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 truned off. 106 Default value for this option is on. So garbage 107 collection is on by default. 108disable_roll_forward Disable the roll-forward recovery routine 109norecovery Disable the roll-forward recovery routine, mounted read- 110 only (i.e., -o ro,disable_roll_forward) 111discard Issue discard/TRIM commands when a segment is cleaned. 112no_heap Disable heap-style segment allocation which finds free 113 segments for data from the beginning of main area, while 114 for node from the end of main area. 115nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 116 by default if CONFIG_F2FS_FS_XATTR is selected. 117noacl Disable POSIX Access Control List. Note: acl is enabled 118 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 119active_logs=%u Support configuring the number of active logs. In the 120 current design, f2fs supports only 2, 4, and 6 logs. 121 Default number is 6. 122disable_ext_identify Disable the extension list configured by mkfs, so f2fs 123 does not aware of cold files such as media files. 124inline_xattr Enable the inline xattrs feature. 125inline_data Enable the inline data feature: New created small(<~3.4k) 126 files can be written into inode block. 127inline_dentry Enable the inline dir feature: data in new created 128 directory entries can be written into inode block. The 129 space of inode block which is used to store inline 130 dentries is limited to ~3.4k. 131flush_merge Merge concurrent cache_flush commands as much as possible 132 to eliminate redundant command issues. If the underlying 133 device handles the cache_flush command relatively slowly, 134 recommend to enable this option. 135nobarrier This option can be used if underlying storage guarantees 136 its cached data should be written to the novolatile area. 137 If this option is set, no cache_flush commands are issued 138 but f2fs still guarantees the write ordering of all the 139 data writes. 140fastboot This option is used when a system wants to reduce mount 141 time as much as possible, even though normal performance 142 can be sacrificed. 143extent_cache Enable an extent cache based on rb-tree, it can cache 144 as many as extent which map between contiguous logical 145 address and physical address per inode, resulting in 146 increasing the cache hit ratio. 147noinline_data Disable the inline data feature, inline data feature is 148 enabled by default. 149 150================================================================================ 151DEBUGFS ENTRIES 152================================================================================ 153 154/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 155f2fs. Each file shows the whole f2fs information. 156 157/sys/kernel/debug/f2fs/status includes: 158 - major file system information managed by f2fs currently 159 - average SIT information about whole segments 160 - current memory footprint consumed by f2fs. 161 162================================================================================ 163SYSFS ENTRIES 164================================================================================ 165 166Information about mounted f2f2 file systems can be found in 167/sys/fs/f2fs. Each mounted filesystem will have a directory in 168/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 169The files in each per-device directory are shown in table below. 170 171Files in /sys/fs/f2fs/<devname> 172(see also Documentation/ABI/testing/sysfs-fs-f2fs) 173.............................................................................. 174 File Content 175 176 gc_max_sleep_time This tuning parameter controls the maximum sleep 177 time for the garbage collection thread. Time is 178 in milliseconds. 179 180 gc_min_sleep_time This tuning parameter controls the minimum sleep 181 time for the garbage collection thread. Time is 182 in milliseconds. 183 184 gc_no_gc_sleep_time This tuning parameter controls the default sleep 185 time for the garbage collection thread. Time is 186 in milliseconds. 187 188 gc_idle This parameter controls the selection of victim 189 policy for garbage collection. Setting gc_idle = 0 190 (default) will disable this option. Setting 191 gc_idle = 1 will select the Cost Benefit approach 192 & setting gc_idle = 2 will select the greedy aproach. 193 194 reclaim_segments This parameter controls the number of prefree 195 segments to be reclaimed. If the number of prefree 196 segments is larger than the number of segments 197 in the proportion to the percentage over total 198 volume size, f2fs tries to conduct checkpoint to 199 reclaim the prefree segments to free segments. 200 By default, 5% over total # of segments. 201 202 max_small_discards This parameter controls the number of discard 203 commands that consist small blocks less than 2MB. 204 The candidates to be discarded are cached until 205 checkpoint is triggered, and issued during the 206 checkpoint. By default, it is disabled with 0. 