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1 | ================================================================================ |
2 | WHAT IS Flash-Friendly File System (F2FS)? | |
3 | ================================================================================ | |
4 | ||
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. | |
10 | ||
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. | |
15 | ||
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. | |
20 | ||
d51a7fba CL |
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). | |
5bb446a2 JK |
23 | >> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git |
24 | ||
25 | For reporting bugs and sending patches, please use the following mailing list: | |
26 | >> linux-f2fs-devel@lists.sourceforge.net | |
98e4da8c JK |
27 | |
28 | ================================================================================ | |
29 | BACKGROUND AND DESIGN ISSUES | |
30 | ================================================================================ | |
31 | ||
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. | |
43 | ||
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. | |
53 | ||
54 | [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/ | |
55 | ||
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. | |
62 | ||
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. | |
69 | ||
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. | |
73 | ||
74 | ================================================================================ | |
75 | KEY FEATURES | |
76 | ================================================================================ | |
77 | ||
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 | |
83 | ||
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. | |
89 | ||
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 | |
96 | ||
97 | ================================================================================ | |
98 | MOUNT OPTIONS | |
99 | ================================================================================ | |
100 | ||
696c018c NJ |
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 | |
4bb9998d | 105 | will be turned off. If background_gc=sync, it will turn |
6aefd93b | 106 | on synchronous garbage collection running in background. |
696c018c NJ |
107 | Default value for this option is on. So garbage |
108 | collection is on by default. | |
98e4da8c | 109 | disable_roll_forward Disable the roll-forward recovery routine |
2d834bf9 JK |
110 | norecovery Disable the roll-forward recovery routine, mounted read- |
111 | only (i.e., -o ro,disable_roll_forward) | |
64058be9 CY |
112 | discard/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. | |
98e4da8c JK |
115 | no_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. | |
118 | nouser_xattr Disable Extended User Attributes. Note: xattr is enabled | |
119 | by default if CONFIG_F2FS_FS_XATTR is selected. | |
120 | noacl Disable POSIX Access Control List. Note: acl is enabled | |
121 | by default if CONFIG_F2FS_FS_POSIX_ACL is selected. | |
122 | active_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. | |
125 | disable_ext_identify Disable the extension list configured by mkfs, so f2fs | |
126 | does not aware of cold files such as media files. | |
66e960c6 | 127 | inline_xattr Enable the inline xattrs feature. |
23cf7212 | 128 | noinline_xattr Disable the inline xattrs feature. |
7321dd97 CY |
129 | inline_xattr_size=%u Support configuring inline xattr size, it depends on |
130 | flexible inline xattr feature. | |
e4024e86 HL |
131 | inline_data Enable the inline data feature: New created small(<~3.4k) |
132 | files can be written into inode block. | |
d37a868f CY |
133 | inline_dentry Enable the inline dir feature: data in new created |
134 | directory entries can be written into inode block. The | |
135 | space of inode block which is used to store inline | |
136 | dentries is limited to ~3.4k. | |
04b9a5f0 | 137 | noinline_dentry Disable the inline dentry feature. |
6b4afdd7 JK |
138 | flush_merge Merge concurrent cache_flush commands as much as possible |
139 | to eliminate redundant command issues. If the underlying | |
140 | device handles the cache_flush command relatively slowly, | |
141 | recommend to enable this option. | |
0f7b2abd JK |
142 | nobarrier This option can be used if underlying storage guarantees |
143 | its cached data should be written to the novolatile area. | |
144 | If this option is set, no cache_flush commands are issued | |
145 | but f2fs still guarantees the write ordering of all the | |
146 | data writes. | |
d5053a34 JK |
147 | fastboot This option is used when a system wants to reduce mount |
148 | time as much as possible, even though normal performance | |
149 | can be sacrificed. | |
89672159 CY |
