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Commit 9268cc3523c120eed04a6ad980753d6e7b82d071

Authored by Huajun Li
Committed by Jaegeuk Kim
1 parent 24c366a9ea
Exists in smarc-l5.0.0_1.0.0-ga and in 5 other branches imx_3.10.17_1.0.1_ga, imx_3.10.53_1.1.0_ga, imx_3.14.28_1.0.0_ga, smarc-imx_3.10.53_1.1.0_ga, smarc-imx_3.14.28_1.0.0_ga

f2fs: update f2fs document to reflect SIT/NAT layout correctly 

document to reflect the layout generated by mkfs.f2fs .

Signed-off-by: Huajun Li <huajun.li.lee@gmail.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>

Showing 1 changed file with 9 additions and 9 deletions Inline Diff

Documentation/filesystems/f2fs.txt
Diff comments   View file @ 9268cc3
1 ================================================================================ 1 ================================================================================
2 WHAT IS Flash-Friendly File System (F2FS)? 2 WHAT IS Flash-Friendly File System (F2FS)?
3 ================================================================================ 3 ================================================================================
4 4
5 NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have 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 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 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 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. 9 changes from the sketch in the design level.
10 10
11 F2FS is a file system exploiting NAND flash memory-based storage devices, which 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 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 13 addressing the fundamental issues in LFS, which are snowball effect of wandering
14 tree and high cleaning overhead. 14 tree and high cleaning overhead.
15 15
16 Since a NAND flash memory-based storage device shows different characteristic 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, 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 18 F2FS and its tools support various parameters not only for configuring on-disk
19 layout, but also for selecting allocation and cleaning algorithms. 19 layout, but also for selecting allocation and cleaning algorithms.
20 20
21 The file system formatting tool, "mkfs.f2fs", is available from the following 21 The file system formatting tool, "mkfs.f2fs", is available from the following
22 git tree: 22 git tree:
23 >> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git 23 >> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
24 24
25 For reporting bugs and sending patches, please use the following mailing list: 25 For reporting bugs and sending patches, please use the following mailing list:
26 >> linux-f2fs-devel@lists.sourceforge.net 26 >> linux-f2fs-devel@lists.sourceforge.net
27 27
28 ================================================================================ 28 ================================================================================
29 BACKGROUND AND DESIGN ISSUES 29 BACKGROUND AND DESIGN ISSUES
30 ================================================================================ 30 ================================================================================
31 31
32 Log-structured File System (LFS) 32 Log-structured File System (LFS)
33 -------------------------------- 33 --------------------------------
34 "A log-structured file system writes all modifications to disk sequentially in 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. 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 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 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 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 39 segment cleaner to compress the live information from heavily fragmented
40 segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and 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 41 implementation of a log-structured file system", ACM Trans. Computer Systems
42 10, 1, 26–52. 42 10, 1, 26–52.
43 43
44 Wandering Tree Problem 44 Wandering Tree Problem
45 ---------------------- 45 ----------------------
46 In LFS, when a file data is updated and written to the end of log, its direct 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 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, 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 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], 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 51 and in order to enhance the performance, it should eliminate or relax the update
52 propagation as much as possible. 52 propagation as much as possible.
53 53
54 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/ 54 [1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
55 55
56 Cleaning Overhead 56 Cleaning Overhead
57 ----------------- 57 -----------------
58 Since LFS is based on out-of-place writes, it produces so many obsolete blocks 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 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 60 needs to reclaim these obsolete blocks seamlessly to users. This job is called
61 as a cleaning process. 61 as a cleaning process.
62 62
63 The process consists of three operations as follows. 63 The process consists of three operations as follows.
64 1. A victim segment is selected through referencing segment usage table. 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 65 2. It loads parent index structures of all the data in the victim identified by
66 segment summary blocks. 66 segment summary blocks.
67 3. It checks the cross-reference between the data and its parent index structure. 67 3. It checks the cross-reference between the data and its parent index structure.
68 4. It moves valid data selectively. 68 4. It moves valid data selectively.
