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Based on kernel version 4.1. Page generated on 2015-06-28 12:12 EST.

1	================================================================================
2	WHAT IS Flash-Friendly File System (F2FS)?
3	================================================================================
4	
5	NAND flash memory-based storage devices, such as SSD, eMMC, and SD cards, have
6	been equipped on a variety systems ranging from mobile to server systems. Since
7	they are known to have different characteristics from the conventional rotating
8	disks, a file system, an upper layer to the storage device, should adapt to the
9	changes from the sketch in the design level.
10	
11	F2FS is a file system exploiting NAND flash memory-based storage devices, which
12	is based on Log-structured File System (LFS). The design has been focused on
13	addressing the fundamental issues in LFS, which are snowball effect of wandering
14	tree and high cleaning overhead.
15	
16	Since a NAND flash memory-based storage device shows different characteristic
17	according to its internal geometry or flash memory management scheme, namely FTL,
18	F2FS and its tools support various parameters not only for configuring on-disk
19	layout, but also for selecting allocation and cleaning algorithms.
20	
21	The following git tree provides the file system formatting tool (mkfs.f2fs),
22	a consistency checking tool (fsck.f2fs), and a debugging tool (dump.f2fs).
23	>> git://git.kernel.org/pub/scm/linux/kernel/git/jaegeuk/f2fs-tools.git
24	
25	For reporting bugs and sending patches, please use the following mailing list:
26	>> linux-f2fs-devel@lists.sourceforge.net
27	
28	================================================================================
29	BACKGROUND AND DESIGN ISSUES
30	================================================================================
31	
32	Log-structured File System (LFS)
33	--------------------------------
34	"A log-structured file system writes all modifications to disk sequentially in
35	a log-like structure, thereby speeding up  both file writing and crash recovery.
36	The log is the only structure on disk; it contains indexing information so that
37	files can be read back from the log efficiently. In order to maintain large free
38	areas on disk for fast writing, we divide  the log into segments and use a
39	segment cleaner to compress the live information from heavily fragmented
40	segments." from Rosenblum, M. and Ousterhout, J. K., 1992, "The design and
41	implementation of a log-structured file system", ACM Trans. Computer Systems
42	10, 1, 26–52.
43	
44	Wandering Tree Problem
45	----------------------
46	In LFS, when a file data is updated and written to the end of log, its direct
47	pointer block is updated due to the changed location. Then the indirect pointer
48	block is also updated due to the direct pointer block update. In this manner,
49	the upper index structures such as inode, inode map, and checkpoint block are
50	also updated recursively. This problem is called as wandering tree problem [1],
51	and in order to enhance the performance, it should eliminate or relax the update
52	propagation as much as possible.
53	
54	[1] Bityutskiy, A. 2005. JFFS3 design issues. http://www.linux-mtd.infradead.org/
55	
56	Cleaning Overhead
57	-----------------
58	Since LFS is based on out-of-place writes, it produces so many obsolete blocks
59	scattered across the whole storage. In order to serve new empty log space, it
60	needs to reclaim these obsolete blocks seamlessly to users. This job is called
61	as a cleaning process.
62	
63	The process consists of three operations as follows.
64	1. A victim segment is selected through referencing segment usage table.
65	2. It loads parent index structures of all the data in the victim identified by
66	   segment summary blocks.
67	3. It checks the cross-reference between the data and its parent index structure.
68	4. It moves valid data selectively.
69	
70	This cleaning job may cause unexpected long delays, so the most important goal
71	is to hide the latencies to users. And also definitely, it should reduce the
72	amount of valid data to be moved, and move them quickly as well.
73	
74	================================================================================
75	KEY FEATURES
76	================================================================================
77	
78	Flash Awareness
79	---------------
80	- Enlarge the random write area for better performance, but provide the high
81	  spatial locality
82	- Align FS data structures to the operational units in FTL as best efforts
83	
84	Wandering Tree Problem
85	----------------------
86	- Use a term, “node”, that represents inodes as well as various pointer blocks
87	- Introduce Node Address Table (NAT) containing the locations of all the “node”
88	  blocks; this will cut off the update propagation.