150 | extent_cache Enable an extent cache based on rb-tree, it can cache |
151 | as many as extent which map between contiguous logical | |
152 | address and physical address per inode, resulting in | |
7daaea25 | 153 | increasing the cache hit ratio. Set by default. |
4bb9998d | 154 | noextent_cache Disable an extent cache based on rb-tree explicitly, see |
7daaea25 | 155 | the above extent_cache mount option. |
75342797 WL |
156 | noinline_data Disable the inline data feature, inline data feature is |
157 | enabled by default. | |
343f40f0 CY |
158 | data_flush Enable data flushing before checkpoint in order to |
159 | persist data of regular and symlink. | |
d0995b53 CY |
160 | reserve_root=%d Support configuring reserved space which is used for |
161 | allocation from a privileged user with specified uid or | |
162 | gid, unit: 4KB, the default limit is 0.2% of user blocks. | |
163 | resuid=%d The user ID which may use the reserved blocks. | |
164 | resgid=%d The group ID which may use the reserved blocks. | |
56412894 CY |
165 | fault_injection=%d Enable fault injection in all supported types with |
166 | specified injection rate. | |
d494500a CY |
167 | fault_type=%d Support configuring fault injection type, should be |
168 | enabled with fault_injection option, fault type value | |
169 | is shown below, it supports single or combined type. | |
170 | Type_Name Type_Value | |
171 | FAULT_KMALLOC 0x000000001 | |
172 | FAULT_KVMALLOC 0x000000002 | |
173 | FAULT_PAGE_ALLOC 0x000000004 | |
174 | FAULT_PAGE_GET 0x000000008 | |
175 | FAULT_ALLOC_BIO 0x000000010 | |
176 | FAULT_ALLOC_NID 0x000000020 | |
177 | FAULT_ORPHAN 0x000000040 | |
178 | FAULT_BLOCK 0x000000080 | |
179 | FAULT_DIR_DEPTH 0x000000100 | |
180 | FAULT_EVICT_INODE 0x000000200 | |
181 | FAULT_TRUNCATE 0x000000400 | |
6f5c2ed0 | 182 | FAULT_READ_IO 0x000000800 |
d494500a CY |
183 | FAULT_CHECKPOINT 0x000001000 |
184 | FAULT_DISCARD 0x000002000 | |
6f5c2ed0 | 185 | FAULT_WRITE_IO 0x000004000 |
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186 | mode=%s Control block allocation mode which supports "adaptive" |
187 | and "lfs". In "lfs" mode, there should be no random | |
188 | writes towards main area. | |
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189 | io_bits=%u Set the bit size of write IO requests. It should be set |
190 | with "mode=lfs". | |
0abd675e CY |
191 | usrquota Enable plain user disk quota accounting. |
192 | grpquota Enable plain group disk quota accounting. | |
5c57132e | 193 | prjquota Enable plain project quota accounting. |
4b2414d0 CY |
194 | usrjquota=<file> Appoint specified file and type during mount, so that quota |
195 | grpjquota=<file> information can be properly updated during recovery flow, | |
196 | prjjquota=<file> <quota file>: must be in root directory; | |
197 | jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. | |
198 | offusrjquota Turn off user journelled quota. | |
199 | offgrpjquota Turn off group journelled quota. | |
200 | offprjjquota Turn off project journelled quota. | |
201 | quota Enable plain user disk quota accounting. | |
202 | noquota Disable all plain disk quota option. | |
8b3a0ca0 HL |
203 | whint_mode=%s Control which write hints are passed down to block |
204 | layer. This supports "off", "user-based", and | |
205 | "fs-based". In "off" mode (default), f2fs does not pass | |
206 | down hints. In "user-based" mode, f2fs tries to pass | |
207 | down hints given by users. And in "fs-based" mode, f2fs | |
208 | passes down hints with its policy. | |
07939627 JK |
209 | alloc_mode=%s Adjust block allocation policy, which supports "reuse" |
210 | and "default". | |
d6290814 JK |
211 | fsync_mode=%s Control the policy of fsync. Currently supports "posix", |
212 | "strict", and "nobarrier". In "posix" mode, which is | |
213 | default, fsync will follow POSIX semantics and does a | |
214 | light operation to improve the filesystem performance. | |
215 | In "strict" mode, fsync will be heavy and behaves in line | |
216 | with xfs, ext4 and btrfs, where xfstest generic/342 will | |
217 | pass, but the performance will regress. "nobarrier" is | |
218 | based on "posix", but doesn't issue flush command for | |
219 | non-atomic files likewise "nobarrier" mount option. | |
ff62af20 SY |
220 | test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt |
221 | context. The fake fscrypt context is used by xfstests. | |
4d3aed70 | 222 | checkpoint=%s[:%u[%]] Set to "disable" to turn off checkpointing. Set to "enable" |
4354994f DR |
223 | to reenable checkpointing. Is enabled by default. While |
224 | disabled, any unmounting or unexpected shutdowns will cause | |
225 | the filesystem contents to appear as they did when the | |
226 | filesystem was mounted with that option. | |
4d3aed70 DR |
