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Based on kernel version 4.7.2. Page generated on 2016-08-22 22:45 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 turned off. If background_gc=sync, it will turn
106	                       on synchronous garbage collection running in background.
107	                       Default value for this option is on. So garbage
108	                       collection is on by default.
109	disable_roll_forward   Disable the roll-forward recovery routine
110	norecovery             Disable the roll-forward recovery routine, mounted read-
111	                       only (i.e., -o ro,disable_roll_forward)
112	discard                Issue discard/TRIM commands when a segment is cleaned.
113	no_heap                Disable heap-style segment allocation which finds free
114	                       segments for data from the beginning of main area, while
115			       for node from the end of main area.
116	nouser_xattr           Disable Extended User Attributes. Note: xattr is enabled
117	                       by default if CONFIG_F2FS_FS_XATTR is selected.
118	noacl                  Disable POSIX Access Control List. Note: acl is enabled
119	                       by default if CONFIG_F2FS_FS_POSIX_ACL is selected.
120	active_logs=%u         Support configuring the number of active logs. In the
121	                       current design, f2fs supports only 2, 4, and 6 logs.
122	                       Default number is 6.
123	disable_ext_identify   Disable the extension list configured by mkfs, so f2fs
124	                       does not aware of cold files such as media files.
125	inline_xattr           Enable the inline xattrs feature.
126	inline_data            Enable the inline data feature: New created small(<~3.4k)
127	                       files can be written into inode block.
128	inline_dentry          Enable the inline dir feature: data in new created
129	                       directory entries can be written into inode block. The
130	                       space of inode block which is used to store inline
131	                       dentries is limited to ~3.4k.
132	flush_merge	       Merge concurrent cache_flush commands as much as possible
133	                       to eliminate redundant command issues. If the underlying
134			       device handles the cache_flush command relatively slowly,
135			       recommend to enable this option.
136	nobarrier              This option can be used if underlying storage guarantees
137	                       its cached data should be written to the novolatile area.
138			       If this option is set, no cache_flush commands are issued
139			       but f2fs still guarantees the write ordering of all the
140			       data writes.
141	fastboot               This option is used when a system wants to reduce mount
142	                       time as much as possible, even though normal performance
143			       can be sacrificed.
144	extent_cache           Enable an extent cache based on rb-tree, it can cache
145	                       as many as extent which map between contiguous logical
146	                       address and physical address per inode, resulting in
147	                       increasing the cache hit ratio. Set by default.
148	noextent_cache         Disable an extent cache based on rb-tree explicitly, see
149	                       the above extent_cache mount option.
150	noinline_data          Disable the inline data feature, inline data feature is
151	                       enabled by default.
152	data_flush             Enable data flushing before checkpoint in order to
153	                       persist data of regular and symlink.
154	
155	================================================================================
156	DEBUGFS ENTRIES
157	================================================================================
158	
159	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
160	f2fs. Each file shows the whole f2fs information.
161	
162	/sys/kernel/debug/f2fs/status includes:
163	 - major file system information managed by f2fs currently
164	 - average SIT information about whole segments
165	 - current memory footprint consumed by f2fs.
166	
167	================================================================================
168	SYSFS ENTRIES
169	================================================================================
170	
171	Information about mounted f2f2 file systems can be found in
172	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
173	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
174	The files in each per-device directory are shown in table below.
175	
176	Files in /sys/fs/f2fs/<devname>
177	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
178	..............................................................................
179	 File                         Content
180	
181	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
182	                              time for the garbage collection thread. Time is
183	                              in milliseconds.
184	
185	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
186	                              time for the garbage collection thread. Time is
187	                              in milliseconds.
188	
189	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
190	                              time for the garbage collection thread. Time is
191	                              in milliseconds.
192	
193	 gc_idle                      This parameter controls the selection of victim
194	                              policy for garbage collection. Setting gc_idle = 0
195	                              (default) will disable this option. Setting
196	                              gc_idle = 1 will select the Cost Benefit approach
197	                              & setting gc_idle = 2 will select the greedy approach.
198	
199	 reclaim_segments             This parameter controls the number of prefree
200	                              segments to be reclaimed. If the number of prefree
201				      segments is larger than the number of segments
202				      in the proportion to the percentage over total
203				      volume size, f2fs tries to conduct checkpoint to
204				      reclaim the prefree segments to free segments.
