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Based on kernel version 3.19. Page generated on 2015-02-13 21:20 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	discard                Issue discard/TRIM commands when a segment is cleaned.
110	no_heap                Disable heap-style segment allocation which finds free
111	                       segments for data from the beginning of main area, while
112			       for node from the end of main area.
113	nouser_xattr           Disable Extended User Attributes. Note: xattr is enabled
114	                       by default if CONFIG_F2FS_FS_XATTR is selected.
115	noacl                  Disable POSIX Access Control List. Note: acl is enabled
116	                       by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
117	active_logs=%u         Support configuring the number of active logs. In the
118	                       current design, f2fs supports only 2, 4, and 6 logs.
119	                       Default number is 6.
120	disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
121	                       does not aware of cold files such as media files.
122	inline_xattr           Enable the inline xattrs feature.
123	inline_data            Enable the inline data feature: New created small(<~3.4k)
124	                       files can be written into inode block.
125	inline_dentry          Enable the inline dir feature: data in new created
126	                       directory entries can be written into inode block. The
127	                       space of inode block which is used to store inline
128	                       dentries is limited to ~3.4k.
129	flush_merge	       Merge concurrent cache_flush commands as much as possible
130	                       to eliminate redundant command issues. If the underlying
131			       device handles the cache_flush command relatively slowly,
132			       recommend to enable this option.
133	nobarrier              This option can be used if underlying storage guarantees
134	                       its cached data should be written to the novolatile area.
135			       If this option is set, no cache_flush commands are issued
136			       but f2fs still guarantees the write ordering of all the
137			       data writes.
138	fastboot               This option is used when a system wants to reduce mount
139	                       time as much as possible, even though normal performance
140			       can be sacrificed.
141	
142	================================================================================
143	DEBUGFS ENTRIES
144	================================================================================
145	
146	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
147	f2fs. Each file shows the whole f2fs information.
148	
149	/sys/kernel/debug/f2fs/status includes:
150	 - major file system information managed by f2fs currently
151	 - average SIT information about whole segments
152	 - current memory footprint consumed by f2fs.
153	
154	================================================================================
155	SYSFS ENTRIES
156	================================================================================
157	
158	Information about mounted f2f2 file systems can be found in
159	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
160	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
161	The files in each per-device directory are shown in table below.
162	
163	Files in /sys/fs/f2fs/<devname>
164	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
165	..............................................................................
166	 File                         Content
167	
168	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
169	                              time for the garbage collection thread. Time is
170	                              in milliseconds.
171	
172	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
173	                              time for the garbage collection thread. Time is
174	                              in milliseconds.
175	
176	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
177	                              time for the garbage collection thread. Time is
178	                              in milliseconds.
179	
180	 gc_idle                      This parameter controls the selection of victim
181	                              policy for garbage collection. Setting gc_idle = 0
182	                              (default) will disable this option. Setting
183	                              gc_idle = 1 will select the Cost Benefit approach
184	                              & setting gc_idle = 2 will select the greedy aproach.
185	
186	 reclaim_segments             This parameter controls the number of prefree
187	                              segments to be reclaimed. If the number of prefree
188				      segments is larger than the number of segments
189				      in the proportion to the percentage over total
190				      volume size, f2fs tries to conduct checkpoint to
191				      reclaim the prefree segments to free segments.
192				      By default, 5% over total # of segments.
193	
194	 max_small_discards	      This parameter controls the number of discard
195				      commands that consist small blocks less than 2MB.
196				      The candidates to be discarded are cached until
197				      checkpoint is triggered, and issued during the
198				      checkpoint. By default, it is disabled with 0.
199	
200	 ipu_policy                   This parameter controls the policy of in-place
201	                              updates in f2fs. There are five policies:
202	                               0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
203	                               0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
204	                               0x10: F2FS_IPU_FSYNC.
