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Based on kernel version 4.3. Page generated on 2015-11-02 12:49 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. Set by default.
147	noextent_cache         Diable an extent cache based on rb-tree explicitly, see
148	                       the above extent_cache mount option.
149	noinline_data          Disable the inline data feature, inline data feature is
150	                       enabled by default.
151	
152	================================================================================
153	DEBUGFS ENTRIES
154	================================================================================
155	
156	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
157	f2fs. Each file shows the whole f2fs information.
158	
159	/sys/kernel/debug/f2fs/status includes:
160	 - major file system information managed by f2fs currently
161	 - average SIT information about whole segments
162	 - current memory footprint consumed by f2fs.
163	
164	================================================================================
165	SYSFS ENTRIES
166	================================================================================
167	
168	Information about mounted f2f2 file systems can be found in
169	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
170	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
171	The files in each per-device directory are shown in table below.
172	
173	Files in /sys/fs/f2fs/<devname>
174	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
175	..............................................................................
176	 File                         Content
177	
178	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
179	                              time for the garbage collection thread. Time is
180	                              in milliseconds.
181	
182	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
183	                              time for the garbage collection thread. Time is
184	                              in milliseconds.
185	
186	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
187	                              time for the garbage collection thread. Time is
188	                              in milliseconds.
189	
190	 gc_idle                      This parameter controls the selection of victim
191	                              policy for garbage collection. Setting gc_idle = 0
192	                              (default) will disable this option. Setting
193	                              gc_idle = 1 will select the Cost Benefit approach
194	                              & setting gc_idle = 2 will select the greedy aproach.
195	
196	 reclaim_segments             This parameter controls the number of prefree
197	                              segments to be reclaimed. If the number of prefree
198				      segments is larger than the number of segments
199				      in the proportion to the percentage over total
200				      volume size, f2fs tries to conduct checkpoint to
201				      reclaim the prefree segments to free segments.
202				      By default, 5% over total # of segments.
203	
204	 max_small_discards	      This parameter controls the number of discard
205				      commands that consist small blocks less than 2MB.
206				      The candidates to be discarded are cached until
207				      checkpoint is triggered, and issued during the
208				      checkpoint. By default, it is disabled with 0.
209	
210	 trim_sections                This parameter controls the number of sections
211	                              to be trimmed out in batch mode when FITRIM
212	                              conducts. 32 sections is set by default.
213	
214	 ipu_policy                   This parameter controls the policy of in-place
215	                              updates in f2fs. There are five policies:
216	                               0x01: F2FS_IPU_FORCE, 0x02: F2FS_IPU_SSR,
217	                               0x04: F2FS_IPU_UTIL,  0x08: F2FS_IPU_SSR_UTIL,
218	                               0x10: F2FS_IPU_FSYNC.
219	
220	 min_ipu_util                 This parameter controls the threshold to trigger
221	                              in-place-updates. The number indicates percentage
222	                              of the filesystem utilization, and used by
223	                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
224	
225	 min_fsync_blocks             This parameter controls the threshold to trigger
226	                              in-place-updates when F2FS_IPU_FSYNC mode is set.
227				      The number indicates the number of dirty pages
228				      when fsync needs to flush on its call path. If
229				      the number is less than this value, it triggers
230				      in-place-updates.
231	
232	 max_victim_search	      This parameter controls the number of trials to
233				      find a victim segment when conducting SSR and
234				      cleaning operations. The default value is 4096
235				      which covers 8GB block address range.
236	
237	 dir_level                    This parameter controls the directory level to
238				      support large directory. If a directory has a
239				      number of files, it can reduce the file lookup
240				      latency by increasing this dir_level value.
241				      Otherwise, it needs to decrease this value to
242				      reduce the space overhead. The default value is 0.
243	
244	 ram_thresh                   This parameter controls the memory footprint used
245				      by free nids and cached nat entries. By default,
246				      10 is set, which indicates 10 MB / 1 GB RAM.
247	
248	================================================================================
249	USAGE
250	================================================================================
251	
252	1. Download userland tools and compile them.
253	
254	2. Skip, if f2fs was compiled statically inside kernel.
255	   Otherwise, insert the f2fs.ko module.
256	 # insmod f2fs.ko
257	
258	3. Create a directory trying to mount
259	 # mkdir /mnt/f2fs
260	
261	4. Format the block device, and then mount as f2fs
262	 # mkfs.f2fs -l label /dev/block_device
263	 # mount -t f2fs /dev/block_device /mnt/f2fs
264	
265	mkfs.f2fs
266	---------
267	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
268	which builds a basic on-disk layout.
269	
270	The options consist of:
271	-l [label]   : Give a volume label, up to 512 unicode name.
272	-a [0 or 1]  : Split start location of each area for heap-based allocation.
273	               1 is set by default, which performs this.
274	-o [int]     : Set overprovision ratio in percent over volume size.
275	               5 is set by default.
276	-s [int]     : Set the number of segments per section.