69 69
70 This cleaning job may cause unexpected long delays, so the most important goal 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 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. 72 amount of valid data to be moved, and move them quickly as well.
73 73
74 ================================================================================ 74 ================================================================================
75 KEY FEATURES 75 KEY FEATURES
76 ================================================================================ 76 ================================================================================
77 77
78 Flash Awareness 78 Flash Awareness
79 --------------- 79 ---------------
80 - Enlarge the random write area for better performance, but provide the high 80 - Enlarge the random write area for better performance, but provide the high
81 spatial locality 81 spatial locality
82 - Align FS data structures to the operational units in FTL as best efforts 82 - Align FS data structures to the operational units in FTL as best efforts
83 83
84 Wandering Tree Problem 84 Wandering Tree Problem
85 ---------------------- 85 ----------------------
86 - Use a term, “node”, that represents inodes as well as various pointer blocks 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” 87 - Introduce Node Address Table (NAT) containing the locations of all the “node”
88 blocks; this will cut off the update propagation. 88 blocks; this will cut off the update propagation.
89 89
90 Cleaning Overhead 90 Cleaning Overhead
91 ----------------- 91 -----------------
92 - Support a background cleaning process 92 - Support a background cleaning process
93 - Support greedy and cost-benefit algorithms for victim selection policies 93 - Support greedy and cost-benefit algorithms for victim selection policies
94 - Support multi-head logs for static/dynamic hot and cold data separation 94 - Support multi-head logs for static/dynamic hot and cold data separation
95 - Introduce adaptive logging for efficient block allocation 95 - Introduce adaptive logging for efficient block allocation
96 96
97 ================================================================================ 97 ================================================================================
98 MOUNT OPTIONS 98 MOUNT OPTIONS
99 ================================================================================ 99 ================================================================================
100 100
101 background_gc_off Turn off cleaning operations, namely garbage collection, 101 background_gc_off Turn off cleaning operations, namely garbage collection,
102 triggered in background when I/O subsystem is idle. 102 triggered in background when I/O subsystem is idle.
103 disable_roll_forward Disable the roll-forward recovery routine 103 disable_roll_forward Disable the roll-forward recovery routine
104 discard Issue discard/TRIM commands when a segment is cleaned. 104 discard Issue discard/TRIM commands when a segment is cleaned.
105 no_heap Disable heap-style segment allocation which finds free 105 no_heap Disable heap-style segment allocation which finds free
106 segments for data from the beginning of main area, while 106 segments for data from the beginning of main area, while
107 for node from the end of main area. 107 for node from the end of main area.
108 nouser_xattr Disable Extended User Attributes. Note: xattr is enabled 108 nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
109 by default if CONFIG_F2FS_FS_XATTR is selected. 109 by default if CONFIG_F2FS_FS_XATTR is selected.
110 noacl Disable POSIX Access Control List. Note: acl is enabled 110 noacl Disable POSIX Access Control List. Note: acl is enabled
111 by default if CONFIG_F2FS_FS_POSIX_ACL is selected. 111 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
112 active_logs=%u Support configuring the number of active logs. In the 112 active_logs=%u Support configuring the number of active logs. In the
113 current design, f2fs supports only 2, 4, and 6 logs. 113 current design, f2fs supports only 2, 4, and 6 logs.
114 Default number is 6. 114 Default number is 6.
115 disable_ext_identify Disable the extension list configured by mkfs, so f2fs 115 disable_ext_identify Disable the extension list configured by mkfs, so f2fs
116 does not aware of cold files such as media files. 116 does not aware of cold files such as media files.
117 117
118 ================================================================================ 118 ================================================================================
119 DEBUGFS ENTRIES 119 DEBUGFS ENTRIES
120 ================================================================================ 120 ================================================================================
121 121
122 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as 122 /sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
123 f2fs. Each file shows the whole f2fs information. 123 f2fs. Each file shows the whole f2fs information.