227 | While mounting with checkpoint=disabled, the filesystem must |
228 | run garbage collection to ensure that all available space can | |
229 | be used. If this takes too much time, the mount may return | |
230 | EAGAIN. You may optionally add a value to indicate how much | |
231 | of the disk you would be willing to temporarily give up to | |
232 | avoid additional garbage collection. This can be given as a | |
233 | number of blocks, or as a percent. For instance, mounting | |
234 | with checkpoint=disable:100% would always succeed, but it may | |
235 | hide up to all remaining free space. The actual space that | |
236 | would be unusable can be viewed at /sys/fs/f2fs/<disk>/unusable | |
237 | This space is reclaimed once checkpoint=enable. | |
98e4da8c JK |
238 | |
239 | ================================================================================ | |
240 | DEBUGFS ENTRIES | |
241 | ================================================================================ | |
242 | ||
243 | /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as | |
244 | f2fs. Each file shows the whole f2fs information. | |
245 | ||
246 | /sys/kernel/debug/f2fs/status includes: | |
247 | - major file system information managed by f2fs currently | |
248 | - average SIT information about whole segments | |
249 | - current memory footprint consumed by f2fs. | |
250 | ||
b59d0bae NJ |
251 | ================================================================================ |
252 | SYSFS ENTRIES | |
253 | ================================================================================ | |
254 | ||
6de3f12e | 255 | Information about mounted f2fs file systems can be found in |
b59d0bae NJ |
256 | /sys/fs/f2fs. Each mounted filesystem will have a directory in |
257 | /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). | |
258 | The files in each per-device directory are shown in table below. | |
259 | ||
260 | Files in /sys/fs/f2fs/<devname> | |
261 | (see also Documentation/ABI/testing/sysfs-fs-f2fs) | |
262 | .............................................................................. | |
263 | File Content | |
264 | ||
4d11d13e JK |
265 | gc_urgent_sleep_time This parameter controls sleep time for gc_urgent. |
266 | 500 ms is set by default. See above gc_urgent. | |
267 | ||
268 | gc_min_sleep_time This tuning parameter controls the minimum sleep | |
b59d0bae NJ |
269 | time for the garbage collection thread. Time is |
270 | in milliseconds. | |
271 | ||
4d11d13e | 272 | gc_max_sleep_time This tuning parameter controls the maximum sleep |
b59d0bae NJ |
273 | time for the garbage collection thread. Time is |
274 | in milliseconds. | |
275 | ||
276 | gc_no_gc_sleep_time This tuning parameter controls the default sleep | |
277 | time for the garbage collection thread. Time is | |
278 | in milliseconds. | |
279 | ||
d2dc095f NJ |
280 | gc_idle This parameter controls the selection of victim |
281 | policy for garbage collection. Setting gc_idle = 0 | |
282 | (default) will disable this option. Setting | |
283 | gc_idle = 1 will select the Cost Benefit approach | |
4bb9998d | 284 | & setting gc_idle = 2 will select the greedy approach. |
d2dc095f | 285 | |
d9872a69 JK |
286 | gc_urgent This parameter controls triggering background GCs |
287 | urgently or not. Setting gc_urgent = 0 [default] | |
288 | makes back to default behavior, while if it is set | |
289 | to 1, background thread starts to do GC by given | |
290 | gc_urgent_sleep_time interval. | |
291 | ||
ea91e9b0 JK |
292 | reclaim_segments This parameter controls the number of prefree |
293 | segments to be reclaimed. If the number of prefree | |
58c41035 JK |
294 | segments is larger than the number of segments |
295 | in the proportion to the percentage over total | |
296 | volume size, f2fs tries to conduct checkpoint to | |
297 | reclaim the prefree segments to free segments. | |
298 | By default, 5% over total # of segments. | |
ea91e9b0 | 299 | |
ba0697ec JK |
300 | max_small_discards This parameter controls the number of discard |
301 | commands that consist small blocks less than 2MB. | |
302 | The candidates to be discarded are cached until | |
303 | checkpoint is triggered, and issued during the | |
304 | checkpoint. By default, it is disabled with 0. | |
305 | ||
4d11d13e JK |
306 | discard_granularity This parameter controls the granularity of discard |
307 | command size. It will issue discard commands iif | |
308 | the size is larger than given granularity. Its | |
309 | unit size is 4KB, and 4 (=16KB) is set by default. | |
310 | The maximum value is 128 (=512KB). | |
311 | ||
312 | reserved_blocks This parameter indicates the number of blocks that | |
313 | f2fs reserves internally for root. | |
314 | ||
315 | batched_trim_sections This parameter controls the number of sections | |
