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 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. 109disable_roll_forward Disable the roll-forward recovery routine 110norecovery Disable the roll-forward recovery routine, mounted read- 111 only (i.e., -o ro,disable_roll_forward) 112discard/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. 115no_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. 118nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 119 by default if CONFIG_F2FS_FS_XATTR is selected. 120noacl Disable POSIX Access Control List. Note: acl is enabled 121 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 122active_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. 125disable_ext_identify Disable the extension list configured by mkfs, so f2fs 126 does not aware of cold files such as media files. 127inline_xattr Enable the inline xattrs feature. 128inline_data Enable the inline data feature: New created small(<~3.4k) 129 files can be written into inode block. 130inline_dentry Enable the inline dir feature: data in new created 131 directory entries can be written into inode block. The 132 space of inode block which is used to store inline 133 dentries is limited to ~3.4k. 134noinline_dentry Diable the inline dentry feature. 135flush_merge Merge concurrent cache_flush commands as much as possible 136 to eliminate redundant command issues. If the underlying 137 device handles the cache_flush command relatively slowly, 138 recommend to enable this option. 139nobarrier This option can be used if underlying storage guarantees 140 its cached data should be written to the novolatile area. 141 If this option is set, no cache_flush commands are issued 142 but f2fs still guarantees the write ordering of all the 143 data writes. 144fastboot This option is used when a system wants to reduce mount 145 time as much as possible, even though normal performance 146 can be sacrificed. 147extent_cache Enable an extent cache based on rb-tree, it can cache 148 as many as extent which map between contiguous logical 149 address and physical address per inode, resulting in 150 increasing the cache hit ratio. Set by default. 151noextent_cache Disable an extent cache based on rb-tree explicitly, see 152 the above extent_cache mount option. 153noinline_data Disable the inline data feature, inline data feature is 154 enabled by default. 155data_flush Enable data flushing before checkpoint in order to 156 persist data of regular and symlink. 157mode=%s Control block allocation mode which supports "adaptive" 158 and "lfs". In "lfs" mode, there should be no random 159 writes towards main area. 160io_bits=%u Set the bit size of write IO requests. It should be set 161 with "mode=lfs". 162usrquota Enable plain user disk quota accounting. 163grpquota Enable plain group disk quota accounting. 164prjquota Enable plain project quota accounting. 165usrjquota=<file> Appoint specified file and type during mount, so that quota 166grpjquota=<file> information can be properly updated during recovery flow, 167prjjquota=<file> <quota file>: must be in root directory; 168jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1]. 169offusrjquota Turn off user journelled quota. 170offgrpjquota Turn off group journelled quota. 171offprjjquota Turn off project journelled quota. 172quota Enable plain user disk quota accounting. 173noquota Disable all plain disk quota option. 174whint_mode=%s Control which write hints are passed down to block 175 layer. This supports "off", "user-based", and 176 "fs-based". In "off" mode (default), f2fs does not pass 177 down hints. In "user-based" mode, f2fs tries to pass 178 down hints given by users. And in "fs-based" mode, f2fs 179 passes down hints with its policy. 180alloc_mode=%s Adjust block allocation policy, which supports "reuse" 181 and "default". 182fsync_mode=%s Control the policy of fsync. Currently supports "posix" 183 and "strict". In "posix" mode, which is default, fsync 184 will follow POSIX semantics and does a light operation 185 to improve the filesystem performance. In "strict" mode, 186 fsync will be heavy and behaves in line with xfs, ext4 187 and btrfs, where xfstest generic/342 will pass, but the 188 performance will regress. 189test_dummy_encryption Enable dummy encryption, which provides a fake fscrypt 190 context. The fake fscrypt context is used by xfstests. 191 192================================================================================ 193DEBUGFS ENTRIES 194================================================================================ 195 196/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 197f2fs. Each file shows the whole f2fs information. 198 199/sys/kernel/debug/f2fs/status includes: 200 - major file system information managed by f2fs currently 201 - average SIT information about whole segments 202 - current memory footprint consumed by f2fs. 203 204================================================================================ 205SYSFS ENTRIES 206================================================================================ 207 208Information about mounted f2f2 file systems can be found in 209/sys/fs/f2fs. Each mounted filesystem will have a directory in 210/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda). 211The files in each per-device directory are shown in table below. 212 213Files in /sys/fs/f2fs/<devname> 214(see also Documentation/ABI/testing/sysfs-fs-f2fs) 215.............................................................................. 