124 124
125 /sys/kernel/debug/f2fs/status includes: 125 /sys/kernel/debug/f2fs/status includes:
126 - major file system information managed by f2fs currently 126 - major file system information managed by f2fs currently
127 - average SIT information about whole segments 127 - average SIT information about whole segments
128 - current memory footprint consumed by f2fs. 128 - current memory footprint consumed by f2fs.
129 129
130 ================================================================================ 130 ================================================================================
131 USAGE 131 USAGE
132 ================================================================================ 132 ================================================================================
133 133
134 1. Download userland tools and compile them. 134 1. Download userland tools and compile them.
135 135
136 2. Skip, if f2fs was compiled statically inside kernel. 136 2. Skip, if f2fs was compiled statically inside kernel.
137 Otherwise, insert the f2fs.ko module. 137 Otherwise, insert the f2fs.ko module.
138 # insmod f2fs.ko 138 # insmod f2fs.ko
139 139
140 3. Create a directory trying to mount 140 3. Create a directory trying to mount
141 # mkdir /mnt/f2fs 141 # mkdir /mnt/f2fs
142 142
143 4. Format the block device, and then mount as f2fs 143 4. Format the block device, and then mount as f2fs
144 # mkfs.f2fs -l label /dev/block_device 144 # mkfs.f2fs -l label /dev/block_device
145 # mount -t f2fs /dev/block_device /mnt/f2fs 145 # mount -t f2fs /dev/block_device /mnt/f2fs
146 146
147 Format options 147 Format options
148 -------------- 148 --------------
149 -l [label] : Give a volume label, up to 256 unicode name. 149 -l [label] : Give a volume label, up to 256 unicode name.
150 -a [0 or 1] : Split start location of each area for heap-based allocation. 150 -a [0 or 1] : Split start location of each area for heap-based allocation.
151 1 is set by default, which performs this. 151 1 is set by default, which performs this.
152 -o [int] : Set overprovision ratio in percent over volume size. 152 -o [int] : Set overprovision ratio in percent over volume size.
153 5 is set by default. 153 5 is set by default.
154 -s [int] : Set the number of segments per section. 154 -s [int] : Set the number of segments per section.
155 1 is set by default. 155 1 is set by default.
156 -z [int] : Set the number of sections per zone. 156 -z [int] : Set the number of sections per zone.
157 1 is set by default. 157 1 is set by default.
158 -e [str] : Set basic extension list. e.g. "mp3,gif,mov" 158 -e [str] : Set basic extension list. e.g. "mp3,gif,mov"
159 159
160 ================================================================================ 160 ================================================================================
161 DESIGN 161 DESIGN
162 ================================================================================ 162 ================================================================================
163 163
164 On-disk Layout 164 On-disk Layout
165 -------------- 165 --------------
166 166
167 F2FS divides the whole volume into a number of segments, each of which is fixed 167 F2FS divides the whole volume into a number of segments, each of which is fixed
168 to 2MB in size. A section is composed of consecutive segments, and a zone 168 to 2MB in size. A section is composed of consecutive segments, and a zone
169 consists of a set of sections. By default, section and zone sizes are set to one 169 consists of a set of sections. By default, section and zone sizes are set to one
170 segment size identically, but users can easily modify the sizes by mkfs. 170 segment size identically, but users can easily modify the sizes by mkfs.
171 171
172 F2FS splits the entire volume into six areas, and all the areas except superblock 172 F2FS splits the entire volume into six areas, and all the areas except superblock
173 consists of multiple segments as described below. 173 consists of multiple segments as described below.
174 174
175 align with the zone size <-| 175 align with the zone size <-|
176 |-> align with the segment size 176 |-> align with the segment size
177 _________________________________________________________________________ 177 _________________________________________________________________________
178 | | | Node | Segment | Segment | | 178 | | | Segment | Node | Segment | |
179 | Superblock | Checkpoint | Address | Info. | Summary | Main | 179 | Superblock | Checkpoint | Info. | Address | Summary | Main |
180 | (SB) | (CP) | Table (NAT) | Table (SIT) | Area (SSA) | | 180 | (SB) | (CP) | Table (SIT) | Table (NAT) | Area (SSA) | |
181 |____________|_____2______|______N______|______N______|______N_____|__N___| 181 |____________|_____2______|______N______|______N______|______N_____|__N___|
182 . . 182 . .