205				      By default, 5% over total # of segments.
206	
207	 max_small_discards	      This parameter controls the number of discard
208				      commands that consist small blocks less than 2MB.
209				      The candidates to be discarded are cached until
210				      checkpoint is triggered, and issued during the
211				      checkpoint. By default, it is disabled with 0.
212	
213	 trim_sections                This parameter controls the number of sections
214	                              to be trimmed out in batch mode when FITRIM
215	                              conducts. 32 sections is set by default.
216	
217	 ipu_policy                   This parameter controls the policy of in-place
218	                              updates in f2fs. There are five policies:
219	                               0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
220	                               0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
221	                               0x10: F2FS_IPU_FSYNC.
222	
223	 min_ipu_util                 This parameter controls the threshold to trigger
224	                              in-place-updates. The number indicates percentage
225	                              of the filesystem utilization, and used by
226	                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
227	
228	 min_fsync_blocks             This parameter controls the threshold to trigger
229	                              in-place-updates when F2FS_IPU_FSYNC mode is set.
230				      The number indicates the number of dirty pages
231				      when fsync needs to flush on its call path. If
232				      the number is less than this value, it triggers
233				      in-place-updates.
234	
235	 max_victim_search	      This parameter controls the number of trials to
236				      find a victim segment when conducting SSR and
237				      cleaning operations. The default value is 4096
238				      which covers 8GB block address range.
239	
240	 dir_level                    This parameter controls the directory level to
241				      support large directory. If a directory has a
242				      number of files, it can reduce the file lookup
243				      latency by increasing this dir_level value.
244				      Otherwise, it needs to decrease this value to
245				      reduce the space overhead. The default value is 0.
246	
247	 ram_thresh                   This parameter controls the memory footprint used
248				      by free nids and cached nat entries. By default,
249				      10 is set, which indicates 10 MB / 1 GB RAM.
250	
251	================================================================================
252	USAGE
253	================================================================================
254	
255	1. Download userland tools and compile them.
256	
257	2. Skip, if f2fs was compiled statically inside kernel.
258	   Otherwise, insert the f2fs.ko module.
259	 # insmod f2fs.ko
260	
261	3. Create a directory trying to mount
262	 # mkdir /mnt/f2fs
263	
264	4. Format the block device, and then mount as f2fs
265	 # mkfs.f2fs -l label /dev/block_device
266	 # mount -t f2fs /dev/block_device /mnt/f2fs
267	
268	mkfs.f2fs
269	---------
270	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
271	which builds a basic on-disk layout.
272	
273	The options consist of:
274	-l [label]   : Give a volume label, up to 512 unicode name.
275	-a [0 or 1]  : Split start location of each area for heap-based allocation.
276	               1 is set by default, which performs this.
277	-o [int]     : Set overprovision ratio in percent over volume size.
278	               5 is set by default.
279	-s [int]     : Set the number of segments per section.
280	               1 is set by default.
281	-z [int]     : Set the number of sections per zone.
282	               1 is set by default.
283	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
284	-t [0 or 1]  : Disable discard command or not.
285	               1 is set by default, which conducts discard.
286	
287	fsck.f2fs
288	---------
289	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
290	partition, which examines whether the filesystem metadata and user-made data
291	are cross-referenced correctly or not.
292	Note that, initial version of the tool does not fix any inconsistency.
293	
294	The options consist of:
295	  -d debug level [default:0]
296	
297	dump.f2fs
298	---------
299	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
300	file. Each file is dump_ssa and dump_sit.
301	
302	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
303	It shows on-disk inode information recognized by a given inode number, and is
304	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
305	./dump_sit respectively.
306	
307	The options consist of:
308	  -d debug level [default:0]
309	  -i inode no (hex)
310	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
311	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
312	
313	Examples:
314	# dump.f2fs -i [ino] /dev/sdx
315	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
316	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
317	
318	================================================================================
319	DESIGN
320	================================================================================
321	
322	On-disk Layout
323	--------------
324	
325	F2FS divides the whole volume into a number of segments, each of which is fixed
326	to 2MB in size. A section is composed of consecutive segments, and a zone
327	consists of a set of sections. By default, section and zone sizes are set to one
328	segment size identically, but users can easily modify the sizes by mkfs.
