Documentation/filesystems/f2fs.txt

   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
  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
  24
  25 For reporting bugs and sending patches, please use the following mailing list:
  26 >> linux-f2fs-devel@lists.sourceforge.net
  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
 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 turned off. If background_gc=sync, it will turn
 106                        on synchronous garbage collection running in background.
 107                        Default value for this option is on. So garbage
 108                        collection is on by default.
 109 disable_roll_forward   Disable the roll-forward recovery routine
 110 norecovery             Disable the roll-forward recovery routine, mounted read-
 111                        only (i.e., -o ro,disable_roll_forward)
 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.
 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.
 127 inline_xattr           Enable the inline xattrs feature.
 128 noinline_xattr         Disable the inline xattrs feature.
 129 inline_data            Enable the inline data feature: New created small(<~3.4k)
 130                        files can be written into inode block.
 131 inline_dentry          Enable the inline dir feature: data in new created
 132                        directory entries can be written into inode block. The
 133                        space of inode block which is used to store inline
 134                        dentries is limited to ~3.4k.
 135 noinline_dentry        Disable the inline dentry feature.
 136 flush_merge            Merge concurrent cache_flush commands as much as possible
 137                        to eliminate redundant command issues. If the underlying
 138                        device handles the cache_flush command relatively slowly,
 139                        recommend to enable this option.
 140 nobarrier              This option can be used if underlying storage guarantees
 141                        its cached data should be written to the novolatile area.
 142                        If this option is set, no cache_flush commands are issued
 143                        but f2fs still guarantees the write ordering of all the
 144                        data writes.
 145 fastboot               This option is used when a system wants to reduce mount
 146                        time as much as possible, even though normal performance
 147                        can be sacrificed.
 148 extent_cache           Enable an extent cache based on rb-tree, it can cache
 149                        as many as extent which map between contiguous logical
 150                        address and physical address per inode, resulting in
 151                        increasing the cache hit ratio. Set by default.
 152 noextent_cache         Disable an extent cache based on rb-tree explicitly, see
 153                        the above extent_cache mount option.
 154 noinline_data          Disable the inline data feature, inline data feature is
 155                        enabled by default.
 156 data_flush             Enable data flushing before checkpoint in order to
 157                        persist data of regular and symlink.
 158 mode=%s                Control block allocation mode which supports "adaptive"
 159                        and "lfs". In "lfs" mode, there should be no random
 160                        writes towards main area.
 161 io_bits=%u             Set the bit size of write IO requests. It should be set
 162                        with "mode=lfs".
 163
 164 ================================================================================
 165 DEBUGFS ENTRIES
 166 ================================================================================
 167
 168 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
 169 f2fs. Each file shows the whole f2fs information.
 170
 171 /sys/kernel/debug/f2fs/status includes:
 172  - major file system information managed by f2fs currently
 173  - average SIT information about whole segments
 174  - current memory footprint consumed by f2fs.
 175
 176 ================================================================================
 177 SYSFS ENTRIES
 178 ================================================================================
 179
 180 Information about mounted f2fs file systems can be found in
 181 /sys/fs/f2fs.  Each mounted filesystem will have a directory in
 182 /sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
 183 The files in each per-device directory are shown in table below.
 184
 185 Files in /sys/fs/f2fs/<devname>
 186 (see also Documentation/ABI/testing/sysfs-fs-f2fs)
 187 ..............................................................................
 188  File                         Content
 189
 190  gc_max_sleep_time            This tuning parameter controls the maximum sleep
 191                               time for the garbage collection thread. Time is
 192                               in milliseconds.
 193
 194  gc_min_sleep_time            This tuning parameter controls the minimum sleep
 195                               time for the garbage collection thread. Time is
 196                               in milliseconds.
 197
 198  gc_no_gc_sleep_time          This tuning parameter controls the default sleep
 199                               time for the garbage collection thread. Time is
 200                               in milliseconds.
 201
 202  gc_idle                      This parameter controls the selection of victim
 203                               policy for garbage collection. Setting gc_idle = 0
 204                               (default) will disable this option. Setting
 205                               gc_idle = 1 will select the Cost Benefit approach
 206                               & setting gc_idle = 2 will select the greedy approach.
 207
 208  reclaim_segments             This parameter controls the number of prefree
 209                               segments to be reclaimed. If the number of prefree
 210                               segments is larger than the number of segments
 211                               in the proportion to the percentage over total
 212                               volume size, f2fs tries to conduct checkpoint to
 213                               reclaim the prefree segments to free segments.