183 . . 183 . .
184 . . 184 . .
185 ._________________________________________. 185 ._________________________________________.
186 |_Segment_|_..._|_Segment_|_..._|_Segment_| 186 |_Segment_|_..._|_Segment_|_..._|_Segment_|
187 . . 187 . .
188 ._________._________ 188 ._________._________
189 |_section_|__...__|_ 189 |_section_|__...__|_
190 . . 190 . .
191 .________. 191 .________.
192 |__zone__| 192 |__zone__|
193 193
194 - Superblock (SB) 194 - Superblock (SB)
195 : It is located at the beginning of the partition, and there exist two copies 195 : It is located at the beginning of the partition, and there exist two copies
196 to avoid file system crash. It contains basic partition information and some 196 to avoid file system crash. It contains basic partition information and some
197 default parameters of f2fs. 197 default parameters of f2fs.
198 198
199 - Checkpoint (CP) 199 - Checkpoint (CP)
200 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan 200 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
201 inode lists, and summary entries of current active segments. 201 inode lists, and summary entries of current active segments.
202 202
203 - Node Address Table (NAT)
204 : It is composed of a block address table for all the node blocks stored in
205 Main area.
206
207 - Segment Information Table (SIT) 203 - Segment Information Table (SIT)
208 : It contains segment information such as valid block count and bitmap for the 204 : It contains segment information such as valid block count and bitmap for the
209 validity of all the blocks. 205 validity of all the blocks.
210 206
207 - Node Address Table (NAT)
208 : It is composed of a block address table for all the node blocks stored in
209 Main area.
210
211 - Segment Summary Area (SSA) 211 - Segment Summary Area (SSA)
212 : It contains summary entries which contains the owner information of all the 212 : It contains summary entries which contains the owner information of all the
213 data and node blocks stored in Main area. 213 data and node blocks stored in Main area.
214 214
215 - Main Area 215 - Main Area
216 : It contains file and directory data including their indices. 216 : It contains file and directory data including their indices.
217 217
218 In order to avoid misalignment between file system and flash-based storage, F2FS 218 In order to avoid misalignment between file system and flash-based storage, F2FS
219 aligns the start block address of CP with the segment size. Also, it aligns the 219 aligns the start block address of CP with the segment size. Also, it aligns the
220 start block address of Main area with the zone size by reserving some segments 220 start block address of Main area with the zone size by reserving some segments
221 in SSA area. 221 in SSA area.
222 222
223 Reference the following survey for additional technical details. 223 Reference the following survey for additional technical details.
224 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey 224 https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
225 225
226 File System Metadata Structure 226 File System Metadata Structure
227 ------------------------------ 227 ------------------------------
228 228
229 F2FS adopts the checkpointing scheme to maintain file system consistency. At 229 F2FS adopts the checkpointing scheme to maintain file system consistency. At
230 mount time, F2FS first tries to find the last valid checkpoint data by scanning 230 mount time, F2FS first tries to find the last valid checkpoint data by scanning
231 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP. 231 CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
232 One of them always indicates the last valid data, which is called as shadow copy 232 One of them always indicates the last valid data, which is called as shadow copy
233 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism. 233 mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
234 234
235 For file system consistency, each CP points to which NAT and SIT copies are 235 For file system consistency, each CP points to which NAT and SIT copies are
236 valid, as shown as below. 236 valid, as shown as below.
237 237
238 +--------+----------+---------+ 238 +--------+----------+---------+
239 | CP | NAT | SIT | 239 | CP | SIT | NAT |
240 +--------+----------+---------+ 240 +--------+----------+---------+
241 . . . . 241 . . . .
242 . . . . 242 . . . .
243 . . . . 243 . . . .