205	
206	 min_ipu_util                 This parameter controls the threshold to trigger
207	                              in-place-updates. The number indicates percentage
208	                              of the filesystem utilization, and used by
209	                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
210	
211	 min_fsync_blocks             This parameter controls the threshold to trigger
212	                              in-place-updates when F2FS_IPU_FSYNC mode is set.
213				      The number indicates the number of dirty pages
214				      when fsync needs to flush on its call path. If
215				      the number is less than this value, it triggers
216				      in-place-updates.
217	
218	 max_victim_search	      This parameter controls the number of trials to
219				      find a victim segment when conducting SSR and
220				      cleaning operations. The default value is 4096
221				      which covers 8GB block address range.
222	
223	 dir_level                    This parameter controls the directory level to
224				      support large directory. If a directory has a
225				      number of files, it can reduce the file lookup
226				      latency by increasing this dir_level value.
227				      Otherwise, it needs to decrease this value to
228				      reduce the space overhead. The default value is 0.
229	
230	 ram_thresh                   This parameter controls the memory footprint used
231				      by free nids and cached nat entries. By default,
232				      10 is set, which indicates 10 MB / 1 GB RAM.
233	
234	================================================================================
235	USAGE
236	================================================================================
237	
238	1. Download userland tools and compile them.
239	
240	2. Skip, if f2fs was compiled statically inside kernel.
241	   Otherwise, insert the f2fs.ko module.
242	 # insmod f2fs.ko
243	
244	3. Create a directory trying to mount
245	 # mkdir /mnt/f2fs
246	
247	4. Format the block device, and then mount as f2fs
248	 # mkfs.f2fs -l label /dev/block_device
249	 # mount -t f2fs /dev/block_device /mnt/f2fs
250	
251	mkfs.f2fs
252	---------
253	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
254	which builds a basic on-disk layout.
255	
256	The options consist of:
257	-l [label]   : Give a volume label, up to 512 unicode name.
258	-a [0 or 1]  : Split start location of each area for heap-based allocation.
259	               1 is set by default, which performs this.
260	-o [int]     : Set overprovision ratio in percent over volume size.
261	               5 is set by default.
262	-s [int]     : Set the number of segments per section.
263	               1 is set by default.
264	-z [int]     : Set the number of sections per zone.
265	               1 is set by default.
266	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
267	-t [0 or 1]  : Disable discard command or not.
268	               1 is set by default, which conducts discard.
269	
270	fsck.f2fs
271	---------
272	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
273	partition, which examines whether the filesystem metadata and user-made data
274	are cross-referenced correctly or not.
275	Note that, initial version of the tool does not fix any inconsistency.
276	
277	The options consist of:
278	  -d debug level [default:0]
279	
280	dump.f2fs
281	---------
282	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
283	file. Each file is dump_ssa and dump_sit.
284	
285	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
286	It shows on-disk inode information reconized by a given inode number, and is
287	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
288	./dump_sit respectively.
289	
290	The options consist of:
291	  -d debug level [default:0]
292	  -i inode no (hex)
293	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
294	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
295	
296	Examples:
297	# dump.f2fs -i [ino] /dev/sdx
298	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
299	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
300	
301	================================================================================
302	DESIGN
303	================================================================================
304	
305	On-disk Layout
306	--------------
307	
308	F2FS divides the whole volume into a number of segments, each of which is fixed
309	to 2MB in size. A section is composed of consecutive segments, and a zone
310	consists of a set of sections. By default, section and zone sizes are set to one
311	segment size identically, but users can easily modify the sizes by mkfs.
312	
313	F2FS splits the entire volume into six areas, and all the areas except superblock
314	consists of multiple segments as described below.
315	
316	                                            align with the zone size <-|
317	                 |-> align with the segment size
318	     _________________________________________________________________________
319	    |            |            |   Segment   |    Node     |   Segment  |      |
320	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
321	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
322	    |____________|_____2______|______N______|______N______|______N_____|__N___|
323	                                                                       .      .
324	                                                             .                .
325	                                                 .                            .
326	                                    ._________________________________________.
327	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
328	                                    .           .