329	
330	F2FS splits the entire volume into six areas, and all the areas except superblock
331	consists of multiple segments as described below.
332	
333	                                            align with the zone size <-|
334	                 |-> align with the segment size
335	     _________________________________________________________________________
336	    |            |            |   Segment   |    Node     |   Segment  |      |
337	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
338	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
339	    |____________|_____2______|______N______|______N______|______N_____|__N___|
340	                                                                       .      .
341	                                                             .                .
342	                                                 .                            .
343	                                    ._________________________________________.
344	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
345	                                    .           .
346	                                    ._________._________
347	                                    |_section_|__...__|_
348	                                    .            .
349			                    .________.
350		                            |__zone__|
351	
352	- Superblock (SB)
353	 : It is located at the beginning of the partition, and there exist two copies
354	   to avoid file system crash. It contains basic partition information and some
355	   default parameters of f2fs.
356	
357	- Checkpoint (CP)
358	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
359	   inode lists, and summary entries of current active segments.
360	
361	- Segment Information Table (SIT)
362	 : It contains segment information such as valid block count and bitmap for the
363	   validity of all the blocks.
364	
365	- Node Address Table (NAT)
366	 : It is composed of a block address table for all the node blocks stored in
367	   Main area.
368	
369	- Segment Summary Area (SSA)
370	 : It contains summary entries which contains the owner information of all the
371	   data and node blocks stored in Main area.
372	
373	- Main Area
374	 : It contains file and directory data including their indices.
375	
376	In order to avoid misalignment between file system and flash-based storage, F2FS
377	aligns the start block address of CP with the segment size. Also, it aligns the
378	start block address of Main area with the zone size by reserving some segments
379	in SSA area.
380	
381	Reference the following survey for additional technical details.
382	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
383	
384	File System Metadata Structure
385	------------------------------
386	
387	F2FS adopts the checkpointing scheme to maintain file system consistency. At
388	mount time, F2FS first tries to find the last valid checkpoint data by scanning
389	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
390	One of them always indicates the last valid data, which is called as shadow copy
391	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
392	
393	For file system consistency, each CP points to which NAT and SIT copies are
394	valid, as shown as below.
395	
396	  +--------+----------+---------+
397	  |   CP   |    SIT   |   NAT   |
398	  +--------+----------+---------+
399	  .         .          .          .
400	  .            .              .              .
401	  .               .                 .                 .
402	  +-------+-------+--------+--------+--------+--------+
403	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
404	  +-------+-------+--------+--------+--------+--------+
405	     |             ^                          ^
406	     |             |                          |
407	     `----------------------------------------'
408	
409	Index Structure
410	---------------
411	
412	The key data structure to manage the data locations is a "node". Similar to
413	traditional file structures, F2FS has three types of node: inode, direct node,
414	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
415	indices, two direct node pointers, two indirect node pointers, and one double
416	indirect node pointer as described below. One direct node block contains 1018
417	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
418	one inode block (i.e., a file) covers:
419	
420	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
421	
422	   Inode block (4KB)
423	     |- data (923)
424	     |- direct node (2)
425	     |          `- data (1018)
426	     |- indirect node (2)
427	     |            `- direct node (1018)
428	     |                       `- data (1018)
429	     `- double indirect node (1)
430	                         `- indirect node (1018)
431				              `- direct node (1018)
432		                                         `- data (1018)
433	
434	Note that, all the node blocks are mapped by NAT which means the location of
435	each node is translated by the NAT table. In the consideration of the wandering
436	tree problem, F2FS is able to cut off the propagation of node updates caused by
437	leaf data writes.
438	
439	Directory Structure
440	-------------------
441	
442	A directory entry occupies 11 bytes, which consists of the following attributes.
443	
444	- hash		hash value of the file name
445	- ino		inode number
446	- len		the length of file name
447	- type		file type such as directory, symlink, etc
448	
449	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
450	used to represent whether each dentry is valid or not. A dentry block occupies
451	4KB with the following composition.
452	
453	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
454		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
455	
456	                         [Bucket]
457	             +--------------------------------+
458	             |dentry block 1 | dentry block 2 |
459	             +--------------------------------+
460	             .               .
461	       .                             .