207 208 trim_sections This parameter controls the number of sections 209 to be trimmed out in batch mode when FITRIM 210 conducts. 32 sections is set by default. 211 212 ipu_policy This parameter controls the policy of in-place 213 updates in f2fs. There are five policies: 214 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR, 215 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL, 216 0x10: F2FS_IPU_FSYNC. 217 218 min_ipu_util This parameter controls the threshold to trigger 219 in-place-updates. The number indicates percentage 220 of the filesystem utilization, and used by 221 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies. 222 223 min_fsync_blocks This parameter controls the threshold to trigger 224 in-place-updates when F2FS_IPU_FSYNC mode is set. 225 The number indicates the number of dirty pages 226 when fsync needs to flush on its call path. If 227 the number is less than this value, it triggers 228 in-place-updates. 229 230 max_victim_search This parameter controls the number of trials to 231 find a victim segment when conducting SSR and 232 cleaning operations. The default value is 4096 233 which covers 8GB block address range. 234 235 dir_level This parameter controls the directory level to 236 support large directory. If a directory has a 237 number of files, it can reduce the file lookup 238 latency by increasing this dir_level value. 239 Otherwise, it needs to decrease this value to 240 reduce the space overhead. The default value is 0. 241 242 ram_thresh This parameter controls the memory footprint used 243 by free nids and cached nat entries. By default, 244 10 is set, which indicates 10 MB / 1 GB RAM. 245 246================================================================================ 247USAGE 248================================================================================ 249 2501. Download userland tools and compile them. 251 2522. Skip, if f2fs was compiled statically inside kernel. 253 Otherwise, insert the f2fs.ko module. 254 # insmod f2fs.ko 255 2563. Create a directory trying to mount 257 # mkdir /mnt/f2fs 258 2594. Format the block device, and then mount as f2fs 260 # mkfs.f2fs -l label /dev/block_device 261 # mount -t f2fs /dev/block_device /mnt/f2fs 262 263mkfs.f2fs 264--------- 265The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 266which builds a basic on-disk layout. 267 268The options consist of: 269-l [label] : Give a volume label, up to 512 unicode name. 270-a [0 or 1] : Split start location of each area for heap-based allocation. 271 1 is set by default, which performs this. 272-o [int] : Set overprovision ratio in percent over volume size. 273 5 is set by default. 274-s [int] : Set the number of segments per section. 275 1 is set by default. 276-z [int] : Set the number of sections per zone. 277 1 is set by default. 278-e [str] : Set basic extension list. e.g. "mp3,gif,mov" 279-t [0 or 1] : Disable discard command or not. 280 1 is set by default, which conducts discard. 281 282fsck.f2fs 283--------- 284The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 285partition, which examines whether the filesystem metadata and user-made data 286are cross-referenced correctly or not. 287Note that, initial version of the tool does not fix any inconsistency. 288 289The options consist of: 290 -d debug level [default:0] 291 292dump.f2fs 293--------- 294The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 295file. Each file is dump_ssa and dump_sit. 296 297The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 298It shows on-disk inode information reconized by a given inode number, and is 299able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 300./dump_sit respectively. 301 302The options consist of: 303 -d debug level [default:0] 304 -i inode no (hex) 305 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 306 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 307 308Examples: 309# dump.f2fs -i [ino] /dev/sdx 310# dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 311# dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 312 313================================================================================ 314DESIGN 315================================================================================ 316 317On-disk Layout 318-------------- 319 320F2FS divides the whole volume into a number of segments, each of which is fixed 321to 2MB in size. A section is composed of consecutive segments, and a zone 322consists of a set of sections. By default, section and zone sizes are set to one 323segment size identically, but users can easily modify the sizes by mkfs. 324 325F2FS splits the entire volume into six areas, and all the areas except superblock 326consists of multiple segments as described below. 