bba681cb JK |
316 | to be trimmed out in batch mode when FITRIM |
317 | conducts. 32 sections is set by default. | |
318 | ||
216fbd64 JK |
319 | ipu_policy This parameter controls the policy of in-place |
320 | updates in f2fs. There are five policies: | |
9b5f136f JK |
321 | 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR, |
322 | 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL, | |
323 | 0x10: F2FS_IPU_FSYNC. | |
216fbd64 JK |
324 | |
325 | min_ipu_util This parameter controls the threshold to trigger | |
326 | in-place-updates. The number indicates percentage | |
327 | of the filesystem utilization, and used by | |
328 | F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies. | |
329 | ||
c1ce1b02 JK |
330 | min_fsync_blocks This parameter controls the threshold to trigger |
331 | in-place-updates when F2FS_IPU_FSYNC mode is set. | |
332 | The number indicates the number of dirty pages | |
333 | when fsync needs to flush on its call path. If | |
334 | the number is less than this value, it triggers | |
335 | in-place-updates. | |
336 | ||
4d11d13e JK |
337 | min_seq_blocks This parameter controls the threshold to serialize |
338 | write IOs issued by multiple threads in parallel. | |
339 | ||
340 | min_hot_blocks This parameter controls the threshold to allocate | |
341 | a hot data log for pending data blocks to write. | |
342 | ||
343 | min_ssr_sections This parameter adds the threshold when deciding | |
344 | SSR block allocation. If this is large, SSR mode | |
345 | will be enabled early. | |
346 | ||
347 | ram_thresh This parameter controls the memory footprint used | |
348 | by free nids and cached nat entries. By default, | |
349 | 10 is set, which indicates 10 MB / 1 GB RAM. | |
350 | ||
351 | ra_nid_pages When building free nids, F2FS reads NAT blocks | |
352 | ahead for speed up. Default is 0. | |
353 | ||
354 | dirty_nats_ratio Given dirty ratio of cached nat entries, F2FS | |
355 | determines flushing them in background. | |
356 | ||
3bac380c JK |
357 | max_victim_search This parameter controls the number of trials to |
358 | find a victim segment when conducting SSR and | |
359 | cleaning operations. The default value is 4096 | |
360 | which covers 8GB block address range. | |
361 | ||
4d11d13e JK |
362 | migration_granularity For large-sized sections, F2FS can stop GC given |
363 | this granularity instead of reclaiming entire | |
364 | section. | |
365 | ||
ab9fa662 JK |
366 | dir_level This parameter controls the directory level to |
367 | support large directory. If a directory has a | |
368 | number of files, it can reduce the file lookup | |
369 | latency by increasing this dir_level value. | |
370 | Otherwise, it needs to decrease this value to | |
371 | reduce the space overhead. The default value is 0. | |
372 | ||
4d11d13e JK |
373 | cp_interval F2FS tries to do checkpoint periodically, 60 secs |
374 | by default. | |
375 | ||
376 | idle_interval F2FS detects system is idle, if there's no F2FS | |
377 | operations during given interval, 5 secs by | |
378 | default. | |
379 | ||
380 | discard_idle_interval F2FS detects the discard thread is idle, given | |
381 | time interval. Default is 5 secs. | |
382 | ||
383 | gc_idle_interval F2FS detects the GC thread is idle, given time | |
384 | interval. Default is 5 secs. | |
385 | ||
386 | umount_discard_timeout When unmounting the disk, F2FS waits for finishing | |
387 | queued discard commands which can take huge time. | |
388 | This gives time out for it, 5 secs by default. | |
389 | ||
390 | iostat_enable This controls to enable/disable iostat in F2FS. | |
391 | ||
392 | readdir_ra This enables/disabled readahead of inode blocks | |
393 | in readdir, and default is enabled. | |
394 | ||
395 | gc_pin_file_thresh This indicates how many GC can be failed for the | |
396 | pinned file. If it exceeds this, F2FS doesn't | |
397 | guarantee its pinning state. 2048 trials is set | |
398 | by default. | |
399 | ||
400 | extension_list This enables to change extension_list for hot/cold | |
401 | files in runtime. | |
402 | ||
403 | inject_rate This controls injection rate of arbitrary faults. | |
404 | ||
405 | inject_type This controls injection type of arbitrary faults. | |
406 | ||
407 | dirty_segments This shows # of dirty segments. | |
408 | ||
409 | lifetime_write_kbytes This shows # of data written to the disk. | |
410 | ||
411 | features This shows current features enabled on F2FS. | |
412 | ||
413 | current_reserved_blocks This shows # of blocks currently reserved. | |
cdfc41c1 | 414 | |
4d3aed70 DR |
415 | unusable If checkpoint=disable, this shows the number of |
416 | blocks that are unusable. | |
417 | If checkpoint=enable it shows the number of blocks | |