462	  .       [Dentry Block Structure: 4KB]       .
463	  +--------+----------+----------+------------+
464	  | bitmap | reserved | dentries | file names |
465	  +--------+----------+----------+------------+
466	  [Dentry Block: 4KB] .   .
467			 .               .
468	            .                          .
469	            +------+------+-----+------+
470	            | hash | ino  | len | type |
471	            +------+------+-----+------+
472	            [Dentry Structure: 11 bytes]
473	
474	F2FS implements multi-level hash tables for directory structure. Each level has
475	a hash table with dedicated number of hash buckets as shown below. Note that
476	"A(2B)" means a bucket includes 2 data blocks.
477	
478	----------------------
479	A : bucket
480	B : block
481	N : MAX_DIR_HASH_DEPTH
482	----------------------
483	
484	level #0   | A(2B)
485	           |
486	level #1   | A(2B) - A(2B)
487	           |
488	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
489	     .     |   .       .       .       .
490	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
491	     .     |   .       .       .       .
492	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
493	
494	The number of blocks and buckets are determined by,
495	
496	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
497	  # of blocks in level #n = |
498	                            `- 4, Otherwise
499	
500	                             ,- 2^(n + dir_level),
501				     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
502	  # of buckets in level #n = |
503	                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
504				              Otherwise
505	
506	When F2FS finds a file name in a directory, at first a hash value of the file
507	name is calculated. Then, F2FS scans the hash table in level #0 to find the
508	dentry consisting of the file name and its inode number. If not found, F2FS
509	scans the next hash table in level #1. In this way, F2FS scans hash tables in
510	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
511	one bucket determined by the following equation, which shows O(log(# of files))
512	complexity.
513	
514	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
515	
516	In the case of file creation, F2FS finds empty consecutive slots that cover the
517	file name. F2FS searches the empty slots in the hash tables of whole levels from
518	1 to N in the same way as the lookup operation.
519	
520	The following figure shows an example of two cases holding children.
521	       --------------> Dir <--------------
522	       |                                 |
523	    child                             child
524	
525	    child - child                     [hole] - child
526	
527	    child - child - child             [hole] - [hole] - child
528	
529	   Case 1:                           Case 2:
530	   Number of children = 6,           Number of children = 3,
531	   File size = 7                     File size = 7
532	
533	Default Block Allocation
534	------------------------
535	
536	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
537	and Hot/Warm/Cold data.
538	
539	- Hot node	contains direct node blocks of directories.
540	- Warm node	contains direct node blocks except hot node blocks.
541	- Cold node	contains indirect node blocks
542	- Hot data	contains dentry blocks
543	- Warm data	contains data blocks except hot and cold data blocks
544	- Cold data	contains multimedia data or migrated data blocks
545	
546	LFS has two schemes for free space management: threaded log and copy-and-compac-
547	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
548	for devices showing very good sequential write performance, since free segments
549	are served all the time for writing new data. However, it suffers from cleaning
550	overhead under high utilization. Contrarily, the threaded log scheme suffers
551	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
552	scheme where the copy-and-compaction scheme is adopted by default, but the
553	policy is dynamically changed to the threaded log scheme according to the file
554	system status.
555	
556	In order to align F2FS with underlying flash-based storage, F2FS allocates a
557	segment in a unit of section. F2FS expects that the section size would be the
558	same as the unit size of garbage collection in FTL. Furthermore, with respect
559	to the mapping granularity in FTL, F2FS allocates each section of the active
560	logs from different zones as much as possible, since FTL can write the data in
561	the active logs into one allocation unit according to its mapping granularity.
562	
563	Cleaning process
564	----------------
565	
566	F2FS does cleaning both on demand and in the background. On-demand cleaning is
567	triggered when there are not enough free segments to serve VFS calls. Background
568	cleaner is operated by a kernel thread, and triggers the cleaning job when the
569	system is idle.
570	
571	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
572	In the greedy algorithm, F2FS selects a victim segment having the smallest number
573	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
574	according to the segment age and the number of valid blocks in order to address
575	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
576	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
577	algorithm.
578	
579	In order to identify whether the data in the victim segment are valid or not,
580	F2FS manages a bitmap. Each bit represents the validity of a block, and the
581	bitmap is composed of a bit stream covering whole blocks in main area.
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