327 328 align with the zone size <-| 329 |-> align with the segment size 330 _________________________________________________________________________ 331 | | | Segment | Node | Segment | | 332 | Superblock | Checkpoint | Info. | Address | Summary | Main | 333 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 334 |____________|_____2______|______N______|______N______|______N_____|__N___| 335 . . 336 . . 337 . . 338 ._________________________________________. 339 |_Segment_|_..._|_Segment_|_..._|_Segment_| 340 . . 341 ._________._________ 342 |_section_|__...__|_ 343 . . 344 .________. 345 |__zone__| 346 347- Superblock (SB) 348 : It is located at the beginning of the partition, and there exist two copies 349 to avoid file system crash. It contains basic partition information and some 350 default parameters of f2fs. 351 352- Checkpoint (CP) 353 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan 354 inode lists, and summary entries of current active segments. 355 356- Segment Information Table (SIT) 357 : It contains segment information such as valid block count and bitmap for the 358 validity of all the blocks. 359 360- Node Address Table (NAT) 361 : It is composed of a block address table for all the node blocks stored in 362 Main area. 363 364- Segment Summary Area (SSA) 365 : It contains summary entries which contains the owner information of all the 366 data and node blocks stored in Main area. 367 368- Main Area 369 : It contains file and directory data including their indices. 370 371In order to avoid misalignment between file system and flash-based storage, F2FS 372aligns the start block address of CP with the segment size. Also, it aligns the 373start block address of Main area with the zone size by reserving some segments 374in SSA area. 375 376Reference the following survey for additional technical details. 377https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 378 379File System Metadata Structure 380------------------------------ 381 382F2FS adopts the checkpointing scheme to maintain file system consistency. At 383mount time, F2FS first tries to find the last valid checkpoint data by scanning 384CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 385One of them always indicates the last valid data, which is called as shadow copy 386mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 387 388For file system consistency, each CP points to which NAT and SIT copies are 389valid, as shown as below. 390 391 +--------+----------+---------+ 392 | CP | SIT | NAT | 393 +--------+----------+---------+ 394 . . . . 395 . . . . 396 . . . . 397 +-------+-------+--------+--------+--------+--------+ 398 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 399 +-------+-------+--------+--------+--------+--------+ 400 | ^ ^ 401 | | | 402 `----------------------------------------' 403 404Index Structure 405--------------- 406 407The key data structure to manage the data locations is a "node". Similar to 408traditional file structures, F2FS has three types of node: inode, direct node, 409indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 410indices, two direct node pointers, two indirect node pointers, and one double 411indirect node pointer as described below. One direct node block contains 1018 412data blocks, and one indirect node block contains also 1018 node blocks. Thus, 413one inode block (i.e., a file) covers: 414 415 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 416 417 Inode block (4KB) 418 |- data (923) 419 |- direct node (2) 420 | `- data (1018) 421 |- indirect node (2) 422 | `- direct node (1018) 423 | `- data (1018) 424 `- double indirect node (1) 425 `- indirect node (1018) 426 `- direct node (1018) 427 `- data (1018) 428 429Note that, all the node blocks are mapped by NAT which means the location of 430each node is translated by the NAT table. In the consideration of the wandering 431tree problem, F2FS is able to cut off the propagation of node updates caused by 432leaf data writes. 433 434Directory Structure 435------------------- 436 437A directory entry occupies 11 bytes, which consists of the following attributes. 438 439- hash hash value of the file name 440- ino inode number 441- len the length of file name 442- type file type such as directory, symlink, etc 443 444A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 445used to represent whether each dentry is valid or not. A dentry block occupies 4464KB with the following composition. 447 448 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 449 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 450 451 [Bucket] 452 +--------------------------------+ 453 |dentry block 1 | dentry block 2 | 454 +--------------------------------+ 455 . . 456 . . 457 . [Dentry Block Structure: 4KB] . 458 +--------+----------+----------+------------+ 459 | bitmap | reserved | dentries | file names | 460 +--------+----------+----------+------------+ 461 [Dentry Block: 4KB] . . 