277	               1 is set by default.
278	-z [int]     : Set the number of sections per zone.
279	               1 is set by default.
280	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
281	-t [0 or 1]  : Disable discard command or not.
282	               1 is set by default, which conducts discard.
283	
284	fsck.f2fs
285	---------
286	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
287	partition, which examines whether the filesystem metadata and user-made data
288	are cross-referenced correctly or not.
289	Note that, initial version of the tool does not fix any inconsistency.
290	
291	The options consist of:
292	  -d debug level [default:0]
293	
294	dump.f2fs
295	---------
296	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
297	file. Each file is dump_ssa and dump_sit.
298	
299	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
300	It shows on-disk inode information reconized by a given inode number, and is
301	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
302	./dump_sit respectively.
303	
304	The options consist of:
305	  -d debug level [default:0]
306	  -i inode no (hex)
307	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
308	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
309	
310	Examples:
311	# dump.f2fs -i [ino] /dev/sdx
312	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
313	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
314	
315	================================================================================
316	DESIGN
317	================================================================================
318	
319	On-disk Layout
320	--------------
321	
322	F2FS divides the whole volume into a number of segments, each of which is fixed
323	to 2MB in size. A section is composed of consecutive segments, and a zone
324	consists of a set of sections. By default, section and zone sizes are set to one
325	segment size identically, but users can easily modify the sizes by mkfs.
326	
327	F2FS splits the entire volume into six areas, and all the areas except superblock
328	consists of multiple segments as described below.
329	
330	                                            align with the zone size <-|
331	                 |-> align with the segment size
332	     _________________________________________________________________________
333	    |            |            |   Segment   |    Node     |   Segment  |      |
334	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
335	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
336	    |____________|_____2______|______N______|______N______|______N_____|__N___|
337	                                                                       .      .
338	                                                             .                .
339	                                                 .                            .
340	                                    ._________________________________________.
341	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
342	                                    .           .
343	                                    ._________._________
344	                                    |_section_|__...__|_
345	                                    .            .
346			                    .________.
347		                            |__zone__|
348	
349	- Superblock (SB)
350	 : It is located at the beginning of the partition, and there exist two copies
351	   to avoid file system crash. It contains basic partition information and some
352	   default parameters of f2fs.
353	
354	- Checkpoint (CP)
355	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
356	   inode lists, and summary entries of current active segments.
357	
358	- Segment Information Table (SIT)
359	 : It contains segment information such as valid block count and bitmap for the
360	   validity of all the blocks.
361	
362	- Node Address Table (NAT)
363	 : It is composed of a block address table for all the node blocks stored in
364	   Main area.
365	
366	- Segment Summary Area (SSA)
367	 : It contains summary entries which contains the owner information of all the
368	   data and node blocks stored in Main area.
369	
370	- Main Area
371	 : It contains file and directory data including their indices.
372	
373	In order to avoid misalignment between file system and flash-based storage, F2FS
374	aligns the start block address of CP with the segment size. Also, it aligns the
375	start block address of Main area with the zone size by reserving some segments
376	in SSA area.
377	
378	Reference the following survey for additional technical details.
379	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
380	
381	File System Metadata Structure
382	------------------------------
383	
384	F2FS adopts the checkpointing scheme to maintain file system consistency. At
385	mount time, F2FS first tries to find the last valid checkpoint data by scanning
386	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
387	One of them always indicates the last valid data, which is called as shadow copy
388	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
389	
390	For file system consistency, each CP points to which NAT and SIT copies are
391	valid, as shown as below.
392	
393	  +--------+----------+---------+
394	  |   CP   |    SIT   |   NAT   |
395	  +--------+----------+---------+
396	  .         .          .          .
397	  .            .              .              .
398	  .               .                 .                 .
399	  +-------+-------+--------+--------+--------+--------+
400	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
401	  +-------+-------+--------+--------+--------+--------+
402	     |             ^                          ^
403	     |             |                          |
404	     `----------------------------------------'
405	
406	Index Structure
407	---------------
408	
409	The key data structure to manage the data locations is a "node". Similar to
410	traditional file structures, F2FS has three types of node: inode, direct node,
411	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
412	indices, two direct node pointers, two indirect node pointers, and one double
413	indirect node pointer as described below. One direct node block contains 1018
414	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
415	one inode block (i.e., a file) covers:
416	
417	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
418	
419	   Inode block (4KB)
420	     |- data (923)
421	     |- direct node (2)
422	     |          `- data (1018)
423	     |- indirect node (2)
424	     |            `- direct node (1018)
425	     |                       `- data (1018)
426	     `- double indirect node (1)
427	                         `- indirect node (1018)
428				              `- direct node (1018)
429		                                         `- data (1018)
430	
431	Note that, all the node blocks are mapped by NAT which means the location of
432	each node is translated by the NAT table. In the consideration of the wandering
433	tree problem, F2FS is able to cut off the propagation of node updates caused by
434	leaf data writes.