244 +-------+-------+--------+--------+--------+--------+ 244 +-------+-------+--------+--------+--------+--------+
245 | CP #0 | CP #1 | NAT #0 | NAT #1 | SIT #0 | SIT #1 | 245 | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
246 +-------+-------+--------+--------+--------+--------+ 246 +-------+-------+--------+--------+--------+--------+
247 | ^ ^ 247 | ^ ^
248 | | | 248 | | |
249 `----------------------------------------' 249 `----------------------------------------'
250 250
251 Index Structure 251 Index Structure
252 --------------- 252 ---------------
253 253
254 The key data structure to manage the data locations is a "node". Similar to 254 The key data structure to manage the data locations is a "node". Similar to
255 traditional file structures, F2FS has three types of node: inode, direct node, 255 traditional file structures, F2FS has three types of node: inode, direct node,
256 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block 256 indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
257 indices, two direct node pointers, two indirect node pointers, and one double 257 indices, two direct node pointers, two indirect node pointers, and one double
258 indirect node pointer as described below. One direct node block contains 1018 258 indirect node pointer as described below. One direct node block contains 1018
259 data blocks, and one indirect node block contains also 1018 node blocks. Thus, 259 data blocks, and one indirect node block contains also 1018 node blocks. Thus,
260 one inode block (i.e., a file) covers: 260 one inode block (i.e., a file) covers:
261 261
262 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB. 262 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
263 263
264 Inode block (4KB) 264 Inode block (4KB)
265 |- data (923) 265 |- data (923)
266 |- direct node (2) 266 |- direct node (2)
267 | `- data (1018) 267 | `- data (1018)
268 |- indirect node (2) 268 |- indirect node (2)
269 | `- direct node (1018) 269 | `- direct node (1018)
270 | `- data (1018) 270 | `- data (1018)
271 `- double indirect node (1) 271 `- double indirect node (1)
272 `- indirect node (1018) 272 `- indirect node (1018)
273 `- direct node (1018) 273 `- direct node (1018)
274 `- data (1018) 274 `- data (1018)
275 275
276 Note that, all the node blocks are mapped by NAT which means the location of 276 Note that, all the node blocks are mapped by NAT which means the location of
277 each node is translated by the NAT table. In the consideration of the wandering 277 each node is translated by the NAT table. In the consideration of the wandering
278 tree problem, F2FS is able to cut off the propagation of node updates caused by 278 tree problem, F2FS is able to cut off the propagation of node updates caused by
279 leaf data writes. 279 leaf data writes.
280 280
281 Directory Structure 281 Directory Structure
282 ------------------- 282 -------------------
283 283
284 A directory entry occupies 11 bytes, which consists of the following attributes. 284 A directory entry occupies 11 bytes, which consists of the following attributes.
285 285
286 - hash hash value of the file name 286 - hash hash value of the file name
287 - ino inode number 287 - ino inode number
288 - len the length of file name 288 - len the length of file name
289 - type file type such as directory, symlink, etc 289 - type file type such as directory, symlink, etc
290 290
291 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is 291 A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
292 used to represent whether each dentry is valid or not. A dentry block occupies 292 used to represent whether each dentry is valid or not. A dentry block occupies
293 4KB with the following composition. 293 4KB with the following composition.
294 294
295 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) + 295 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
296 dentries(11 * 214 bytes) + file name (8 * 214 bytes) 296 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
297 297
298 [Bucket] 298 [Bucket]
299 +--------------------------------+ 299 +--------------------------------+
300 |dentry block 1 | dentry block 2 | 300 |dentry block 1 | dentry block 2 |
301 +--------------------------------+ 301 +--------------------------------+
302 . . 302 . .
303 . . 303 . .
304 . [Dentry Block Structure: 4KB] . 304 . [Dentry Block Structure: 4KB] .
305 +--------+----------+----------+------------+ 305 +--------+----------+----------+------------+
306 | bitmap | reserved | dentries | file names | 306 | bitmap | reserved | dentries | file names |
307 +--------+----------+----------+------------+ 307 +--------+----------+----------+------------+
308 [Dentry Block: 4KB] . . 308 [Dentry Block: 4KB] . .