 214                               By default, 5% over total # of segments.
 215
 216  max_small_discards           This parameter controls the number of discard
 217                               commands that consist small blocks less than 2MB.
 218                               The candidates to be discarded are cached until
 219                               checkpoint is triggered, and issued during the
 220                               checkpoint. By default, it is disabled with 0.
 221
 222  trim_sections                This parameter controls the number of sections
 223                               to be trimmed out in batch mode when FITRIM
 224                               conducts. 32 sections is set by default.
 225
 226  ipu_policy                   This parameter controls the policy of in-place
 227                               updates in f2fs. There are five policies:
 228                                0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
 229                                0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
 230                                0x10: F2FS_IPU_FSYNC.
 231
 232  min_ipu_util                 This parameter controls the threshold to trigger
 233                               in-place-updates. The number indicates percentage
 234                               of the filesystem utilization, and used by
 235                               F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
 236
 237  min_fsync_blocks             This parameter controls the threshold to trigger
 238                               in-place-updates when F2FS_IPU_FSYNC mode is set.
 239                               The number indicates the number of dirty pages
 240                               when fsync needs to flush on its call path. If
 241                               the number is less than this value, it triggers
 242                               in-place-updates.
 243
 244  max_victim_search            This parameter controls the number of trials to
 245                               find a victim segment when conducting SSR and
 246                               cleaning operations. The default value is 4096
 247                               which covers 8GB block address range.
 248
 249  dir_level                    This parameter controls the directory level to
 250                               support large directory. If a directory has a
 251                               number of files, it can reduce the file lookup
 252                               latency by increasing this dir_level value.
 253                               Otherwise, it needs to decrease this value to
 254                               reduce the space overhead. The default value is 0.
 255
 256  ram_thresh                   This parameter controls the memory footprint used
 257                               by free nids and cached nat entries. By default,
 258                               10 is set, which indicates 10 MB / 1 GB RAM.
 259
 260 ================================================================================
 261 USAGE
 262 ================================================================================
 263
 264 1. Download userland tools and compile them.
 265
 266 2. Skip, if f2fs was compiled statically inside kernel.
 267    Otherwise, insert the f2fs.ko module.
 268  # insmod f2fs.ko
 269
 270 3. Create a directory trying to mount
 271  # mkdir /mnt/f2fs
 272
 273 4. Format the block device, and then mount as f2fs
 274  # mkfs.f2fs -l label /dev/block_device
 275  # mount -t f2fs /dev/block_device /mnt/f2fs
 276
 277 mkfs.f2fs
 278 ---------
 279 The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
 280 which builds a basic on-disk layout.
 281
 282 The options consist of:
 283 -l [label]   : Give a volume label, up to 512 unicode name.
 284 -a [0 or 1]  : Split start location of each area for heap-based allocation.
 285                1 is set by default, which performs this.
 286 -o [int]     : Set overprovision ratio in percent over volume size.
 287                5 is set by default.
 288 -s [int]     : Set the number of segments per section.
 289                1 is set by default.
 290 -z [int]     : Set the number of sections per zone.
 291                1 is set by default.
 292 -e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
 293 -t [0 or 1]  : Disable discard command or not.
 294                1 is set by default, which conducts discard.
 295
 296 fsck.f2fs
 297 ---------
 298 The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
 299 partition, which examines whether the filesystem metadata and user-made data
 300 are cross-referenced correctly or not.
 301 Note that, initial version of the tool does not fix any inconsistency.
 302
 303 The options consist of:
 304   -d debug level [default:0]
 305
 306 dump.f2fs
 307 ---------
 308 The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
 309 file. Each file is dump_ssa and dump_sit.
 310
 311 The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
 312 It shows on-disk inode information recognized by a given inode number, and is
 313 able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
 314 ./dump_sit respectively.
 315
 316 The options consist of:
 317   -d debug level [default:0]
 318   -i inode no (hex)
 319   -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
 320   -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
 321
 322 Examples:
 323 # dump.f2fs -i [ino] /dev/sdx
 324 # dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
 325 # dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
 326
 327 ================================================================================
 328 DESIGN
 329 ================================================================================
 330
 331 On-disk Layout
 332 --------------
 333
 334 F2FS divides the whole volume into a number of segments, each of which is fixed
 335 to 2MB in size. A section is composed of consecutive segments, and a zone
 336 consists of a set of sections. By default, section and zone sizes are set to one
 337 segment size identically, but users can easily modify the sizes by mkfs.
