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authorLinus Torvalds2012-12-20 15:54:51 -0600
committerLinus Torvalds2012-12-20 15:54:52 -0600
commita13eea6bd9ee62ceacfc5243d54c84396bc86cb4 (patch)
tree46192468880c144f2b367deb5188267866ee1fac /Documentation
parentfcc16882ac4532aaa644bff444f0c5d6228ba71e (diff)
parent6666e6aa9f36b2bfd6b30072c07b34f2a24becf1 (diff)
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Merge tag 'for-3.8-merge' of git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs
Pull new F2FS filesystem from Jaegeuk Kim: "Introduce a new file system, Flash-Friendly File System (F2FS), to Linux 3.8. Highlights: - Add initial f2fs source codes - Fix an endian conversion bug - Fix build failures on random configs - Fix the power-off-recovery routine - Minor cleanup, coding style, and typos patches" From the Kconfig help text: F2FS is based on Log-structured File System (LFS), which supports versatile "flash-friendly" features. The design has been focused on addressing the fundamental issues in LFS, which are snowball effect of wandering tree and high cleaning overhead. Since flash-based storages show different characteristics according to the internal geometry or flash memory management schemes aka FTL, F2FS and tools support various parameters not only for configuring on-disk layout, but also for selecting allocation and cleaning algorithms. and there's an article by Neil Brown about it on lwn.net: http://lwn.net/Articles/518988/ * tag 'for-3.8-merge' of git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs: (36 commits) f2fs: fix tracking parent inode number f2fs: cleanup the f2fs_bio_alloc routine f2fs: introduce accessor to retrieve number of dentry slots f2fs: remove redundant call to f2fs_put_page in delete entry f2fs: make use of GFP_F2FS_ZERO for setting gfp_mask f2fs: rewrite f2fs_bio_alloc to make it simpler f2fs: fix a typo in f2fs documentation f2fs: remove unused variable f2fs: move error condition for mkdir at proper place f2fs: remove unneeded initialization f2fs: check read only condition before beginning write out f2fs: remove unneeded memset from init_once f2fs: show error in case of invalid mount arguments f2fs: fix the compiler warning for uninitialized use of variable f2fs: resolve build failures f2fs: adjust kernel coding style f2fs: fix endian conversion bugs reported by sparse f2fs: remove unneeded version.h header file from f2fs.h f2fs: update the f2fs document f2fs: update Kconfig and Makefile ...
Diffstat (limited to 'Documentation')
-rw-r--r--Documentation/filesystems/00-INDEX2
-rw-r--r--Documentation/filesystems/f2fs.txt421
2 files changed, 423 insertions, 0 deletions
diff --git a/Documentation/filesystems/00-INDEX b/Documentation/filesystems/00-INDEX
index 7b52ba7bf32a..8042050eb265 100644
--- a/Documentation/filesystems/00-INDEX
+++ b/Documentation/filesystems/00-INDEX
@@ -50,6 +50,8 @@ ext4.txt
50 - info, mount options and specifications for the Ext4 filesystem. 50 - info, mount options and specifications for the Ext4 filesystem.
51files.txt 51files.txt
52 - info on file management in the Linux kernel. 52 - info on file management in the Linux kernel.
53f2fs.txt
54 - info and mount options for the F2FS filesystem.
53fuse.txt 55fuse.txt
54 - info on the Filesystem in User SpacE including mount options. 56 - info on the Filesystem in User SpacE including mount options.
55gfs2.txt 57gfs2.txt
diff --git a/Documentation/filesystems/f2fs.txt b/Documentation/filesystems/f2fs.txt
new file mode 100644
index 000000000000..8fbd8b46ee34
--- /dev/null
+++ b/Documentation/filesystems/f2fs.txt
@@ -0,0 +1,421 @@
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 file system formatting tool, "mkfs.f2fs", is available from the following
22git tree:
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_off Turn off cleaning operations, namely garbage collection,
102 triggered in background when I/O subsystem is idle.
103disable_roll_forward Disable the roll-forward recovery routine
104discard Issue discard/TRIM commands when a segment is cleaned.
105no_heap Disable heap-style segment allocation which finds free
106 segments for data from the beginning of main area, while
107 for node from the end of main area.
108nouser_xattr Disable Extended User Attributes. Note: xattr is enabled
109 by default if CONFIG_F2FS_FS_XATTR is selected.
110noacl Disable POSIX Access Control List. Note: acl is enabled
111 by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
112active_logs=%u Support configuring the number of active logs. In the
113 current design, f2fs supports only 2, 4, and 6 logs.
114 Default number is 6.
115disable_ext_identify Disable the extension list configured by mkfs, so f2fs
116 does not aware of cold files such as media files.
117
118================================================================================
119DEBUGFS ENTRIES
120================================================================================
121
122/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
123f2fs. Each file shows the whole f2fs information.