89	
90	Cleaning Overhead
91	-----------------
92	- Support a background cleaning process
93	- Support greedy and cost-benefit algorithms for victim selection policies
94	- Support multi-head logs for static/dynamic hot and cold data separation
95	- Introduce adaptive logging for efficient block allocation
96	
97	================================================================================
98	MOUNT OPTIONS
99	================================================================================
100	
101	background_gc=%s       Turn on/off cleaning operations, namely garbage
102	                       collection, triggered in background when I/O subsystem is
103	                       idle. If background_gc=on, it will turn on the garbage
104	                       collection and if background_gc=off, garbage collection
105	                       will be truned off.
106	                       Default value for this option is on. So garbage
107	                       collection is on by default.
108	disable_roll_forward   Disable the roll-forward recovery routine
109	norecovery             Disable the roll-forward recovery routine, mounted read-
110	                       only (i.e., -o ro,disable_roll_forward)
111	discard                Issue discard/TRIM commands when a segment is cleaned.
112	no_heap                Disable heap-style segment allocation which finds free
113	                       segments for data from the beginning of main area, while
114			       for node from the end of main area.
115	nouser_xattr           Disable Extended User Attributes. Note: xattr is enabled
116	                       by default if CONFIG_F2FS_FS_XATTR is selected.
117	noacl                  Disable POSIX Access Control List. Note: acl is enabled
118	                       by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
119	active_logs=%u         Support configuring the number of active logs. In the
120	                       current design, f2fs supports only 2, 4, and 6 logs.
121	                       Default number is 6.
122	disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
123	                       does not aware of cold files such as media files.
124	inline_xattr           Enable the inline xattrs feature.
125	inline_data            Enable the inline data feature: New created small(<~3.4k)
126	                       files can be written into inode block.
127	inline_dentry          Enable the inline dir feature: data in new created
128	                       directory entries can be written into inode block. The
129	                       space of inode block which is used to store inline
130	                       dentries is limited to ~3.4k.
131	flush_merge	       Merge concurrent cache_flush commands as much as possible
132	                       to eliminate redundant command issues. If the underlying
133			       device handles the cache_flush command relatively slowly,
134			       recommend to enable this option.
135	nobarrier              This option can be used if underlying storage guarantees
136	                       its cached data should be written to the novolatile area.
137			       If this option is set, no cache_flush commands are issued
138			       but f2fs still guarantees the write ordering of all the
139			       data writes.
140	fastboot               This option is used when a system wants to reduce mount
141	                       time as much as possible, even though normal performance
142			       can be sacrificed.
143	extent_cache           Enable an extent cache based on rb-tree, it can cache
144	                       as many as extent which map between contiguous logical
145	                       address and physical address per inode, resulting in
146	                       increasing the cache hit ratio.
147	noinline_data          Disable the inline data feature, inline data feature is
148	                       enabled by default.
149	
150	================================================================================
151	DEBUGFS ENTRIES
152	================================================================================
153	
154	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
155	f2fs. Each file shows the whole f2fs information.
156	
157	/sys/kernel/debug/f2fs/status includes:
158	 - major file system information managed by f2fs currently
159	 - average SIT information about whole segments
160	 - current memory footprint consumed by f2fs.
161	
162	================================================================================
163	SYSFS ENTRIES
164	================================================================================
165	
166	Information about mounted f2f2 file systems can be found in
167	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
168	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
169	The files in each per-device directory are shown in table below.
170	
171	Files in /sys/fs/f2fs/<devname>
172	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
173	..............................................................................
174	 File                         Content
175	
176	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
177	                              time for the garbage collection thread. Time is
178	                              in milliseconds.
179	
180	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
181	                              time for the garbage collection thread. Time is
182	                              in milliseconds.
183	
184	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
185	                              time for the garbage collection thread. Time is
186	                              in milliseconds.
187	
188	 gc_idle                      This parameter controls the selection of victim
189	                              policy for garbage collection. Setting gc_idle = 0
190	                              (default) will disable this option. Setting
191	                              gc_idle = 1 will select the Cost Benefit approach
192	                              & setting gc_idle = 2 will select the greedy aproach.
193	
194	 reclaim_segments             This parameter controls the number of prefree
195	                              segments to be reclaimed. If the number of prefree
196				      segments is larger than the number of segments
197				      in the proportion to the percentage over total
198				      volume size, f2fs tries to conduct checkpoint to
199				      reclaim the prefree segments to free segments.