418 | that would be unusable if checkpoint=disable were | |
419 | to be set. | |
420 | ||
5aba5430 DR |
421 | encoding This shows the encoding used for casefolding. |
422 | If casefolding is not enabled, returns (none) | |
423 | ||
98e4da8c JK |
424 | ================================================================================ |
425 | USAGE | |
426 | ================================================================================ | |
427 | ||
428 | 1. Download userland tools and compile them. | |
429 | ||
430 | 2. Skip, if f2fs was compiled statically inside kernel. | |
431 | Otherwise, insert the f2fs.ko module. | |
432 | # insmod f2fs.ko | |
433 | ||
434 | 3. Create a directory trying to mount | |
435 | # mkdir /mnt/f2fs | |
436 | ||
437 | 4. Format the block device, and then mount as f2fs | |
438 | # mkfs.f2fs -l label /dev/block_device | |
439 | # mount -t f2fs /dev/block_device /mnt/f2fs | |
440 | ||
d51a7fba CL |
441 | mkfs.f2fs |
442 | --------- | |
443 | The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, | |
444 | which builds a basic on-disk layout. | |
445 | ||
446 | The options consist of: | |
1571f84a | 447 | -l [label] : Give a volume label, up to 512 unicode name. |
98e4da8c JK |
448 | -a [0 or 1] : Split start location of each area for heap-based allocation. |
449 | 1 is set by default, which performs this. | |
450 | -o [int] : Set overprovision ratio in percent over volume size. | |
451 | 5 is set by default. | |
452 | -s [int] : Set the number of segments per section. | |
453 | 1 is set by default. | |
454 | -z [int] : Set the number of sections per zone. | |
455 | 1 is set by default. | |
456 | -e [str] : Set basic extension list. e.g. "mp3,gif,mov" | |
1571f84a CL |
457 | -t [0 or 1] : Disable discard command or not. |
458 | 1 is set by default, which conducts discard. | |
98e4da8c | 459 | |
d51a7fba CL |
460 | fsck.f2fs |
461 | --------- | |
462 | The fsck.f2fs is a tool to check the consistency of an f2fs-formatted | |
463 | partition, which examines whether the filesystem metadata and user-made data | |
464 | are cross-referenced correctly or not. | |
465 | Note that, initial version of the tool does not fix any inconsistency. | |
466 | ||
467 | The options consist of: | |
468 | -d debug level [default:0] | |
469 | ||
470 | dump.f2fs | |
471 | --------- | |
472 | The dump.f2fs shows the information of specific inode and dumps SSA and SIT to | |
473 | file. Each file is dump_ssa and dump_sit. | |
474 | ||
475 | The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. | |
4bb9998d | 476 | It shows on-disk inode information recognized by a given inode number, and is |
d51a7fba CL |
477 | able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and |
478 | ./dump_sit respectively. | |
479 | ||
480 | The options consist of: | |
481 | -d debug level [default:0] | |
482 | -i inode no (hex) | |
483 | -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] | |
484 | -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] | |
485 | ||
486 | Examples: | |
487 | # dump.f2fs -i [ino] /dev/sdx | |
488 | # dump.f2fs -s 0~-1 /dev/sdx (SIT dump) | |
489 | # dump.f2fs -a 0~-1 /dev/sdx (SSA dump) | |
490 | ||
98e4da8c JK |
491 | ================================================================================ |
492 | DESIGN | |
493 | ================================================================================ | |
494 | ||
495 | On-disk Layout | |
496 | -------------- | |
497 | ||
498 | F2FS divides the whole volume into a number of segments, each of which is fixed | |
499 | to 2MB in size. A section is composed of consecutive segments, and a zone | |
500 | consists of a set of sections. By default, section and zone sizes are set to one | |
501 | segment size identically, but users can easily modify the sizes by mkfs. | |
502 | ||
503 | F2FS splits the entire volume into six areas, and all the areas except superblock | |
504 | consists of multiple segments as described below. | |
505 | ||
506 | align with the zone size <-| | |
507 | |-> align with the segment size | |
508 | _________________________________________________________________________ | |
9268cc35 HL |
509 | | | | Segment | Node | Segment | | |
510 | | Superblock | Checkpoint | Info. | Address | Summary | Main | | |
511 | | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | | |
98e4da8c JK |
512 | |____________|_____2______|______N______|______N______|______N_____|__N___| |
513 | . . | |
514 | . . | |
515 | . . | |
516 | ._________________________________________. | |
517 | |_Segment_|_..._|_Segment_|_..._|_Segment_| | |
518 | . . | |
519 | ._________._________ | |
520 | |_section_|__...__|_ | |
521 | . . | |
522 | .________. | |
523 | |__zone__| | |
524 | ||
525 | - Superblock (SB) | |
526 | : It is located at the beginning of the partition, and there exist two copies | |