435	
436	Directory Structure
437	-------------------
438	
439	A directory entry occupies 11 bytes, which consists of the following attributes.
440	
441	- hash		hash value of the file name
442	- ino		inode number
443	- len		the length of file name
444	- type		file type such as directory, symlink, etc
445	
446	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
447	used to represent whether each dentry is valid or not. A dentry block occupies
448	4KB with the following composition.
449	
450	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
451		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
452	
453	                         [Bucket]
454	             +--------------------------------+
455	             |dentry block 1 | dentry block 2 |
456	             +--------------------------------+
457	             .               .
458	       .                             .
459	  .       [Dentry Block Structure: 4KB]       .
460	  +--------+----------+----------+------------+
461	  | bitmap | reserved | dentries | file names |
462	  +--------+----------+----------+------------+
463	  [Dentry Block: 4KB] .   .
464			 .               .
465	            .                          .
466	            +------+------+-----+------+
467	            | hash | ino  | len | type |
468	            +------+------+-----+------+
469	            [Dentry Structure: 11 bytes]
470	
471	F2FS implements multi-level hash tables for directory structure. Each level has
472	a hash table with dedicated number of hash buckets as shown below. Note that
473	"A(2B)" means a bucket includes 2 data blocks.
474	
475	----------------------
476	A : bucket
477	B : block
478	N : MAX_DIR_HASH_DEPTH
479	----------------------
480	
481	level #0   | A(2B)
482	           |
483	level #1   | A(2B) - A(2B)
484	           |
485	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
486	     .     |   .       .       .       .
487	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
488	     .     |   .       .       .       .
489	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
490	
491	The number of blocks and buckets are determined by,
492	
493	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
494	  # of blocks in level #n = |
495	                            `- 4, Otherwise
496	
497	                             ,- 2^(n + dir_level),
498				     |        if n + dir_level < MAX_DIR_HASH_DEPTH / 2,
499	  # of buckets in level #n = |
500	                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1),
501				              Otherwise
502	
503	When F2FS finds a file name in a directory, at first a hash value of the file
504	name is calculated. Then, F2FS scans the hash table in level #0 to find the
505	dentry consisting of the file name and its inode number. If not found, F2FS
506	scans the next hash table in level #1. In this way, F2FS scans hash tables in
507	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
508	one bucket determined by the following equation, which shows O(log(# of files))
509	complexity.
510	
511	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
512	
513	In the case of file creation, F2FS finds empty consecutive slots that cover the
514	file name. F2FS searches the empty slots in the hash tables of whole levels from
515	1 to N in the same way as the lookup operation.
516	
517	The following figure shows an example of two cases holding children.
518	       --------------> Dir <--------------
519	       |                                 |
520	    child                             child
521	
522	    child - child                     [hole] - child
523	
524	    child - child - child             [hole] - [hole] - child
525	
526	   Case 1:                           Case 2:
527	   Number of children = 6,           Number of children = 3,
528	   File size = 7                     File size = 7
529	
530	Default Block Allocation
531	------------------------
532	
533	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
534	and Hot/Warm/Cold data.
535	
536	- Hot node	contains direct node blocks of directories.
537	- Warm node	contains direct node blocks except hot node blocks.
538	- Cold node	contains indirect node blocks
539	- Hot data	contains dentry blocks
540	- Warm data	contains data blocks except hot and cold data blocks
541	- Cold data	contains multimedia data or migrated data blocks
542	
543	LFS has two schemes for free space management: threaded log and copy-and-compac-
544	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
545	for devices showing very good sequential write performance, since free segments
546	are served all the time for writing new data. However, it suffers from cleaning
547	overhead under high utilization. Contrarily, the threaded log scheme suffers
548	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
549	scheme where the copy-and-compaction scheme is adopted by default, but the
550	policy is dynamically changed to the threaded log scheme according to the file
551	system status.
552	
553	In order to align F2FS with underlying flash-based storage, F2FS allocates a
554	segment in a unit of section. F2FS expects that the section size would be the
555	same as the unit size of garbage collection in FTL. Furthermore, with respect
556	to the mapping granularity in FTL, F2FS allocates each section of the active
557	logs from different zones as much as possible, since FTL can write the data in
558	the active logs into one allocation unit according to its mapping granularity.
559	
560	Cleaning process
561	----------------
562	
563	F2FS does cleaning both on demand and in the background. On-demand cleaning is
564	triggered when there are not enough free segments to serve VFS calls. Background
565	cleaner is operated by a kernel thread, and triggers the cleaning job when the
566	system is idle.
567	
568	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
569	In the greedy algorithm, F2FS selects a victim segment having the smallest number
570	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
571	according to the segment age and the number of valid blocks in order to address
572	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
573	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
574	algorithm.
575	
576	In order to identify whether the data in the victim segment are valid or not,
577	F2FS manages a bitmap. Each bit represents the validity of a block, and the
578	bitmap is composed of a bit stream covering whole blocks in main area.
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