200				      By default, 5% over total # of segments.
201	
202	 max_small_discards	      This parameter controls the number of discard
203				      commands that consist small blocks less than 2MB.
204				      The candidates to be discarded are cached until
205				      checkpoint is triggered, and issued during the
206				      checkpoint. By default, it is disabled with 0.
207	
208	 trim_sections                This parameter controls the number of sections
209	                              to be trimmed out in batch mode when FITRIM
210	                              conducts. 32 sections is set by default.
211	
212	 ipu_policy                   This parameter controls the policy of in-place
213	                              updates in f2fs. There are five policies:
214	                               0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
215	                               0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
216	                               0x10: F2FS_IPU_FSYNC.
217	
218	 min_ipu_util                 This parameter controls the threshold to trigger
219	                              in-place-updates. The number indicates percentage
220	                              of the filesystem utilization, and used by
221	                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
222	
223	 min_fsync_blocks             This parameter controls the threshold to trigger
224	                              in-place-updates when F2FS_IPU_FSYNC mode is set.
225				      The number indicates the number of dirty pages
226				      when fsync needs to flush on its call path. If
227				      the number is less than this value, it triggers
228				      in-place-updates.
229	
230	 max_victim_search	      This parameter controls the number of trials to
231				      find a victim segment when conducting SSR and
232				      cleaning operations. The default value is 4096
233				      which covers 8GB block address range.
234	
235	 dir_level                    This parameter controls the directory level to
236				      support large directory. If a directory has a
237				      number of files, it can reduce the file lookup
238				      latency by increasing this dir_level value.
239				      Otherwise, it needs to decrease this value to
240				      reduce the space overhead. The default value is 0.
241	
242	 ram_thresh                   This parameter controls the memory footprint used
243				      by free nids and cached nat entries. By default,
244				      10 is set, which indicates 10 MB / 1 GB RAM.
245	
246	================================================================================
247	USAGE
248	================================================================================
249	
250	1. Download userland tools and compile them.
251	
252	2. Skip, if f2fs was compiled statically inside kernel.
253	   Otherwise, insert the f2fs.ko module.
254	 # insmod f2fs.ko
255	
256	3. Create a directory trying to mount
257	 # mkdir /mnt/f2fs
258	
259	4. Format the block device, and then mount as f2fs
260	 # mkfs.f2fs -l label /dev/block_device
261	 # mount -t f2fs /dev/block_device /mnt/f2fs
262	
263	mkfs.f2fs
264	---------
265	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
266	which builds a basic on-disk layout.
267	
268	The options consist of:
269	-l [label]   : Give a volume label, up to 512 unicode name.
270	-a [0 or 1]  : Split start location of each area for heap-based allocation.
271	               1 is set by default, which performs this.
272	-o [int]     : Set overprovision ratio in percent over volume size.
273	               5 is set by default.
274	-s [int]     : Set the number of segments per section.
275	               1 is set by default.
276	-z [int]     : Set the number of sections per zone.
277	               1 is set by default.
278	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
279	-t [0 or 1]  : Disable discard command or not.
280	               1 is set by default, which conducts discard.
281	
282	fsck.f2fs
283	---------
284	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
285	partition, which examines whether the filesystem metadata and user-made data
286	are cross-referenced correctly or not.
287	Note that, initial version of the tool does not fix any inconsistency.
288	
289	The options consist of:
290	  -d debug level [default:0]
291	
292	dump.f2fs
293	---------
294	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
295	file. Each file is dump_ssa and dump_sit.
296	
297	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
298	It shows on-disk inode information reconized by a given inode number, and is
299	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
300	./dump_sit respectively.
301	
302	The options consist of:
303	  -d debug level [default:0]
304	  -i inode no (hex)
305	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
306	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
307	
308	Examples:
309	# dump.f2fs -i [ino] /dev/sdx
310	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
311	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
312	
313	================================================================================
314	DESIGN
315	================================================================================
316	
317	On-disk Layout
318	--------------
319	
320	F2FS divides the whole volume into a number of segments, each of which is fixed
321	to 2MB in size. A section is composed of consecutive segments, and a zone
322	consists of a set of sections. By default, section and zone sizes are set to one
323	segment size identically, but users can easily modify the sizes by mkfs.