124
125/sys/kernel/debug/f2fs/status includes:
126 - major file system information managed by f2fs currently
127 - average SIT information about whole segments
128 - current memory footprint consumed by f2fs.
129
130================================================================================
131USAGE
132================================================================================
133
1341. Download userland tools and compile them.
135
1362. Skip, if f2fs was compiled statically inside kernel.
137 Otherwise, insert the f2fs.ko module.
138 # insmod f2fs.ko
139
1403. Create a directory trying to mount
141 # mkdir /mnt/f2fs
142
1434. Format the block device, and then mount as f2fs
144 # mkfs.f2fs -l label /dev/block_device
145 # mount -t f2fs /dev/block_device /mnt/f2fs
146
147Format options
148--------------
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.
151 1 is set by default, which performs this.
152-o [int] : Set overprovision ratio in percent over volume size.
153 5 is set by default.
154-s [int] : Set the number of segments per section.
155 1 is set by default.
156-z [int] : Set the number of sections per zone.
157 1 is set by default.
158-e [str] : Set basic extension list. e.g. "mp3,gif,mov"
159
160================================================================================
161DESIGN
162================================================================================
163
164On-disk Layout
165--------------
166
167F2FS divides the whole volume into a number of segments, each of which is fixed
168to 2MB in size. A section is composed of consecutive segments, and a zone
169consists of a set of sections. By default, section and zone sizes are set to one
170segment size identically, but users can easily modify the sizes by mkfs.
171
172F2FS splits the entire volume into six areas, and all the areas except superblock
173consists of multiple segments as described below.
174
175 align with the zone size <-|
176 |-> align with the segment size
177 _________________________________________________________________________
178 | | | Node | Segment | Segment | |
179 | Superblock | Checkpoint | Address | Info. | Summary | Main |
180 | (SB) | (CP) | Table (NAT) | Table (SIT) | Area (SSA) | |
181 |____________|_____2______|______N______|______N______|______N_____|__N___|
182 . .
183 . .
184 . .
185 ._________________________________________.
186 |_Segment_|_..._|_Segment_|_..._|_Segment_|
187 . .
188 ._________._________
189 |_section_|__...__|_
190 . .
191 .________.
192 |__zone__|
193
194- Superblock (SB)
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
197 default parameters of f2fs.
198
199- Checkpoint (CP)
200 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
201 inode lists, and summary entries of current active segments.
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)
208 : It contains segment information such as valid block count and bitmap for the
209 validity of all the blocks.
210
211- Segment Summary Area (SSA)
212 : It contains summary entries which contains the owner information of all the
213 data and node blocks stored in Main area.
214
215- Main Area
216 : It contains file and directory data including their indices.
217
218In order to avoid misalignment between file system and flash-based storage, F2FS
219aligns the start block address of CP with the segment size. Also, it aligns the
220start block address of Main area with the zone size by reserving some segments
221in SSA area.
222
223Reference the following survey for additional technical details.
224https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
225
226File System Metadata Structure
227------------------------------
228
229F2FS adopts the checkpointing scheme to maintain file system consistency. At
230mount time, F2FS first tries to find the last valid checkpoint data by scanning
231CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
232One of them always indicates the last valid data, which is called as shadow copy
233mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
234
235For file system consistency, each CP points to which NAT and SIT copies are
236valid, as shown as below.
237
238 +--------+----------+---------+
239 | CP | NAT | SIT |
240 +--------+----------+---------+
241 . . . .
242 . . . .
243 . . . .
244 +-------+-------+--------+--------+--------+--------+
245 | CP #0 | CP #1 | NAT #0 | NAT #1 | SIT #0 | SIT #1 |
246 +-------+-------+--------+--------+--------+--------+
247 | ^ ^
248 | | |
249 `----------------------------------------'
250
251Index Structure
252---------------
253
254The key data structure to manage the data locations is a "node". Similar to
255traditional file structures, F2FS has three types of node: inode, direct node,
256indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
257indices, two direct node pointers, two indirect node pointers, and one double
258indirect node pointer as described below. One direct node block contains 1018
259data blocks, and one indirect node block contains also 1018 node blocks. Thus,
260one inode block (i.e., a file) covers:
261
262 4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
263
264 Inode block (4KB)
265 |- data (923)
266 |- direct node (2)
267 | `- data (1018)
268 |- indirect node (2)
269 | `- direct node (1018)
270 | `- data (1018)
271 `- double indirect node (1)
272 `- indirect node (1018)
273 `- direct node (1018)
274 `- data (1018)
275
276Note that, all the node blocks are mapped by NAT which means the location of
277each node is translated by the NAT table. In the consideration of the wandering
278tree problem, F2FS is able to cut off the propagation of node updates caused by
279leaf data writes.