216 File Content 217 218 gc_max_sleep_time This tuning parameter controls the maximum sleep 219 time for the garbage collection thread. Time is 220 in milliseconds. 221 222 gc_min_sleep_time This tuning parameter controls the minimum sleep 223 time for the garbage collection thread. Time is 224 in milliseconds. 225 226 gc_no_gc_sleep_time This tuning parameter controls the default sleep 227 time for the garbage collection thread. Time is 228 in milliseconds. 229 230 gc_idle This parameter controls the selection of victim 231 policy for garbage collection. Setting gc_idle = 0 232 (default) will disable this option. Setting 233 gc_idle = 1 will select the Cost Benefit approach 234 & setting gc_idle = 2 will select the greedy approach. 235 236 gc_urgent This parameter controls triggering background GCs 237 urgently or not. Setting gc_urgent = 0 [default] 238 makes back to default behavior, while if it is set 239 to 1, background thread starts to do GC by given 240 gc_urgent_sleep_time interval. 241 242 gc_urgent_sleep_time This parameter controls sleep time for gc_urgent. 243 500 ms is set by default. See above gc_urgent. 244 245 reclaim_segments This parameter controls the number of prefree 246 segments to be reclaimed. If the number of prefree 247 segments is larger than the number of segments 248 in the proportion to the percentage over total 249 volume size, f2fs tries to conduct checkpoint to 250 reclaim the prefree segments to free segments. 251 By default, 5% over total # of segments. 252 253 max_small_discards This parameter controls the number of discard 254 commands that consist small blocks less than 2MB. 255 The candidates to be discarded are cached until 256 checkpoint is triggered, and issued during the 257 checkpoint. By default, it is disabled with 0. 258 259 trim_sections This parameter controls the number of sections 260 to be trimmed out in batch mode when FITRIM 261 conducts. 32 sections is set by default. 262 263 ipu_policy This parameter controls the policy of in-place 264 updates in f2fs. There are five policies: 265 0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR, 266 0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL, 267 0x10: F2FS_IPU_FSYNC. 268 269 min_ipu_util This parameter controls the threshold to trigger 270 in-place-updates. The number indicates percentage 271 of the filesystem utilization, and used by 272 F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies. 273 274 min_fsync_blocks This parameter controls the threshold to trigger 275 in-place-updates when F2FS_IPU_FSYNC mode is set. 276 The number indicates the number of dirty pages 277 when fsync needs to flush on its call path. If 278 the number is less than this value, it triggers 279 in-place-updates. 280 281 max_victim_search This parameter controls the number of trials to 282 find a victim segment when conducting SSR and 283 cleaning operations. The default value is 4096 284 which covers 8GB block address range. 285 286 dir_level This parameter controls the directory level to 287 support large directory. If a directory has a 288 number of files, it can reduce the file lookup 289 latency by increasing this dir_level value. 290 Otherwise, it needs to decrease this value to 291 reduce the space overhead. The default value is 0. 292 293 ram_thresh This parameter controls the memory footprint used 294 by free nids and cached nat entries. By default, 295 10 is set, which indicates 10 MB / 1 GB RAM. 296 297================================================================================ 298USAGE 299================================================================================ 300 3011. Download userland tools and compile them. 302 3032. Skip, if f2fs was compiled statically inside kernel. 304 Otherwise, insert the f2fs.ko module. 305 # insmod f2fs.ko 306 3073. Create a directory trying to mount 308 # mkdir /mnt/f2fs 309 3104. Format the block device, and then mount as f2fs 311 # mkfs.f2fs -l label /dev/block_device 312 # mount -t f2fs /dev/block_device /mnt/f2fs 313 314mkfs.f2fs 315--------- 316The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem, 317which builds a basic on-disk layout. 318 319The options consist of: 320-l [label] : Give a volume label, up to 512 unicode name. 321-a [0 or 1] : Split start location of each area for heap-based allocation. 322 1 is set by default, which performs this. 323-o [int] : Set overprovision ratio in percent over volume size. 324 5 is set by default. 325-s [int] : Set the number of segments per section. 326 1 is set by default. 327-z [int] : Set the number of sections per zone. 328 1 is set by default. 329-e [str] : Set basic extension list. e.g. "mp3,gif,mov" 330-t [0 or 1] : Disable discard command or not. 331 1 is set by default, which conducts discard. 332 333fsck.f2fs 334--------- 335The fsck.f2fs is a tool to check the consistency of an f2fs-formatted 336partition, which examines whether the filesystem metadata and user-made data 337are cross-referenced correctly or not. 338Note that, initial version of the tool does not fix any inconsistency. 339 340The options consist of: 341 -d debug level [default:0] 342 343dump.f2fs 344--------- 345The dump.f2fs shows the information of specific inode and dumps SSA and SIT to 346file. Each file is dump_ssa and dump_sit. 347 348The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem. 349It shows on-disk inode information recognized by a given inode number, and is 350able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and 351./dump_sit respectively. 