527 | to avoid file system crash. It contains basic partition information and some | |
528 | default parameters of f2fs. | |
529 | ||
530 | - Checkpoint (CP) | |
531 | : It contains file system information, bitmaps for valid NAT/SIT sets, orphan | |
532 | inode lists, and summary entries of current active segments. | |
533 | ||
98e4da8c JK |
534 | - Segment Information Table (SIT) |
535 | : It contains segment information such as valid block count and bitmap for the | |
536 | validity of all the blocks. | |
537 | ||
9268cc35 HL |
538 | - Node Address Table (NAT) |
539 | : It is composed of a block address table for all the node blocks stored in | |
540 | Main area. | |
541 | ||
98e4da8c JK |
542 | - Segment Summary Area (SSA) |
543 | : It contains summary entries which contains the owner information of all the | |
544 | data and node blocks stored in Main area. | |
545 | ||
546 | - Main Area | |
547 | : It contains file and directory data including their indices. | |
548 | ||
549 | In order to avoid misalignment between file system and flash-based storage, F2FS | |
550 | aligns the start block address of CP with the segment size. Also, it aligns the | |
551 | start block address of Main area with the zone size by reserving some segments | |
552 | in SSA area. | |
553 | ||
554 | Reference the following survey for additional technical details. | |
555 | https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey | |
556 | ||
557 | File System Metadata Structure | |
558 | ------------------------------ | |
559 | ||
560 | F2FS adopts the checkpointing scheme to maintain file system consistency. At | |
561 | mount time, F2FS first tries to find the last valid checkpoint data by scanning | |
562 | CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. | |
563 | One of them always indicates the last valid data, which is called as shadow copy | |
564 | mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. | |
565 | ||
566 | For file system consistency, each CP points to which NAT and SIT copies are | |
567 | valid, as shown as below. | |
568 | ||
569 | +--------+----------+---------+ | |
9268cc35 | 570 | | CP | SIT | NAT | |
98e4da8c JK |
571 | +--------+----------+---------+ |
572 | . . . . | |
573 | . . . . | |
574 | . . . . | |
575 | +-------+-------+--------+--------+--------+--------+ | |
9268cc35 | 576 | | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | |
98e4da8c JK |
577 | +-------+-------+--------+--------+--------+--------+ |
578 | | ^ ^ | |
579 | | | | | |
580 | `----------------------------------------' | |
581 | ||
582 | Index Structure | |
583 | --------------- | |
584 | ||
585 | The key data structure to manage the data locations is a "node". Similar to | |
586 | traditional file structures, F2FS has three types of node: inode, direct node, | |
d08ab08d | 587 | indirect node. F2FS assigns 4KB to an inode block which contains 923 data block |
98e4da8c JK |
588 | indices, two direct node pointers, two indirect node pointers, and one double |
589 | indirect node pointer as described below. One direct node block contains 1018 | |
590 | data blocks, and one indirect node block contains also 1018 node blocks. Thus, | |
591 | one inode block (i.e., a file) covers: | |
592 | ||
593 | 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. | |
594 | ||
595 | Inode block (4KB) | |
596 | |- data (923) | |
597 | |- direct node (2) | |
598 | | `- data (1018) | |
599 | |- indirect node (2) | |
600 | | `- direct node (1018) | |
601 | | `- data (1018) | |
602 | `- double indirect node (1) | |
603 | `- indirect node (1018) | |
604 | `- direct node (1018) | |
605 | `- data (1018) | |
606 | ||
607 | Note that, all the node blocks are mapped by NAT which means the location of | |
608 | each node is translated by the NAT table. In the consideration of the wandering | |
609 | tree problem, F2FS is able to cut off the propagation of node updates caused by | |
610 | leaf data writes. | |
611 | ||
612 | Directory Structure | |
613 | ------------------- | |
614 | ||
615 | A directory entry occupies 11 bytes, which consists of the following attributes. | |
616 | ||
617 | - hash hash value of the file name | |
618 | - ino inode number | |
619 | - len the length of file name | |
620 | - type file type such as directory, symlink, etc | |
621 | ||
622 | A dentry block consists of 214 dentry slots and file names. Therein a bitmap is | |
623 | used to represent whether each dentry is valid or not. A dentry block occupies | |
624 | 4KB with the following composition. | |
625 | ||
626 | Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + | |
627 | dentries(11 * 214 bytes) + file name (8 * 214 bytes) | |
628 | ||
629 | [Bucket] | |
630 | +--------------------------------+ | |
631 | |dentry block 1 | dentry block 2 | | |