 338
 339 F2FS splits the entire volume into six areas, and all the areas except superblock
 340 consists of multiple segments as described below.
 341
 342                                             align with the zone size <-|
 343                  |-> align with the segment size
 344      _________________________________________________________________________
 345     |            |            |   Segment   |    Node     |   Segment  |      |
 346     | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
 347     |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
 348     |____________|_____2______|______N______|______N______|______N_____|__N___|
 349                                                                        .      .
 350                                                              .                .
 351                                                  .                            .
 352                                     ._________________________________________.
 353                                     |_Segment_|_..._|_Segment_|_..._|_Segment_|
 354                                     .           .
 355                                     ._________._________
 356                                     |_section_|__...__|_
 357                                     .            .
 358                                     .________.
 359                                     |__zone__|
 360
 361 - Superblock (SB)
 362  : It is located at the beginning of the partition, and there exist two copies
 363    to avoid file system crash. It contains basic partition information and some
 364    default parameters of f2fs.
 365
 366 - Checkpoint (CP)
 367  : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
 368    inode lists, and summary entries of current active segments.
 369
 370 - Segment Information Table (SIT)
 371  : It contains segment information such as valid block count and bitmap for the
 372    validity of all the blocks.
 373
 374 - Node Address Table (NAT)
 375  : It is composed of a block address table for all the node blocks stored in
 376    Main area.
 377
 378 - Segment Summary Area (SSA)
 379  : It contains summary entries which contains the owner information of all the
 380    data and node blocks stored in Main area.
 381
 382 - Main Area
 383  : It contains file and directory data including their indices.
 384
 385 In order to avoid misalignment between file system and flash-based storage, F2FS
 386 aligns the start block address of CP with the segment size. Also, it aligns the
 387 start block address of Main area with the zone size by reserving some segments
 388 in SSA area.
 389
 390 Reference the following survey for additional technical details.
 391 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
 392
 393 File System Metadata Structure
 394 ------------------------------
 395
 396 F2FS adopts the checkpointing scheme to maintain file system consistency. At
 397 mount time, F2FS first tries to find the last valid checkpoint data by scanning
 398 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
 399 One of them always indicates the last valid data, which is called as shadow copy
 400 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
 401
 402 For file system consistency, each CP points to which NAT and SIT copies are
 403 valid, as shown as below.
 404
 405   +--------+----------+---------+
 406   |   CP   |    SIT   |   NAT   |
 407   +--------+----------+---------+
 408   .         .          .          .
 409   .            .              .              .
 410   .               .                 .                 .
 411   +-------+-------+--------+--------+--------+--------+
 412   | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
 413   +-------+-------+--------+--------+--------+--------+
 414      |             ^                          ^
 415      |             |                          |
 416      `----------------------------------------'
 417
 418 Index Structure
 419 ---------------
 420
 421 The key data structure to manage the data locations is a "node". Similar to
 422 traditional file structures, F2FS has three types of node: inode, direct node,
 423 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
 424 indices, two direct node pointers, two indirect node pointers, and one double
 425 indirect node pointer as described below. One direct node block contains 1018
 426 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
 427 one inode block (i.e., a file) covers:
 428
 429   4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
 430
 431    Inode block (4KB)
 432      |- data (923)
 433      |- direct node (2)
 434      |          `- data (1018)
 435      |- indirect node (2)
 436      |            `- direct node (1018)
 437      |                       `- data (1018)
 438      `- double indirect node (1)
 439                          `- indirect node (1018)
 440                                       `- direct node (1018)
 441                                                  `- data (1018)
 442
 443 Note that, all the node blocks are mapped by NAT which means the location of
 444 each node is translated by the NAT table. In the consideration of the wandering
 445 tree problem, F2FS is able to cut off the propagation of node updates caused by
 446 leaf data writes.
 447
 448 Directory Structure
 449 -------------------
 450
 451 A directory entry occupies 11 bytes, which consists of the following attributes.
 452
 453 - hash          hash value of the file name
 454 - ino           inode number
 455 - len           the length of file name
 456 - type          file type such as directory, symlink, etc
 457
 458 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
 459 used to represent whether each dentry is valid or not. A dentry block occupies
 460 4KB with the following composition.
 461
 462   Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
 463                       dentries(11 * 214 bytes) + file name (8 * 214 bytes)
 464
 465                          [Bucket]
 466              +--------------------------------+
 467              |dentry block 1 | dentry block 2 |
 468              +--------------------------------+
 469              .               .
 470        .                             .
 471   .       [Dentry Block Structure: 4KB]       .
