaded log and copy-and-compac-
492 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
493 for devices showing very good sequential write performance, since free segments
494 are served all the time for writing new data. However, it suffers from cleaning
495 overhead under high utilization. Contrarily, the threaded log scheme suffers
496 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
497 scheme where the copy-and-compaction scheme is adopted by default, but the
498 policy is dynamically changed to the threaded log scheme according to the file
499 system status.
501 In order to align F2FS with underlying flash-based storage, F2FS allocates a
502 segment in a unit of section. F2FS expects that the section size would be the
503 same as the unit size of garbage collection in FTL. Furthermore, with respect
504 to the mapping granularity in FTL, F2FS allocates each section of the active
505 logs from different zones as much as possible, since FTL can write the data in
506 the active logs into one allocation unit according to its mapping granularity.
508 Cleaning process
509 ----------------
511 F2FS does cleaning both on demand and in the background. On-demand cleaning is
512 triggered when there are not enough free segments to serve VFS calls. Background
513 cleaner is operated by a kernel thread, and triggers the cleaning job when the
514 system is idle.
516 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
517 In the greedy algorithm, F2FS selects a victim segment having the smallest number
518 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
519 according to the segment age and the number of valid blocks in order to address
520 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
521 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
522 algorithm.
524 In order to identify whether the data in the victim segment are valid or not,
525 F2FS manages a bitmap. Each bit represents the validity of a block, and the
526 bitmap is composed of a bit stream covering whole blocks in main area.