324	
325	F2FS splits the entire volume into six areas, and all the areas except superblock
326	consists of multiple segments as described below.
327	
328	                                            align with the zone size <-|
329	                 |-> align with the segment size
330	     _________________________________________________________________________
331	    |            |            |   Segment   |    Node     |   Segment  |      |
332	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
333	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
334	    |____________|_____2______|______N______|______N______|______N_____|__N___|
335	                                                                       .      .
336	                                                             .                .
337	                                                 .                            .
338	                                    ._________________________________________.
339	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
340	                                    .           .
341	                                    ._________._________
342	                                    |_section_|__...__|_
343	                                    .            .
344			                    .________.
345		                            |__zone__|
346	
347	- Superblock (SB)
348	 : It is located at the beginning of the partition, and there exist two copies
349	   to avoid file system crash. It contains basic partition information and some
350	   default parameters of f2fs.
351	
352	- Checkpoint (CP)
353	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
354	   inode lists, and summary entries of current active segments.
355	
356	- Segment Information Table (SIT)
357	 : It contains segment information such as valid block count and bitmap for the
358	   validity of all the blocks.
359	
360	- Node Address Table (NAT)
361	 : It is composed of a block address table for all the node blocks stored in
362	   Main area.
363	
364	- Segment Summary Area (SSA)
365	 : It contains summary entries which contains the owner information of all the
366	   data and node blocks stored in Main area.
367	
368	- Main Area
369	 : It contains file and directory data including their indices.
370	
371	In order to avoid misalignment between file system and flash-based storage, F2FS
372	aligns the start block address of CP with the segment size. Also, it aligns the
373	start block address of Main area with the zone size by reserving some segments
374	in SSA area.
375	
376	Reference the following survey for additional technical details.
377	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
378	
379	File System Metadata Structure
380	------------------------------
381	
382	F2FS adopts the checkpointing scheme to maintain file system consistency. At
383	mount time, F2FS first tries to find the last valid checkpoint data by scanning
384	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
385	One of them always indicates the last valid data, which is called as shadow copy
386	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
387	
388	For file system consistency, each CP points to which NAT and SIT copies are
389	valid, as shown as below.
390	
391	  +--------+----------+---------+
392	  |   CP   |    SIT   |   NAT   |
393	  +--------+----------+---------+
394	  .         .          .          .
395	  .            .              .              .
396	  .               .                 .                 .
397	  +-------+-------+--------+--------+--------+--------+
398	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
399	  +-------+-------+--------+--------+--------+--------+
400	     |             ^                          ^
401	     |             |                          |
402	     `----------------------------------------'
403	
404	Index Structure
405	---------------
406	
407	The key data structure to manage the data locations is a "node". Similar to
408	traditional file structures, F2FS has three types of node: inode, direct node,
409	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
410	indices, two direct node pointers, two indirect node pointers, and one double
411	indirect node pointer as described below. One direct node block contains 1018
412	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
413	one inode block (i.e., a file) covers:
414	
415	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
416	
417	   Inode block (4KB)
418	     |- data (923)
419	     |- direct node (2)
420	     |          `- data (1018)
421	     |- indirect node (2)
422	     |            `- direct node (1018)
423	     |                       `- data (1018)
424	     `- double indirect node (1)
425	                         `- indirect node (1018)
426				              `- direct node (1018)
427		                                         `- data (1018)
428	
429	Note that, all the node blocks are mapped by NAT which means the location of
430	each node is translated by the NAT table. In the consideration of the wandering
431	tree problem, F2FS is able to cut off the propagation of node updates caused by
432	leaf data writes.
433	
434	Directory Structure
435	-------------------
436	
437	A directory entry occupies 11 bytes, which consists of the following attributes.
438	
439	- hash		hash value of the file name
440	- ino		inode number
441	- len		the length of file name
442	- type		file type such as directory, symlink, etc
443	
444	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
445	used to represent whether each dentry is valid or not. A dentry block occupies
446	4KB with the following composition.
447	
448	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
449		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
450	
451	                         [Bucket]
452	             +--------------------------------+
453	             |dentry block 1 | dentry block 2 |
454	             +--------------------------------+
455	             .               .
456	       .                             .