 472   +--------+----------+----------+------------+
 473   | bitmap | reserved | dentries | file names |
 474   +--------+----------+----------+------------+
 475   [Dentry Block: 4KB] .   .
 476                  .               .
 477             .                          .
 478             +------+------+-----+------+
 479             | hash | ino  | len | type |
 480             +------+------+-----+------+
 481             [Dentry Structure: 11 bytes]
 482
 483 F2FS implements multi-level hash tables for directory structure. Each level has
 484 a hash table with dedicated number of hash buckets as shown below. Note that
 485 "A(2B)" means a bucket includes 2 data blocks.
 486
 487 ----------------------
 488 A : bucket
 489 B : block
 490 N : MAX_DIR_HASH_DEPTH
 491 ----------------------
 492
 493 level #0   | A(2B)
 494            |
 495 level #1   | A(2B) - A(2B)
 496            |
 497 level #2   | A(2B) - A(2B) - A(2B) - A(2B)
 498      .     |   .       .       .       .
 499 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
 500      .     |   .       .       .       .
 501 level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
 502
 503 The number of blocks and buckets are determined by,
 504
 505                             ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
 506   # of blocks in level #n = |
 507                             `- 4, Otherwise
 508
 509                              ,- 2^(n + dir_level),
 510                              |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
 511   # of buckets in level #n = |
 512                              `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
 513                                       Otherwise
 514
 515 When F2FS finds a file name in a directory, at first a hash value of the file
 516 name is calculated. Then, F2FS scans the hash table in level #0 to find the
 517 dentry consisting of the file name and its inode number. If not found, F2FS
 518 scans the next hash table in level #1. In this way, F2FS scans hash tables in
 519 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
 520 one bucket determined by the following equation, which shows O(log(# of files))
 521 complexity.
 522
 523   bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
 524
 525 In the case of file creation, F2FS finds empty consecutive slots that cover the
 526 file name. F2FS searches the empty slots in the hash tables of whole levels from
 527 1 to N in the same way as the lookup operation.
 528
 529 The following figure shows an example of two cases holding children.
 530        --------------> Dir <--------------
 531        |                                 |
 532     child                             child
 533
 534     child - child                     [hole] - child
 535
 536     child - child - child             [hole] - [hole] - child
 537
 538    Case 1:                           Case 2:
 539    Number of children = 6,           Number of children = 3,
 540    File size = 7                     File size = 7
 541
 542 Default Block Allocation
 543 ------------------------
 544
 545 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
 546 and Hot/Warm/Cold data.
 547
 548 - Hot node      contains direct node blocks of directories.
 549 - Warm node     contains direct node blocks except hot node blocks.
 550 - Cold node     contains indirect node blocks
 551 - Hot data      contains dentry blocks
 552 - Warm data     contains data blocks except hot and cold data blocks
 553 - Cold data     contains multimedia data or migrated data blocks
 554
 555 LFS has two schemes for free space management: threaded log and copy-and-compac-
 556 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
 557 for devices showing very good sequential write performance, since free segments
 558 are served all the time for writing new data. However, it suffers from cleaning
 559 overhead under high utilization. Contrarily, the threaded log scheme suffers
 560 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
 561 scheme where the copy-and-compaction scheme is adopted by default, but the
 562 policy is dynamically changed to the threaded log scheme according to the file
 563 system status.
 564
 565 In order to align F2FS with underlying flash-based storage, F2FS allocates a
 566 segment in a unit of section. F2FS expects that the section size would be the
 567 same as the unit size of garbage collection in FTL. Furthermore, with respect
 568 to the mapping granularity in FTL, F2FS allocates each section of the active
 569 logs from different zones as much as possible, since FTL can write the data in
 570 the active logs into one allocation unit according to its mapping granularity.
 571
 572 Cleaning process
 573 ----------------
 574
 575 F2FS does cleaning both on demand and in the background. On-demand cleaning is
 576 triggered when there are not enough free segments to serve VFS calls. Background
 577 cleaner is operated by a kernel thread, and triggers the cleaning job when the
 578 system is idle.
 579
 580 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
 581 In the greedy algorithm, F2FS selects a victim segment having the smallest number
 582 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
 583 according to the segment age and the number of valid blocks in order to address
 584 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
 585 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
 586 algorithm.
 587
 588 In order to identify whether the data in the victim segment are valid or not,
 589 F2FS manages a bitmap. Each bit represents the validity of a block, and the
 590 bitmap is composed of a bit stream covering whole blocks in main area.