309 . . 309 . .
310 . . 310 . .
311 +------+------+-----+------+ 311 +------+------+-----+------+
312 | hash | ino | len | type | 312 | hash | ino | len | type |
313 +------+------+-----+------+ 313 +------+------+-----+------+
314 [Dentry Structure: 11 bytes] 314 [Dentry Structure: 11 bytes]
315 315
316 F2FS implements multi-level hash tables for directory structure. Each level has 316 F2FS implements multi-level hash tables for directory structure. Each level has
317 a hash table with dedicated number of hash buckets as shown below. Note that 317 a hash table with dedicated number of hash buckets as shown below. Note that
318 "A(2B)" means a bucket includes 2 data blocks. 318 "A(2B)" means a bucket includes 2 data blocks.
319 319
320 ---------------------- 320 ----------------------
321 A : bucket 321 A : bucket
322 B : block 322 B : block
323 N : MAX_DIR_HASH_DEPTH 323 N : MAX_DIR_HASH_DEPTH
324 ---------------------- 324 ----------------------
325 325
326 level #0 | A(2B) 326 level #0 | A(2B)
327 | 327 |
328 level #1 | A(2B) - A(2B) 328 level #1 | A(2B) - A(2B)
329 | 329 |
330 level #2 | A(2B) - A(2B) - A(2B) - A(2B) 330 level #2 | A(2B) - A(2B) - A(2B) - A(2B)
331 . | . . . . 331 . | . . . .
332 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B) 332 level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
333 . | . . . . 333 . | . . . .
334 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B) 334 level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
335 335
336 The number of blocks and buckets are determined by, 336 The number of blocks and buckets are determined by,
337 337
338 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2, 338 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
339 # of blocks in level #n = | 339 # of blocks in level #n = |
340 `- 4, Otherwise 340 `- 4, Otherwise
341 341
342 ,- 2^n, if n < MAX_DIR_HASH_DEPTH / 2, 342 ,- 2^n, if n < MAX_DIR_HASH_DEPTH / 2,
343 # of buckets in level #n = | 343 # of buckets in level #n = |
344 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise 344 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise
345 345
346 When F2FS finds a file name in a directory, at first a hash value of the file 346 When F2FS finds a file name in a directory, at first a hash value of the file
347 name is calculated. Then, F2FS scans the hash table in level #0 to find the 347 name is calculated. Then, F2FS scans the hash table in level #0 to find the
348 dentry consisting of the file name and its inode number. If not found, F2FS 348 dentry consisting of the file name and its inode number. If not found, F2FS
349 scans the next hash table in level #1. In this way, F2FS scans hash tables in 349 scans the next hash table in level #1. In this way, F2FS scans hash tables in
350 each levels incrementally from 1 to N. In each levels F2FS needs to scan only 350 each levels incrementally from 1 to N. In each levels F2FS needs to scan only
351 one bucket determined by the following equation, which shows O(log(# of files)) 351 one bucket determined by the following equation, which shows O(log(# of files))
352 complexity. 352 complexity.
353 353
354 bucket number to scan in level #n = (hash value) % (# of buckets in level #n) 354 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
355 355
356 In the case of file creation, F2FS finds empty consecutive slots that cover the 356 In the case of file creation, F2FS finds empty consecutive slots that cover the
357 file name. F2FS searches the empty slots in the hash tables of whole levels from 357 file name. F2FS searches the empty slots in the hash tables of whole levels from
358 1 to N in the same way as the lookup operation. 358 1 to N in the same way as the lookup operation.
359 359
360 The following figure shows an example of two cases holding children. 360 The following figure shows an example of two cases holding children.