329	                                    ._________._________
330	                                    |_section_|__...__|_
331	                                    .            .
332			                    .________.
333		                            |__zone__|
334	
335	- Superblock (SB)
336	 : It is located at the beginning of the partition, and there exist two copies
337	   to avoid file system crash. It contains basic partition information and some
338	   default parameters of f2fs.
339	
340	- Checkpoint (CP)
341	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
342	   inode lists, and summary entries of current active segments.
343	
344	- Segment Information Table (SIT)
345	 : It contains segment information such as valid block count and bitmap for the
346	   validity of all the blocks.
347	
348	- Node Address Table (NAT)
349	 : It is composed of a block address table for all the node blocks stored in
350	   Main area.
351	
352	- Segment Summary Area (SSA)
353	 : It contains summary entries which contains the owner information of all the
354	   data and node blocks stored in Main area.
355	
356	- Main Area
357	 : It contains file and directory data including their indices.
358	
359	In order to avoid misalignment between file system and flash-based storage, F2FS
360	aligns the start block address of CP with the segment size. Also, it aligns the
361	start block address of Main area with the zone size by reserving some segments
362	in SSA area.
363	
364	Reference the following survey for additional technical details.
365	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
366	
367	File System Metadata Structure
368	------------------------------
369	
370	F2FS adopts the checkpointing scheme to maintain file system consistency. At
371	mount time, F2FS first tries to find the last valid checkpoint data by scanning
372	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
373	One of them always indicates the last valid data, which is called as shadow copy
374	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
375	
376	For file system consistency, each CP points to which NAT and SIT copies are
377	valid, as shown as below.
378	
379	  +--------+----------+---------+
380	  |   CP   |    SIT   |   NAT   |
381	  +--------+----------+---------+
382	  .         .          .          .
383	  .            .              .              .
384	  .               .                 .                 .
385	  +-------+-------+--------+--------+--------+--------+
386	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
387	  +-------+-------+--------+--------+--------+--------+
388	     |             ^                          ^
389	     |             |                          |
390	     `----------------------------------------'
391	
392	Index Structure
393	---------------
394	
395	The key data structure to manage the data locations is a "node". Similar to
396	traditional file structures, F2FS has three types of node: inode, direct node,
397	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
398	indices, two direct node pointers, two indirect node pointers, and one double
399	indirect node pointer as described below. One direct node block contains 1018
400	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
401	one inode block (i.e., a file) covers:
402	
403	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
404	
405	   Inode block (4KB)
406	     |- data (923)
407	     |- direct node (2)
408	     |          `- data (1018)
409	     |- indirect node (2)
410	     |            `- direct node (1018)
411	     |                       `- data (1018)
412	     `- double indirect node (1)
413	                         `- indirect node (1018)
414				              `- direct node (1018)
415		                                         `- data (1018)
416	
417	Note that, all the node blocks are mapped by NAT which means the location of
418	each node is translated by the NAT table. In the consideration of the wandering
419	tree problem, F2FS is able to cut off the propagation of node updates caused by
420	leaf data writes.
421	
422	Directory Structure
423	-------------------
424	
425	A directory entry occupies 11 bytes, which consists of the following attributes.
426	
427	- hash		hash value of the file name
428	- ino		inode number
429	- len		the length of file name
430	- type		file type such as directory, symlink, etc
431	
432	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
433	used to represent whether each dentry is valid or not. A dentry block occupies
434	4KB with the following composition.
435	
436	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
437		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
438	
439	                         [Bucket]
440	             +--------------------------------+
441	             |dentry block 1 | dentry block 2 |
442	             +--------------------------------+
443	             .               .
444	       .                             .
445	  .       [Dentry Block Structure: 4KB]       .
446	  +--------+----------+----------+------------+
447	  | bitmap | reserved | dentries | file names |
448	  +--------+----------+----------+------------+
449	  [Dentry Block: 4KB] .   .
450			 .               .
451	            .                          .
