forked from torvalds/linux
-
Notifications
You must be signed in to change notification settings - Fork 0
/
Copy pathf2fs.txt
613 lines (511 loc) · 28.9 KB
/
f2fs.txt
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
================================================================================
WHAT IS Flash-Friendly File System (F2FS)?
================================================================================
NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
been equipped on a variety systems ranging from mobile to server systems. Since
they are known to have different characteristics from the conventional rotating
disks, a file system, an upper layer to the storage device, should adapt to the
changes from the sketch in the design level.
F2FS is a file system exploiting NAND flash memory-based storage devices, which
is based on Log-structured File System (LFS). The design has been focused on
addressing the fundamental issues in LFS, which are snowball effect of wandering
tree and high cleaning overhead.
Since a NAND flash memory-based storage device shows different characteristic
according to its internal geometry or flash memory management scheme, namely FTL,
F2FS and its tools support various parameters not only for configuring on-disk
layout, but also for selecting allocation and cleaning algorithms.
The following git tree provides the file system formatting tool (mkfs.f2fs),
a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
>> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
For reporting bugs and sending patches, please use the following mailing list:
================================================================================
BACKGROUND AND DESIGN ISSUES
================================================================================
Log-structured File System (LFS)
--------------------------------
"A log-structured file system writes all modifications to disk sequentially in
a log-like structure, thereby speeding up both file writing and crash recovery.
The log is the only structure on disk; it contains indexing information so that
files can be read back from the log efficiently. In order to maintain large free
areas on disk for fast writing, we divide the log into segments and use a
segment cleaner to compress the live information from heavily fragmented
segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
implementation of a log-structured file system", ACM Trans. Computer Systems
10, 1, 26–52.
Wandering Tree Problem
----------------------
In LFS, when a file data is updated and written to the end of log, its direct
pointer block is updated due to the changed location. Then the indirect pointer
block is also updated due to the direct pointer block update. In this manner,
the upper index structures such as inode, inode map, and checkpoint block are
also updated recursively. This problem is called as wandering tree problem [1],
and in order to enhance the performance, it should eliminate or relax the update
propagation as much as possible.
[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
Cleaning Overhead
-----------------
Since LFS is based on out-of-place writes, it produces so many obsolete blocks
scattered across the whole storage. In order to serve new empty log space, it
needs to reclaim these obsolete blocks seamlessly to users. This job is called
as a cleaning process.
The process consists of three operations as follows.
1. A victim segment is selected through referencing segment usage table.
2. It loads parent index structures of all the data in the victim identified by
segment summary blocks.
3. It checks the cross-reference between the data and its parent index structure.
4. It moves valid data selectively.
This cleaning job may cause unexpected long delays, so the most important goal
is to hide the latencies to users. And also definitely, it should reduce the
amount of valid data to be moved, and move them quickly as well.
================================================================================
KEY FEATURES
================================================================================
Flash Awareness
---------------
- Enlarge the random write area for better performance, but provide the high
spatial locality
- Align FS data structures to the operational units in FTL as best efforts
Wandering Tree Problem
----------------------
- Use a term, “node”, that represents inodes as well as various pointer blocks
- Introduce Node Address Table (NAT) containing the locations of all the “node”
blocks; this will cut off the update propagation.
Cleaning Overhead
-----------------
- Support a background cleaning process
- Support greedy and cost-benefit algorithms for victim selection policies
- Support multi-head logs for static/dynamic hot and cold data separation
- Introduce adaptive logging for efficient block allocation
================================================================================
MOUNT OPTIONS
================================================================================
background_gc=%s Turn on/off cleaning operations, namely garbage
collection, triggered in background when I/O subsystem is
idle. If background_gc=on, it will turn on the garbage
collection and if background_gc=off, garbage collection
will be turned off. If background_gc=sync, it will turn
on synchronous garbage collection running in background.