462 . . 463 . . 464 +------+------+-----+------+ 465 | hash | ino | len | type | 466 +------+------+-----+------+ 467 [Dentry Structure: 11 bytes] 468 469F2FS implements multi-level hash tables for directory structure. Each level has 470a hash table with dedicated number of hash buckets as shown below. Note that 471"A(2B)" means a bucket includes 2 data blocks. 472 473---------------------- 474A : bucket 475B : block 476N : MAX_DIR_HASH_DEPTH 477---------------------- 478 479level #0 | A(2B) 480 | 481level #1 | A(2B) - A(2B) 482 | 483level #2 | A(2B) - A(2B) - A(2B) - A(2B) 484 . | . . . . 485level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 486 . | . . . . 487level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 488 489The number of blocks and buckets are determined by, 490 491 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 492 # of blocks in level #n = | 493 `- 4, Otherwise 494 495 ,- 2^(n + dir_level), 496 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 497 # of buckets in level #n = | 498 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 499 Otherwise 500 501When F2FS finds a file name in a directory, at first a hash value of the file 502name is calculated. Then, F2FS scans the hash table in level #0 to find the 503dentry consisting of the file name and its inode number. If not found, F2FS 504scans the next hash table in level #1. In this way, F2FS scans hash tables in 505each levels incrementally from 1 to N. In each levels F2FS needs to scan only 506one bucket determined by the following equation, which shows O(log(# of files)) 507complexity. 508 509 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 510 511In the case of file creation, F2FS finds empty consecutive slots that cover the 512file name. F2FS searches the empty slots in the hash tables of whole levels from 5131 to N in the same way as the lookup operation. 514 515The following figure shows an example of two cases holding children. 516 --------------> Dir <-------------- 517 | | 518 child child 519 520 child - child [hole] - child 521 522 child - child - child [hole] - [hole] - child 523 524 Case 1: Case 2: 525 Number of children = 6, Number of children = 3, 526 File size = 7 File size = 7 527 528Default Block Allocation 529------------------------ 530 531At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 532and Hot/Warm/Cold data. 533 534- Hot node contains direct node blocks of directories. 535- Warm node contains direct node blocks except hot node blocks. 536- Cold node contains indirect node blocks 537- Hot data contains dentry blocks 538- Warm data contains data blocks except hot and cold data blocks 539- Cold data contains multimedia data or migrated data blocks 540 541LFS has two schemes for free space management: threaded log and copy-and-compac- 542tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 543for devices showing very good sequential write performance, since free segments 544are served all the time for writing new data. However, it suffers from cleaning 545overhead under high utilization. Contrarily, the threaded log scheme suffers 546from random writes, but no cleaning process is needed. F2FS adopts a hybrid 547scheme where the copy-and-compaction scheme is adopted by default, but the 548policy is dynamically changed to the threaded log scheme according to the file 549system status. 550 551In order to align F2FS with underlying flash-based storage, F2FS allocates a 552segment in a unit of section. F2FS expects that the section size would be the 553same as the unit size of garbage collection in FTL. Furthermore, with respect 554to the mapping granularity in FTL, F2FS allocates each section of the active 555logs from different zones as much as possible, since FTL can write the data in 556the active logs into one allocation unit according to its mapping granularity. 557 558Cleaning process 559---------------- 560 561F2FS does cleaning both on demand and in the background. On-demand cleaning is 562triggered when there are not enough free segments to serve VFS calls. Background 563cleaner is operated by a kernel thread, and triggers the cleaning job when the 564system is idle. 565 566F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 567In the greedy algorithm, F2FS selects a victim segment having the smallest number 568of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 569according to the segment age and the number of valid blocks in order to address 570log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 571algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 572algorithm. 573 574In order to identify whether the data in the victim segment are valid or not, 575F2FS manages a bitmap. Each bit represents the validity of a block, and the 576bitmap is composed of a bit stream covering whole blocks in main area. 577