632 | +--------------------------------+ | |
633 | . . | |
634 | . . | |
635 | . [Dentry Block Structure: 4KB] . | |
636 | +--------+----------+----------+------------+ | |
637 | | bitmap | reserved | dentries | file names | | |
638 | +--------+----------+----------+------------+ | |
639 | [Dentry Block: 4KB] . . | |
640 | . . | |
641 | . . | |
642 | +------+------+-----+------+ | |
643 | | hash | ino | len | type | | |
644 | +------+------+-----+------+ | |
645 | [Dentry Structure: 11 bytes] | |
646 | ||
647 | F2FS implements multi-level hash tables for directory structure. Each level has | |
648 | a hash table with dedicated number of hash buckets as shown below. Note that | |
649 | "A(2B)" means a bucket includes 2 data blocks. | |
650 | ||
651 | ---------------------- | |
652 | A : bucket | |
653 | B : block | |
654 | N : MAX_DIR_HASH_DEPTH | |
655 | ---------------------- | |
656 | ||
657 | level #0 | A(2B) | |
658 | | | |
659 | level #1 | A(2B) - A(2B) | |
660 | | | |
661 | level #2 | A(2B) - A(2B) - A(2B) - A(2B) | |
662 | . | . . . . | |
663 | level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) | |
664 | . | . . . . | |
665 | level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) | |
666 | ||
667 | The number of blocks and buckets are determined by, | |
668 | ||
669 | ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, | |
670 | # of blocks in level #n = | | |
671 | `- 4, Otherwise | |
672 | ||
bfec07d0 CY |
673 | ,- 2^(n + dir_level), |
674 | | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, | |
98e4da8c | 675 | # of buckets in level #n = | |
bfec07d0 CY |
676 | `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), |
677 | Otherwise | |
98e4da8c JK |
678 | |
679 | When F2FS finds a file name in a directory, at first a hash value of the file | |
680 | name is calculated. Then, F2FS scans the hash table in level #0 to find the | |
681 | dentry consisting of the file name and its inode number. If not found, F2FS | |
682 | scans the next hash table in level #1. In this way, F2FS scans hash tables in | |
683 | each levels incrementally from 1 to N. In each levels F2FS needs to scan only | |
684 | one bucket determined by the following equation, which shows O(log(# of files)) | |
685 | complexity. | |
686 | ||
687 | bucket number to scan in level #n = (hash value) % (# of buckets in level #n) | |
688 | ||
689 | In the case of file creation, F2FS finds empty consecutive slots that cover the | |
690 | file name. F2FS searches the empty slots in the hash tables of whole levels from | |
691 | 1 to N in the same way as the lookup operation. | |
692 | ||
693 | The following figure shows an example of two cases holding children. | |
694 | --------------> Dir <-------------- | |
695 | | | | |
696 | child child | |
697 | ||
698 | child - child [hole] - child | |
699 | ||
700 | child - child - child [hole] - [hole] - child | |
701 | ||
702 | Case 1: Case 2: | |
703 | Number of children = 6, Number of children = 3, | |
704 | File size = 7 File size = 7 | |
705 | ||
706 | Default Block Allocation | |
707 | ------------------------ | |
708 | ||
709 | At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node | |
710 | and Hot/Warm/Cold data. | |
711 | ||
712 | - Hot node contains direct node blocks of directories. | |
713 | - Warm node contains direct node blocks except hot node blocks. | |
714 | - Cold node contains indirect node blocks | |
715 | - Hot data contains dentry blocks | |
716 | - Warm data contains data blocks except hot and cold data blocks | |
717 | - Cold data contains multimedia data or migrated data blocks | |
718 | ||
719 | LFS has two schemes for free space management: threaded log and copy-and-compac- | |
720 | tion. The copy-and-compaction scheme which is known as cleaning, is well-suited | |
721 | for devices showing very good sequential write performance, since free segments | |
722 | are served all the time for writing new data. However, it suffers from cleaning | |
723 | overhead under high utilization. Contrarily, the threaded log scheme suffers | |
724 | from random writes, but no cleaning process is needed. F2FS adopts a hybrid | |
725 | scheme where the copy-and-compaction scheme is adopted by default, but the | |
726 | policy is dynamically changed to the threaded log scheme according to the file | |
727 | system status. | |
728 | ||
729 | In order to align F2FS with underlying flash-based storage, F2FS allocates a | |
730 | segment in a unit of section. F2FS expects that the section size would be the | |
731 | same as the unit size of garbage collection in FTL. Furthermore, with respect | |
732 | to the mapping granularity in FTL, F2FS allocates each section of the active | |
733 | logs from different zones as much as possible, since FTL can write the data in | |