Default value for this option is on. So garbage
collection is on by default.
disable_roll_forward Disable the roll-forward recovery routine
norecovery Disable the roll-forward recovery routine, mounted read-
only (i.e., -o ro,disable_roll_forward)
discard/nodiscard Enable/disable real-time discard in f2fs, if discard is
enabled, f2fs will issue discard/TRIM commands when a
segment is cleaned.
no_heap Disable heap-style segment allocation which finds free
segments for data from the beginning of main area, while
for node from the end of main area.
nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
by default if CONFIG_F2FS_FS_XATTR is selected.
noacl Disable POSIX Access Control List. Note: acl is enabled
by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
active_logs=%u Support configuring the number of active logs. In the
current design, f2fs supports only 2, 4, and 6 logs.
Default number is 6.
disable_ext_identify Disable the extension list configured by mkfs, so f2fs
does not aware of cold files such as media files.
inline_xattr Enable the inline xattrs feature.
noinline_xattr Disable the inline xattrs feature.
inline_data Enable the inline data feature: New created small(<~3.4k)
files can be written into inode block.
inline_dentry Enable the inline dir feature: data in new created
directory entries can be written into inode block. The
space of inode block which is used to store inline
dentries is limited to ~3.4k.
noinline_dentry Disable the inline dentry feature.
flush_merge Merge concurrent cache_flush commands as much as possible
to eliminate redundant command issues. If the underlying
device handles the cache_flush command relatively slowly,
recommend to enable this option.
nobarrier This option can be used if underlying storage guarantees
its cached data should be written to the novolatile area.
If this option is set, no cache_flush commands are issued
but f2fs still guarantees the write ordering of all the
data writes.
fastboot This option is used when a system wants to reduce mount
time as much as possible, even though normal performance
can be sacrificed.
extent_cache Enable an extent cache based on rb-tree, it can cache
as many as extent which map between contiguous logical
address and physical address per inode, resulting in
increasing the cache hit ratio. Set by default.
noextent_cache Disable an extent cache based on rb-tree explicitly, see
the above extent_cache mount option.
noinline_data Disable the inline data feature, inline data feature is
enabled by default.
data_flush Enable data flushing before checkpoint in order to
persist data of regular and symlink.
fault_injection=%d Enable fault injection in all supported types with
specified injection rate.
mode=%s Control block allocation mode which supports "adaptive"
and "lfs". In "lfs" mode, there should be no random
writes towards main area.
io_bits=%u Set the bit size of write IO requests. It should be set
with "mode=lfs".
usrquota Enable plain user disk quota accounting.
grpquota Enable plain group disk quota accounting.
prjquota Enable plain project quota accounting.
usrjquota=<file> Appoint specified file and type during mount, so that quota
grpjquota=<file> information can be properly updated during recovery flow,
prjjquota=<file> <quota file>: must be in root directory;
jqfmt=<quota type> <quota type>: [vfsold,vfsv0,vfsv1].
offusrjquota Turn off user journelled quota.
offgrpjquota Turn off group journelled quota.
offprjjquota Turn off project journelled quota.
quota Enable plain user disk quota accounting.
noquota Disable all plain disk quota option.
================================================================================
DEBUGFS ENTRIES
================================================================================
/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
f2fs. Each file shows the whole f2fs information.
/sys/kernel/debug/f2fs/status includes:
- major file system information managed by f2fs currently
- average SIT information about whole segments
- current memory footprint consumed by f2fs.
================================================================================
SYSFS ENTRIES
================================================================================
Information about mounted f2fs file systems can be found in
/sys/fs/f2fs. Each mounted filesystem will have a directory in
/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
The files in each per-device directory are shown in table below.
Files in /sys/fs/f2fs/<devname>
(see also Documentation/ABI/testing/sysfs-fs-f2fs)
..............................................................................