457	  .       [Dentry Block Structure: 4KB]       .
458	  +--------+----------+----------+------------+
459	  | bitmap | reserved | dentries | file names |
460	  +--------+----------+----------+------------+
461	  [Dentry Block: 4KB] .   .
462			 .               .
463	            .                          .
464	            +------+------+-----+------+
465	            | hash | ino  | len | type |
466	            +------+------+-----+------+
467	            [Dentry Structure: 11 bytes]
468	
469	F2FS implements multi-level hash tables for directory structure. Each level has
470	a hash table with dedicated number of hash buckets as shown below. Note that
471	"A(2B)" means a bucket includes 2 data blocks.
472	
473	----------------------
474	A : bucket
475	B : block
476	N : MAX_DIR_HASH_DEPTH
477	----------------------
478	
479	level #0   | A(2B)
480	           |
481	level #1   | A(2B) - A(2B)
482	           |
483	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
484	     .     |   .       .       .       .
485	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
486	     .     |   .       .       .       .
487	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
488	
489	The number of blocks and buckets are determined by,
490	
491	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
492	  # of blocks in level #n = |
493	                            `- 4, Otherwise
494	
495	                             ,- 2^(n + dir_level),
496				     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
497	  # of buckets in level #n = |
498	                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
499				              Otherwise
500	
501	When F2FS finds a file name in a directory, at first a hash value of the file
502	name is calculated. Then, F2FS scans the hash table in level #0 to find the
503	dentry consisting of the file name and its inode number. If not found, F2FS
504	scans the next hash table in level #1. In this way, F2FS scans hash tables in
505	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
506	one bucket determined by the following equation, which shows O(log(# of files))
507	complexity.
508	
509	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
510	
511	In the case of file creation, F2FS finds empty consecutive slots that cover the
512	file name. F2FS searches the empty slots in the hash tables of whole levels from
513	1 to N in the same way as the lookup operation.
514	
515	The following figure shows an example of two cases holding children.
516	       --------------> Dir <--------------
517	       |                                 |
518	    child                             child
519	
520	    child - child                     [hole] - child
521	
522	    child - child - child             [hole] - [hole] - child
523	
524	   Case 1:                           Case 2:
525	   Number of children = 6,           Number of children = 3,
526	   File size = 7                     File size = 7
527	
528	Default Block Allocation
529	------------------------
530	
531	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
532	and Hot/Warm/Cold data.
533	
534	- Hot node	contains direct node blocks of directories.
535	- Warm node	contains direct node blocks except hot node blocks.
536	- Cold node	contains indirect node blocks
537	- Hot data	contains dentry blocks
538	- Warm data	contains data blocks except hot and cold data blocks
539	- Cold data	contains multimedia data or migrated data blocks
540	
541	LFS has two schemes for free space management: threaded log and copy-and-compac-
542	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
543	for devices showing very good sequential write performance, since free segments
544	are served all the time for writing new data. However, it suffers from cleaning
545	overhead under high utilization. Contrarily, the threaded log scheme suffers
546	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
547	scheme where the copy-and-compaction scheme is adopted by default, but the
548	policy is dynamically changed to the threaded log scheme according to the file
549	system status.
550	
551	In order to align F2FS with underlying flash-based storage, F2FS allocates a
552	segment in a unit of section. F2FS expects that the section size would be the
553	same as the unit size of garbage collection in FTL. Furthermore, with respect
554	to the mapping granularity in FTL, F2FS allocates each section of the active
555	logs from different zones as much as possible, since FTL can write the data in
556	the active logs into one allocation unit according to its mapping granularity.
557	
558	Cleaning process
559	----------------
560	
561	F2FS does cleaning both on demand and in the background. On-demand cleaning is
562	triggered when there are not enough free segments to serve VFS calls. Background
563	cleaner is operated by a kernel thread, and triggers the cleaning job when the
564	system is idle.
565	
566	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
567	In the greedy algorithm, F2FS selects a victim segment having the smallest number
568	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
569	according to the segment age and the number of valid blocks in order to address
570	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
571	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
572	algorithm.
573	
574	In order to identify whether the data in the victim segment are valid or not,
575	F2FS manages a bitmap. Each bit represents the validity of a block, and the
576	bitmap is composed of a bit stream covering whole blocks in main area.
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