352 353The options consist of: 354 -d debug level [default:0] 355 -i inode no (hex) 356 -s [SIT dump segno from #1~#2 (decimal), for all 0~-1] 357 -a [SSA dump segno from #1~#2 (decimal), for all 0~-1] 358 359Examples: 360# dump.f2fs -i [ino] /dev/sdx 361# dump.f2fs -s 0~-1 /dev/sdx (SIT dump) 362# dump.f2fs -a 0~-1 /dev/sdx (SSA dump) 363 364================================================================================ 365DESIGN 366================================================================================ 367 368On-disk Layout 369-------------- 370 371F2FS divides the whole volume into a number of segments, each of which is fixed 372to 2MB in size. A section is composed of consecutive segments, and a zone 373consists of a set of sections. By default, section and zone sizes are set to one 374segment size identically, but users can easily modify the sizes by mkfs. 375 376F2FS splits the entire volume into six areas, and all the areas except superblock 377consists of multiple segments as described below. 378 379 align with the zone size <-| 380 |-> align with the segment size 381 _________________________________________________________________________ 382 | | | Segment | Node | Segment | | 383 | Superblock | Checkpoint | Info. | Address | Summary | Main | 384 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | | 385 |____________|_____2______|______N______|______N______|______N_____|__N___| 386 . . 387 . . 388 . . 389 ._________________________________________. 390 |_Segment_|_..._|_Segment_|_..._|_Segment_| 391 . . 392 ._________._________ 393 |_section_|__...__|_ 394 . . 395 .________. 396 |__zone__| 397 398- Superblock (SB) 399 : It is located at the beginning of the partition, and there exist two copies 400 to avoid file system crash. It contains basic partition information and some 401 default parameters of f2fs. 402 403- Checkpoint (CP) 404 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan 405 inode lists, and summary entries of current active segments. 406 407- Segment Information Table (SIT) 408 : It contains segment information such as valid block count and bitmap for the 409 validity of all the blocks. 410 411- Node Address Table (NAT) 412 : It is composed of a block address table for all the node blocks stored in 413 Main area. 414 415- Segment Summary Area (SSA) 416 : It contains summary entries which contains the owner information of all the 417 data and node blocks stored in Main area. 418 419- Main Area 420 : It contains file and directory data including their indices. 421 422In order to avoid misalignment between file system and flash-based storage, F2FS 423aligns the start block address of CP with the segment size. Also, it aligns the 424start block address of Main area with the zone size by reserving some segments 425in SSA area. 426 427Reference the following survey for additional technical details. 428https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 429 430File System Metadata Structure 431------------------------------ 432 433F2FS adopts the checkpointing scheme to maintain file system consistency. At 434mount time, F2FS first tries to find the last valid checkpoint data by scanning 435CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 436One of them always indicates the last valid data, which is called as shadow copy 437mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 438 439For file system consistency, each CP points to which NAT and SIT copies are 440valid, as shown as below. 441 442 +--------+----------+---------+ 443 | CP | SIT | NAT | 444 +--------+----------+---------+ 445 . . . . 446 . . . . 447 . . . . 448 +-------+-------+--------+--------+--------+--------+ 449 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 | 450 +-------+-------+--------+--------+--------+--------+ 451 | ^ ^ 452 | | | 453 `----------------------------------------' 454 455Index Structure 456--------------- 457 458The key data structure to manage the data locations is a "node". Similar to 459traditional file structures, F2FS has three types of node: inode, direct node, 460indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 461indices, two direct node pointers, two indirect node pointers, and one double 462indirect node pointer as described below. One direct node block contains 1018 463data blocks, and one indirect node block contains also 1018 node blocks. Thus, 464one inode block (i.e., a file) covers: 465 466 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 467 468 Inode block (4KB) 469 |- data (923) 470 |- direct node (2) 471 | `- data (1018) 472 |- indirect node (2) 473 | `- direct node (1018) 474 | `- data (1018) 475 `- double indirect node (1) 476 `- indirect node (1018) 477 `- direct node (1018) 478 `- data (1018) 479 480Note that, all the node blocks are mapped by NAT which means the location of 481each node is translated by the NAT table. In the consideration of the wandering 482tree problem, F2FS is able to cut off the propagation of node updates caused by 483leaf data writes. 484 485Directory Structure 486------------------- 487 488A directory entry occupies 11 bytes, which consists of the following attributes. 489 490- hash hash value of the file name 491- ino inode number 492- len the length of file name 493- type file type such as directory, symlink, etc 494 495A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 496used to represent whether each dentry is valid or not. A dentry block occupies 4974KB with the following composition. 