File Content
gc_max_sleep_time This tuning parameter controls the maximum sleep
time for the garbage collection thread. Time is
in milliseconds.
gc_min_sleep_time This tuning parameter controls the minimum sleep
time for the garbage collection thread. Time is
in milliseconds.
gc_no_gc_sleep_time This tuning parameter controls the default sleep
time for the garbage collection thread. Time is
in milliseconds.
gc_idle This parameter controls the selection of victim
policy for garbage collection. Setting gc_idle = 0
(default) will disable this option. Setting
gc_idle = 1 will select the Cost Benefit approach
& setting gc_idle = 2 will select the greedy approach.
gc_urgent This parameter controls triggering background GCs
urgently or not. Setting gc_urgent = 0 [default]
makes back to default behavior, while if it is set
to 1, background thread starts to do GC by given
gc_urgent_sleep_time interval.
gc_urgent_sleep_time This parameter controls sleep time for gc_urgent.
500 ms is set by default. See above gc_urgent.
reclaim_segments This parameter controls the number of prefree
segments to be reclaimed. If the number of prefree
segments is larger than the number of segments
in the proportion to the percentage over total
volume size, f2fs tries to conduct checkpoint to
reclaim the prefree segments to free segments.
By default, 5% over total # of segments.
max_small_discards This parameter controls the number of discard
commands that consist small blocks less than 2MB.
The candidates to be discarded are cached until
checkpoint is triggered, and issued during the
checkpoint. By default, it is disabled with 0.
trim_sections This parameter controls the number of sections
to be trimmed out in batch mode when FITRIM
conducts. 32 sections is set by default.
ipu_policy This parameter controls the policy of in-place
updates in f2fs. There are five policies:
0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
0x04: F2FS_IPU_UTIL, 0x08: F2FS_IPU_SSR_UTIL,
0x10: F2FS_IPU_FSYNC.
min_ipu_util This parameter controls the threshold to trigger
in-place-updates. The number indicates percentage
of the filesystem utilization, and used by
F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
min_fsync_blocks This parameter controls the threshold to trigger
in-place-updates when F2FS_IPU_FSYNC mode is set.
The number indicates the number of dirty pages
when fsync needs to flush on its call path. If
the number is less than this value, it triggers
in-place-updates.
max_victim_search This parameter controls the number of trials to
find a victim segment when conducting SSR and
cleaning operations. The default value is 4096
which covers 8GB block address range.
dir_level This parameter controls the directory level to
support large directory. If a directory has a
number of files, it can reduce the file lookup
latency by increasing this dir_level value.
Otherwise, it needs to decrease this value to
reduce the space overhead. The default value is 0.
ram_thresh This parameter controls the memory footprint used
by free nids and cached nat entries. By default,
10 is set, which indicates 10 MB / 1 GB RAM.
================================================================================
USAGE
================================================================================
1. Download userland tools and compile them.
2. Skip, if f2fs was compiled statically inside kernel.
Otherwise, insert the f2fs.ko module.
# insmod f2fs.ko
3. Create a directory trying to mount
# mkdir /mnt/f2fs
4. Format the block device, and then mount as f2fs
# mkfs.f2fs -l label /dev/block_device
# mount -t f2fs /dev/block_device /mnt/f2fs
mkfs.f2fs
---------
The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
which builds a basic on-disk layout.
The options consist of:
-l [label] : Give a volume label, up to 512 unicode name.
-a [0 or 1] : Split start location of each area for heap-based allocation.
1 is set by default, which performs this.
-o [int] : Set overprovision ratio in percent over volume size.
5 is set by default.
-s [int] : Set the number of segments per section.
1 is set by default.
-z [int] : Set the number of sections per zone.
1 is set by default.
-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
-t [0 or 1] : Disable discard command or not.
1 is set by default, which conducts discard.
fsck.f2fs
---------
The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
partition, which examines whether the filesystem metadata and user-made data
are cross-referenced correctly or not.
Note that, initial version of the tool does not fix any inconsistency.
The options consist of:
-d debug level [default:0]
dump.f2fs
---------
The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
file. Each file is dump_ssa and dump_sit.