452	            +------+------+-----+------+
453	            | hash | ino  | len | type |
454	            +------+------+-----+------+
455	            [Dentry Structure: 11 bytes]
456	
457	F2FS implements multi-level hash tables for directory structure. Each level has
458	a hash table with dedicated number of hash buckets as shown below. Note that
459	"A(2B)" means a bucket includes 2 data blocks.
460	
461	----------------------
462	A : bucket
463	B : block
464	N : MAX_DIR_HASH_DEPTH
465	----------------------
466	
467	level #0   | A(2B)
468	           |
469	level #1   | A(2B) - A(2B)
470	           |
471	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
472	     .     |   .       .       .       .
473	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
474	     .     |   .       .       .       .
475	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
476	
477	The number of blocks and buckets are determined by,
478	
479	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
480	  # of blocks in level #n = |
481	                            `- 4, Otherwise
482	
483	                             ,- 2^(n + dir_level),
484				     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
485	  # of buckets in level #n = |
486	                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
487				              Otherwise
488	
489	When F2FS finds a file name in a directory, at first a hash value of the file
490	name is calculated. Then, F2FS scans the hash table in level #0 to find the
491	dentry consisting of the file name and its inode number. If not found, F2FS
492	scans the next hash table in level #1. In this way, F2FS scans hash tables in
493	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
494	one bucket determined by the following equation, which shows O(log(# of files))
495	complexity.
496	
497	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
498	
499	In the case of file creation, F2FS finds empty consecutive slots that cover the
500	file name. F2FS searches the empty slots in the hash tables of whole levels from
501	1 to N in the same way as the lookup operation.
502	
503	The following figure shows an example of two cases holding children.
504	       --------------> Dir <--------------
505	       |                                 |
506	    child                             child
507	
508	    child - child                     [hole] - child
509	
510	    child - child - child             [hole] - [hole] - child
511	
512	   Case 1:                           Case 2:
513	   Number of children = 6,           Number of children = 3,
514	   File size = 7                     File size = 7
515	
516	Default Block Allocation
517	------------------------
518	
519	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
520	and Hot/Warm/Cold data.
521	
522	- Hot node	contains direct node blocks of directories.
523	- Warm node	contains direct node blocks except hot node blocks.
524	- Cold node	contains indirect node blocks
525	- Hot data	contains dentry blocks
526	- Warm data	contains data blocks except hot and cold data blocks
527	- Cold data	contains multimedia data or migrated data blocks
528	
529	LFS has two schemes for free space management: threaded log and copy-and-compac-
530	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
531	for devices showing very good sequential write performance, since free segments
532	are served all the time for writing new data. However, it suffers from cleaning
533	overhead under high utilization. Contrarily, the threaded log scheme suffers
534	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
535	scheme where the copy-and-compaction scheme is adopted by default, but the
536	policy is dynamically changed to the threaded log scheme according to the file
537	system status.
538	
539	In order to align F2FS with underlying flash-based storage, F2FS allocates a
540	segment in a unit of section. F2FS expects that the section size would be the
541	same as the unit size of garbage collection in FTL. Furthermore, with respect
542	to the mapping granularity in FTL, F2FS allocates each section of the active
543	logs from different zones as much as possible, since FTL can write the data in
544	the active logs into one allocation unit according to its mapping granularity.
545	
546	Cleaning process
547	----------------
548	
549	F2FS does cleaning both on demand and in the background. On-demand cleaning is
550	triggered when there are not enough free segments to serve VFS calls. Background
551	cleaner is operated by a kernel thread, and triggers the cleaning job when the
552	system is idle.
553	
554	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
555	In the greedy algorithm, F2FS selects a victim segment having the smallest number
556	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
557	according to the segment age and the number of valid blocks in order to address
558	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
559	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
560	algorithm.
561	
562	In order to identify whether the data in the victim segment are valid or not,
563	F2FS manages a bitmap. Each bit represents the validity of a block, and the
564	bitmap is composed of a bit stream covering whole blocks in main area.
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