498 499 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 500 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 501 502 [Bucket] 503 +--------------------------------+ 504 |dentry block 1 | dentry block 2 | 505 +--------------------------------+ 506 . . 507 . . 508 . [Dentry Block Structure: 4KB] . 509 +--------+----------+----------+------------+ 510 | bitmap | reserved | dentries | file names | 511 +--------+----------+----------+------------+ 512 [Dentry Block: 4KB] . . 513 . . 514 . . 515 +------+------+-----+------+ 516 | hash | ino | len | type | 517 +------+------+-----+------+ 518 [Dentry Structure: 11 bytes] 519 520F2FS implements multi-level hash tables for directory structure. Each level has 521a hash table with dedicated number of hash buckets as shown below. Note that 522"A(2B)" means a bucket includes 2 data blocks. 523 524---------------------- 525A : bucket 526B : block 527N : MAX_DIR_HASH_DEPTH 528---------------------- 529 530level #0 | A(2B) 531 | 532level #1 | A(2B) - A(2B) 533 | 534level #2 | A(2B) - A(2B) - A(2B) - A(2B) 535 . | . . . . 536level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 537 . | . . . . 538level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 539 540The number of blocks and buckets are determined by, 541 542 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 543 # of blocks in level #n = | 544 `- 4, Otherwise 545 546 ,- 2^(n + dir_level), 547 | if n + dir_level < MAX_DIR_HASH_DEPTH / 2, 548 # of buckets in level #n = | 549 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), 550 Otherwise 551 552When F2FS finds a file name in a directory, at first a hash value of the file 553name is calculated. Then, F2FS scans the hash table in level #0 to find the 554dentry consisting of the file name and its inode number. If not found, F2FS 555scans the next hash table in level #1. In this way, F2FS scans hash tables in 556each levels incrementally from 1 to N. In each levels F2FS needs to scan only 557one bucket determined by the following equation, which shows O(log(# of files)) 558complexity. 559 560 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 561 562In the case of file creation, F2FS finds empty consecutive slots that cover the 563file name. F2FS searches the empty slots in the hash tables of whole levels from 5641 to N in the same way as the lookup operation. 565 566The following figure shows an example of two cases holding children. 567 --------------> Dir <-------------- 568 | | 569 child child 570 571 child - child [hole] - child 572 573 child - child - child [hole] - [hole] - child 574 575 Case 1: Case 2: 576 Number of children = 6, Number of children = 3, 577 File size = 7 File size = 7 578 579Default Block Allocation 580------------------------ 581 582At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 583and Hot/Warm/Cold data. 584 585- Hot node contains direct node blocks of directories. 586- Warm node contains direct node blocks except hot node blocks. 587- Cold node contains indirect node blocks 588- Hot data contains dentry blocks 589- Warm data contains data blocks except hot and cold data blocks 590- Cold data contains multimedia data or migrated data blocks 591 592LFS has two schemes for free space management: threaded log and copy-and-compac- 593tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 594for devices showing very good sequential write performance, since free segments 595are served all the time for writing new data. However, it suffers from cleaning 596overhead under high utilization. Contrarily, the threaded log scheme suffers 597from random writes, but no cleaning process is needed. F2FS adopts a hybrid 598scheme where the copy-and-compaction scheme is adopted by default, but the 599policy is dynamically changed to the threaded log scheme according to the file 600system status. 601 602In order to align F2FS with underlying flash-based storage, F2FS allocates a 603segment in a unit of section. F2FS expects that the section size would be the 604same as the unit size of garbage collection in FTL. Furthermore, with respect 605to the mapping granularity in FTL, F2FS allocates each section of the active 606logs from different zones as much as possible, since FTL can write the data in 607the active logs into one allocation unit according to its mapping granularity. 608 609Cleaning process 610---------------- 611 612F2FS does cleaning both on demand and in the background. On-demand cleaning is 613triggered when there are not enough free segments to serve VFS calls. Background 614cleaner is operated by a kernel thread, and triggers the cleaning job when the 615system is idle. 616 617F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 618In the greedy algorithm, F2FS selects a victim segment having the smallest number 619of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 620according to the segment age and the number of valid blocks in order to address 621log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 622algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 623algorithm. 624 625In order to identify whether the data in the victim segment are valid or not, 626F2FS manages a bitmap. Each bit represents the validity of a block, and the 627bitmap is composed of a bit stream covering whole blocks in main area. 628