The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
It shows on-disk inode information recognized by a given inode number, and is
able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
./dump_sit respectively.
The options consist of:
-d debug level [default:0]
-i inode no (hex)
-s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
-a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
Examples:
# dump.f2fs -i [ino] /dev/sdx
# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
================================================================================
DESIGN
================================================================================
On-disk Layout
--------------
F2FS divides the whole volume into a number of segments, each of which is fixed
to 2MB in size. A section is composed of consecutive segments, and a zone
consists of a set of sections. By default, section and zone sizes are set to one
segment size identically, but users can easily modify the sizes by mkfs.
F2FS splits the entire volume into six areas, and all the areas except superblock
consists of multiple segments as described below.
align with the zone size <-|
|-> align with the segment size
_________________________________________________________________________
| | | Segment | Node | Segment | |
| Superblock | Checkpoint | Info. | Address | Summary | Main |
| (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
|____________|_____2______|______N______|______N______|______N_____|__N___|
. .
. .
. .
._________________________________________.
|_Segment_|_..._|_Segment_|_..._|_Segment_|
. .
._________._________
|_section_|__...__|_
. .
.________.
|__zone__|
- Superblock (SB)
: It is located at the beginning of the partition, and there exist two copies
to avoid file system crash. It contains basic partition information and some
default parameters of f2fs.
- Checkpoint (CP)
: It contains file system information, bitmaps for valid NAT/SIT sets, orphan
inode lists, and summary entries of current active segments.
- Segment Information Table (SIT)
: It contains segment information such as valid block count and bitmap for the
validity of all the blocks.
- Node Address Table (NAT)
: It is composed of a block address table for all the node blocks stored in
Main area.
- Segment Summary Area (SSA)
: It contains summary entries which contains the owner information of all the
data and node blocks stored in Main area.
- Main Area
: It contains file and directory data including their indices.
In order to avoid misalignment between file system and flash-based storage, F2FS
aligns the start block address of CP with the segment size. Also, it aligns the
start block address of Main area with the zone size by reserving some segments
in SSA area.
Reference the following survey for additional technical details.
https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
File System Metadata Structure
------------------------------
F2FS adopts the checkpointing scheme to maintain file system consistency. At
mount time, F2FS first tries to find the last valid checkpoint data by scanning
CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
One of them always indicates the last valid data, which is called as shadow copy
mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
For file system consistency, each CP points to which NAT and SIT copies are
valid, as shown as below.
+--------+----------+---------+
| CP | SIT | NAT |
+--------+----------+---------+
. . . .
. . . .
. . . .
+-------+-------+--------+--------+--------+--------+
| CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
+-------+-------+--------+--------+--------+--------+
| ^ ^
| | |
`----------------------------------------'
Index Structure
---------------
The key data structure to manage the data locations is a "node". Similar to
traditional file structures, F2FS has three types of node: inode, direct node,
indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
indices, two direct node pointers, two indirect node pointers, and one double
indirect node pointer as described below. One direct node block contains 1018
data blocks, and one indirect node block contains also 1018 node blocks. Thus,
one inode block (i.e., a file) covers:
4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
Inode block (4KB)
|- data (923)
|- direct node (2)
| `- data (1018)
|- indirect node (2)
| `- direct node (1018)
| `- data (1018)
`- double indirect node (1)
`- indirect node (1018)
`- direct node (1018)
`- data (1018)
Note that, all the node blocks are mapped by NAT which means the location of
each node is translated by the NAT table. In the consideration of the wandering
tree problem, F2FS is able to cut off the propagation of node updates caused by
leaf data writes.
Directory Structure
-------------------
A directory entry occupies 11 bytes, which consists of the following attributes.
- hash hash value of the file name
- ino inode number
- len the length of file name
- type file type such as directory, symlink, etc
A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
used to represent whether each dentry is valid or not. A dentry block occupies
4KB with the following composition.
Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
dentries(11 * 214 bytes) + file name (8 * 214 bytes)
[Bucket]
+--------------------------------+
|dentry block 1 | dentry block 2 |
+--------------------------------+
. .
. .
. [Dentry Block Structure: 4KB] .
+--------+----------+----------+------------+
| bitmap | reserved | dentries | file names |
+--------+----------+----------+------------+
[Dentry Block: 4KB] . .
. .
. .
+------+------+-----+------+
| hash | ino | len | type |
+------+------+-----+------+
[Dentry Structure: 11 bytes]
F2FS implements multi-level hash tables for directory structure. Each level has
a hash table with dedicated number of hash buckets as shown below. Note that
"A(2B)" means a bucket includes 2 data blocks.
----------------------
A : bucket
B : block
N : MAX_DIR_HASH_DEPTH
----------------------
level #0 | A(2B)
|
level #1 | A(2B) - A(2B)
|
level #2 | A(2B) - A(2B) - A(2B) - A(2B)
. | . . . .
level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
. | . . . .
level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
The number of blocks and buckets are determined by,
,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
# of blocks in level #n = |
`- 4, Otherwise
,- 2^(n + dir_level),
| if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
# of buckets in level #n = |
`- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
Otherwise
When F2FS finds a file name in a directory, at first a hash value of the file
name is calculated. Then, F2FS scans the hash table in level #0 to find the
dentry consisting of the file name and its inode number. If not found, F2FS
scans the next hash table in level #1. In this way, F2FS scans hash tables in
each levels incrementally from 1 to N. In each levels F2FS needs to scan only
one bucket determined by the following equation, which shows O(log(# of files))
complexity.
bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
In the case of file creation, F2FS finds empty consecutive slots that cover the
file name. F2FS searches the empty slots in the hash tables of whole levels from
1 to N in the same way as the lookup operation.
The following figure shows an example of two cases holding children.
--------------> Dir <--------------
| |
child child
child - child [hole] - child
child - child - child [hole] - [hole] - child
Case 1: Case 2:
Number of children = 6, Number of children = 3,
File size = 7 File size = 7
Default Block Allocation
------------------------
At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
and Hot/Warm/Cold data.
- Hot node contains direct node blocks of directories.
- Warm node contains direct node blocks except hot node blocks.
- Cold node contains indirect node blocks
- Hot data contains dentry blocks
- Warm data contains data blocks except hot and cold data blocks
- Cold data contains multimedia data or migrated data blocks
LFS has two schemes for free space management: threaded log and copy-and-compac-
tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
for devices showing very good sequential write performance, since free segments
are served all the time for writing new data. However, it suffers from cleaning
overhead under high utilization. Contrarily, the threaded log scheme suffers
from random writes, but no cleaning process is needed. F2FS adopts a hybrid
scheme where the copy-and-compaction scheme is adopted by default, but the
policy is dynamically changed to the threaded log scheme according to the file
system status.
In order to align F2FS with underlying flash-based storage, F2FS allocates a
segment in a unit of section. F2FS expects that the section size would be the
same as the unit size of garbage collection in FTL. Furthermore, with respect
to the mapping granularity in FTL, F2FS allocates each section of the active
logs from different zones as much as possible, since FTL can write the data in
the active logs into one allocation unit according to its mapping granularity.
Cleaning process
----------------
F2FS does cleaning both on demand and in the background. On-demand cleaning is
triggered when there are not enough free segments to serve VFS calls. Background
cleaner is operated by a kernel thread, and triggers the cleaning job when the
system is idle.
F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
In the greedy algorithm, F2FS selects a victim segment having the smallest number
of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
according to the segment age and the number of valid blocks in order to address
log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
algorithm.
In order to identify whether the data in the victim segment are valid or not,
F2FS manages a bitmap. Each bit represents the validity of a block, and the
bitmap is composed of a bit stream covering whole blocks in main area.