280
281Directory Structure
282-------------------
283
284A directory entry occupies 11 bytes, which consists of the following attributes.
285
286- hash hash value of the file name
287- ino inode number
288- len the length of file name
289- type file type such as directory, symlink, etc
290
291A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
292used to represent whether each dentry is valid or not. A dentry block occupies
2934KB with the following composition.
294
295 Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
296 dentries(11 * 214 bytes) + file name (8 * 214 bytes)
297
298 [Bucket]
299 +--------------------------------+
300 |dentry block 1 | dentry block 2 |
301 +--------------------------------+
302 . .
303 . .
304 . [Dentry Block Structure: 4KB] .
305 +--------+----------+----------+------------+
306 | bitmap | reserved | dentries | file names |
307 +--------+----------+----------+------------+
308 [Dentry Block: 4KB] . .
309 . .
310 . .
311 +------+------+-----+------+
312 | hash | ino | len | type |
313 +------+------+-----+------+
314 [Dentry Structure: 11 bytes]
315
316F2FS implements multi-level hash tables for directory structure. Each level has
317a hash table with dedicated number of hash buckets as shown below. Note that
318"A(2B)" means a bucket includes 2 data blocks.
319
320----------------------
321A : bucket
322B : block
323N : MAX_DIR_HASH_DEPTH
324----------------------
325
326level #0 | A(2B)
327 |
328level #1 | A(2B) - A(2B)
329 |
330level #2 | A(2B) - A(2B) - A(2B) - A(2B)
331 . | . . . .
332level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
333 . | . . . .
334level #N | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
335
336The number of blocks and buckets are determined by,
337
338 ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
339 # of blocks in level #n = |
340 `- 4, Otherwise
341
342 ,- 2^n, if n < MAX_DIR_HASH_DEPTH / 2,
343 # of buckets in level #n = |
344 `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise
345
346When F2FS finds a file name in a directory, at first a hash value of the file
347name is calculated. Then, F2FS scans the hash table in level #0 to find the
348dentry consisting of the file name and its inode number. If not found, F2FS
349scans the next hash table in level #1. In this way, F2FS scans hash tables in
350each levels incrementally from 1 to N. In each levels F2FS needs to scan only
351one bucket determined by the following equation, which shows O(log(# of files))
352complexity.
353
354 bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
355
356In the case of file creation, F2FS finds empty consecutive slots that cover the
357file name. F2FS searches the empty slots in the hash tables of whole levels from
3581 to N in the same way as the lookup operation.
359
360The following figure shows an example of two cases holding children.
361 --------------> Dir <--------------
362 | |
363 child child
364
365 child - child [hole] - child
366
367 child - child - child [hole] - [hole] - child
368
369 Case 1: Case 2:
370 Number of children = 6, Number of children = 3,
371 File size = 7 File size = 7
372
373Default Block Allocation
374------------------------
375
376At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
377and Hot/Warm/Cold data.
378
379- Hot node contains direct node blocks of directories.
380- Warm node contains direct node blocks except hot node blocks.
381- Cold node contains indirect node blocks
382- Hot data contains dentry blocks
383- Warm data contains data blocks except hot and cold data blocks
384- Cold data contains multimedia data or migrated data blocks
385
386LFS has two schemes for free space management: threaded log and copy-and-compac-
387tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
388for devices showing very good sequential write performance, since free segments
389are served all the time for writing new data. However, it suffers from cleaning
390overhead under high utilization. Contrarily, the threaded log scheme suffers
391from random writes, but no cleaning process is needed. F2FS adopts a hybrid
392scheme where the copy-and-compaction scheme is adopted by default, but the
393policy is dynamically changed to the threaded log scheme according to the file
394system status.
395
396In order to align F2FS with underlying flash-based storage, F2FS allocates a
397segment in a unit of section. F2FS expects that the section size would be the
398same as the unit size of garbage collection in FTL. Furthermore, with respect
399to the mapping granularity in FTL, F2FS allocates each section of the active
400logs from different zones as much as possible, since FTL can write the data in
401the active logs into one allocation unit according to its mapping granularity.
402
403Cleaning process
404----------------
405
406F2FS does cleaning both on demand and in the background. On-demand cleaning is
407triggered when there are not enough free segments to serve VFS calls. Background
408cleaner is operated by a kernel thread, and triggers the cleaning job when the
409system is idle.
410
411F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
412In the greedy algorithm, F2FS selects a victim segment having the smallest number
413of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
414according to the segment age and the number of valid blocks in order to address
415log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
416algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
417algorithm.
418
419In order to identify whether the data in the victim segment are valid or not,
420F2FS manages a bitmap. Each bit represents the validity of a block, and the
421bitmap is composed of a bit stream covering whole blocks in main area.