361 --------------> Dir <-------------- 361 --------------> Dir <--------------
362 | | 362 | |
363 child child 363 child child
364 364
365 child - child [hole] - child 365 child - child [hole] - child
366 366
367 child - child - child [hole] - [hole] - child 367 child - child - child [hole] - [hole] - child
368 368
369 Case 1: Case 2: 369 Case 1: Case 2:
370 Number of children = 6, Number of children = 3, 370 Number of children = 6, Number of children = 3,
371 File size = 7 File size = 7 371 File size = 7 File size = 7
372 372
373 Default Block Allocation 373 Default Block Allocation
374 ------------------------ 374 ------------------------
375 375
376 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node 376 At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
377 and Hot/Warm/Cold data. 377 and Hot/Warm/Cold data.
378 378
379 - Hot node contains direct node blocks of directories. 379 - Hot node contains direct node blocks of directories.
380 - Warm node contains direct node blocks except hot node blocks. 380 - Warm node contains direct node blocks except hot node blocks.
381 - Cold node contains indirect node blocks 381 - Cold node contains indirect node blocks
382 - Hot data contains dentry blocks 382 - Hot data contains dentry blocks
383 - Warm data contains data blocks except hot and cold data blocks 383 - Warm data contains data blocks except hot and cold data blocks
384 - Cold data contains multimedia data or migrated data blocks 384 - Cold data contains multimedia data or migrated data blocks
385 385
386 LFS has two schemes for free space management: threaded log and copy-and-compac- 386 LFS has two schemes for free space management: threaded log and copy-and-compac-
387 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited 387 tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
388 for devices showing very good sequential write performance, since free segments 388 for devices showing very good sequential write performance, since free segments
389 are served all the time for writing new data. However, it suffers from cleaning 389 are served all the time for writing new data. However, it suffers from cleaning
390 overhead under high utilization. Contrarily, the threaded log scheme suffers 390 overhead under high utilization. Contrarily, the threaded log scheme suffers
391 from random writes, but no cleaning process is needed. F2FS adopts a hybrid 391 from random writes, but no cleaning process is needed. F2FS adopts a hybrid
392 scheme where the copy-and-compaction scheme is adopted by default, but the 392 scheme where the copy-and-compaction scheme is adopted by default, but the
393 policy is dynamically changed to the threaded log scheme according to the file 393 policy is dynamically changed to the threaded log scheme according to the file
394 system status. 394 system status.
395 395
396 In order to align F2FS with underlying flash-based storage, F2FS allocates a 396 In order to align F2FS with underlying flash-based storage, F2FS allocates a
397 segment in a unit of section. F2FS expects that the section size would be the 397 segment in a unit of section. F2FS expects that the section size would be the
398 same as the unit size of garbage collection in FTL. Furthermore, with respect 398 same as the unit size of garbage collection in FTL. Furthermore, with respect
399 to the mapping granularity in FTL, F2FS allocates each section of the active 399 to the mapping granularity in FTL, F2FS allocates each section of the active
400 logs from different zones as much as possible, since FTL can write the data in 400 logs from different zones as much as possible, since FTL can write the data in
401 the active logs into one allocation unit according to its mapping granularity. 401 the active logs into one allocation unit according to its mapping granularity.
402 402
403 Cleaning process 403 Cleaning process
404 ---------------- 404 ----------------
405 405
406 F2FS does cleaning both on demand and in the background. On-demand cleaning is 406 F2FS does cleaning both on demand and in the background. On-demand cleaning is
407 triggered when there are not enough free segments to serve VFS calls. Background 407 triggered when there are not enough free segments to serve VFS calls. Background
408 cleaner is operated by a kernel thread, and triggers the cleaning job when the 408 cleaner is operated by a kernel thread, and triggers the cleaning job when the
409 system is idle. 409 system is idle.
410 410
411 F2FS supports two victim selection policies: greedy and cost-benefit algorithms. 411 F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
412 In the greedy algorithm, F2FS selects a victim segment having the smallest number 412 In the greedy algorithm, F2FS selects a victim segment having the smallest number
413 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment 413 of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
414 according to the segment age and the number of valid blocks in order to address 414 according to the segment age and the number of valid blocks in order to address
415 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy 415 log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
416 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit 416 algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
417 algorithm. 417 algorithm.
418 418