734 | the active logs into one allocation unit according to its mapping granularity. | |
735 | ||
736 | Cleaning process | |
737 | ---------------- | |
738 | ||
739 | F2FS does cleaning both on demand and in the background. On-demand cleaning is | |
740 | triggered when there are not enough free segments to serve VFS calls. Background | |
741 | cleaner is operated by a kernel thread, and triggers the cleaning job when the | |
742 | system is idle. | |
743 | ||
744 | F2FS supports two victim selection policies: greedy and cost-benefit algorithms. | |
745 | In the greedy algorithm, F2FS selects a victim segment having the smallest number | |
746 | of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment | |
747 | according to the segment age and the number of valid blocks in order to address | |
748 | log block thrashing problem in the greedy algorithm. F2FS adopts the greedy | |
749 | algorithm for on-demand cleaner, while background cleaner adopts cost-benefit | |
750 | algorithm. | |
751 | ||
752 | In order to identify whether the data in the victim segment are valid or not, | |
753 | F2FS manages a bitmap. Each bit represents the validity of a block, and the | |
754 | bitmap is composed of a bit stream covering whole blocks in main area. | |
8b3a0ca0 HL |
755 | |
756 | Write-hint Policy | |
757 | ----------------- | |
758 | ||
759 | 1) whint_mode=off. F2FS only passes down WRITE_LIFE_NOT_SET. | |
760 | ||
761 | 2) whint_mode=user-based. F2FS tries to pass down hints given by | |
762 | users. | |
763 | ||
764 | User F2FS Block | |
765 | ---- ---- ----- | |
766 | META WRITE_LIFE_NOT_SET | |
767 | HOT_NODE " | |
768 | WARM_NODE " | |
769 | COLD_NODE " | |
770 | *ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME | |
771 | *extension list " " | |
772 | ||
773 | -- buffered io | |
774 | WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME | |
775 | WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT | |
776 | WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET | |
777 | WRITE_LIFE_NONE " " | |
778 | WRITE_LIFE_MEDIUM " " | |
779 | WRITE_LIFE_LONG " " | |
780 | ||
781 | -- direct io | |
782 | WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME | |
783 | WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT | |
784 | WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET | |
785 | WRITE_LIFE_NONE " WRITE_LIFE_NONE | |
786 | WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM | |
787 | WRITE_LIFE_LONG " WRITE_LIFE_LONG | |
788 | ||
789 | 3) whint_mode=fs-based. F2FS passes down hints with its policy. | |
790 | ||
791 | User F2FS Block | |
792 | ---- ---- ----- | |
793 | META WRITE_LIFE_MEDIUM; | |
794 | HOT_NODE WRITE_LIFE_NOT_SET | |
795 | WARM_NODE " | |
796 | COLD_NODE WRITE_LIFE_NONE | |
797 | ioctl(COLD) COLD_DATA WRITE_LIFE_EXTREME | |
798 | extension list " " | |
799 | ||
800 | -- buffered io | |
801 | WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME | |
802 | WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT | |
803 | WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_LONG | |
804 | WRITE_LIFE_NONE " " | |
805 | WRITE_LIFE_MEDIUM " " | |
806 | WRITE_LIFE_LONG " " | |
807 | ||
808 | -- direct io | |
809 | WRITE_LIFE_EXTREME COLD_DATA WRITE_LIFE_EXTREME | |
810 | WRITE_LIFE_SHORT HOT_DATA WRITE_LIFE_SHORT | |
811 | WRITE_LIFE_NOT_SET WARM_DATA WRITE_LIFE_NOT_SET | |
812 | WRITE_LIFE_NONE " WRITE_LIFE_NONE | |
813 | WRITE_LIFE_MEDIUM " WRITE_LIFE_MEDIUM | |
814 | WRITE_LIFE_LONG " WRITE_LIFE_LONG | |
cad3836f JK |
815 | |
816 | Fallocate(2) Policy | |
817 | ------------------- | |
818 | ||
819 | The default policy follows the below posix rule. | |
820 | ||
821 | Allocating disk space | |
822 | The default operation (i.e., mode is zero) of fallocate() allocates | |
823 | the disk space within the range specified by offset and len. The | |
824 | file size (as reported by stat(2)) will be changed if offset+len is | |
825 | greater than the file size. Any subregion within the range specified | |
826 | by offset and len that did not contain data before the call will be | |
827 | initialized to zero. This default behavior closely resembles the | |
828 | behavior of the posix_fallocate(3) library function, and is intended | |
829 | as a method of optimally implementing that function. | |
830 | ||
831 | However, once F2FS receives ioctl(fd, F2FS_IOC_SET_PIN_FILE) in prior to | |
832 | fallocate(fd, DEFAULT_MODE), it allocates on-disk blocks addressess having | |
833 | zero or random data, which is useful to the below scenario where: | |
834 | 1. create(fd) | |
835 | 2. ioctl(fd, F2FS_IOC_SET_PIN_FILE) | |
836 | 3. fallocate(fd, 0, 0, size) | |
837 | 4. address = fibmap(fd, offset) | |
838 | 5. open(blkdev) | |
839 | 6. write(blkdev, address) |