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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:02 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	flush_merge	       Merge concurrent cache_flush commands as much as possible
126	                       to eliminate redundant command issues. If the underlying
127			       device handles the cache_flush command relatively slowly,
128			       recommend to enable this option.
129	
130	================================================================================
131	DEBUGFS ENTRIES
132	================================================================================
133	
134	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
135	f2fs. Each file shows the whole f2fs information.
136	
137	/sys/kernel/debug/f2fs/status includes:
138	 - major file system information managed by f2fs currently
139	 - average SIT information about whole segments
140	 - current memory footprint consumed by f2fs.
141	
142	================================================================================
143	SYSFS ENTRIES
144	================================================================================
145	
146	Information about mounted f2f2 file systems can be found in
147	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
148	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
149	The files in each per-device directory are shown in table below.
150	
151	Files in /sys/fs/f2fs/<devname>
152	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
153	..............................................................................
154	 File                         Content
155	
156	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
157	                              time for the garbage collection thread. Time is
158	                              in milliseconds.
159	
160	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
161	                              time for the garbage collection thread. Time is
162	                              in milliseconds.
163	
164	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
165	                              time for the garbage collection thread. Time is
166	                              in milliseconds.
167	
168	 gc_idle                      This parameter controls the selection of victim
169	                              policy for garbage collection. Setting gc_idle = 0
170	                              (default) will disable this option. Setting
171	                              gc_idle = 1 will select the Cost Benefit approach
172	                              & setting gc_idle = 2 will select the greedy aproach.
173	
174	 reclaim_segments             This parameter controls the number of prefree
175	                              segments to be reclaimed. If the number of prefree
176				      segments is larger than the number of segments
177				      in the proportion to the percentage over total
178				      volume size, f2fs tries to conduct checkpoint to
179				      reclaim the prefree segments to free segments.
180				      By default, 5% over total # of segments.
181	
182	 max_small_discards	      This parameter controls the number of discard
183				      commands that consist small blocks less than 2MB.
184				      The candidates to be discarded are cached until
185				      checkpoint is triggered, and issued during the
186				      checkpoint. By default, it is disabled with 0.
187	
188	 ipu_policy                   This parameter controls the policy of in-place
189	                              updates in f2fs. There are five policies:
190	                               0: F2FS_IPU_FORCE, 1: F2FS_IPU_SSR,
191	                               2: F2FS_IPU_UTIL,  3: F2FS_IPU_SSR_UTIL,
192	                               4: F2FS_IPU_DISABLE.
193	
194	 min_ipu_util                 This parameter controls the threshold to trigger
195	                              in-place-updates. The number indicates percentage
196	                              of the filesystem utilization, and used by
197	                              F2FS_IPU_UTIL and F2FS_IPU_SSR_UTIL policies.
198	
199	 max_victim_search	      This parameter controls the number of trials to
200				      find a victim segment when conducting SSR and
201				      cleaning operations. The default value is 4096
202				      which covers 8GB block address range.
203	
204	 dir_level                    This parameter controls the directory level to
205				      support large directory. If a directory has a
206				      number of files, it can reduce the file lookup
207				      latency by increasing this dir_level value.
208				      Otherwise, it needs to decrease this value to
209				      reduce the space overhead. The default value is 0.
210	
211	 ram_thresh                   This parameter controls the memory footprint used
212				      by free nids and cached nat entries. By default,
213				      10 is set, which indicates 10 MB / 1 GB RAM.
214	
215	================================================================================
216	USAGE
217	================================================================================
218	
219	1. Download userland tools and compile them.
220	
221	2. Skip, if f2fs was compiled statically inside kernel.
222	   Otherwise, insert the f2fs.ko module.
223	 # insmod f2fs.ko
224	
225	3. Create a directory trying to mount
226	 # mkdir /mnt/f2fs
227	
228	4. Format the block device, and then mount as f2fs
229	 # mkfs.f2fs -l label /dev/block_device
230	 # mount -t f2fs /dev/block_device /mnt/f2fs
231	
232	mkfs.f2fs
233	---------
234	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
235	which builds a basic on-disk layout.
236	
237	The options consist of:
238	-l [label]   : Give a volume label, up to 512 unicode name.
239	-a [0 or 1]  : Split start location of each area for heap-based allocation.
240	               1 is set by default, which performs this.
241	-o [int]     : Set overprovision ratio in percent over volume size.
242	               5 is set by default.
243	-s [int]     : Set the number of segments per section.
244	               1 is set by default.
245	-z [int]     : Set the number of sections per zone.
246	               1 is set by default.
247	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
248	-t [0 or 1]  : Disable discard command or not.
249	               1 is set by default, which conducts discard.
250	
251	fsck.f2fs
252	---------
253	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
254	partition, which examines whether the filesystem metadata and user-made data
255	are cross-referenced correctly or not.
256	Note that, initial version of the tool does not fix any inconsistency.
257	
258	The options consist of:
259	  -d debug level [default:0]
260	
261	dump.f2fs
262	---------
263	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
264	file. Each file is dump_ssa and dump_sit.
265	
266	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
267	It shows on-disk inode information reconized by a given inode number, and is
268	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
269	./dump_sit respectively.
270	
271	The options consist of:
272	  -d debug level [default:0]
273	  -i inode no (hex)
274	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
275	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
276	
277	Examples:
278	# dump.f2fs -i [ino] /dev/sdx
279	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
280	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
281	
282	================================================================================
283	DESIGN
284	================================================================================
285	
286	On-disk Layout
287	--------------
288	
289	F2FS divides the whole volume into a number of segments, each of which is fixed
290	to 2MB in size. A section is composed of consecutive segments, and a zone
291	consists of a set of sections. By default, section and zone sizes are set to one
292	segment size identically, but users can easily modify the sizes by mkfs.
293	
294	F2FS splits the entire volume into six areas, and all the areas except superblock
295	consists of multiple segments as described below.
296	
297	                                            align with the zone size <-|
298	                 |-> align with the segment size
299	     _________________________________________________________________________
300	    |            |            |   Segment   |    Node     |   Segment  |      |
301	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
302	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
303	    |____________|_____2______|______N______|______N______|______N_____|__N___|
304	                                                                       .      .
305	                                                             .                .
306	                                                 .                            .
307	                                    ._________________________________________.
308	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
309	                                    .           .
310	                                    ._________._________
311	                                    |_section_|__...__|_
312	                                    .            .
313			                    .________.
314		                            |__zone__|
315	
316	- Superblock (SB)
317	 : It is located at the beginning of the partition, and there exist two copies
318	   to avoid file system crash. It contains basic partition information and some
319	   default parameters of f2fs.
320	
321	- Checkpoint (CP)
322	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
323	   inode lists, and summary entries of current active segments.
324	
325	- Segment Information Table (SIT)
326	 : It contains segment information such as valid block count and bitmap for the
327	   validity of all the blocks.
328	
329	- Node Address Table (NAT)
330	 : It is composed of a block address table for all the node blocks stored in
331	   Main area.
332	
333	- Segment Summary Area (SSA)
334	 : It contains summary entries which contains the owner information of all the
335	   data and node blocks stored in Main area.
336	
337	- Main Area
338	 : It contains file and directory data including their indices.
339	
340	In order to avoid misalignment between file system and flash-based storage, F2FS
341	aligns the start block address of CP with the segment size. Also, it aligns the
342	start block address of Main area with the zone size by reserving some segments
343	in SSA area.
344	
345	Reference the following survey for additional technical details.
346	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
347	
348	File System Metadata Structure
349	------------------------------
350	
351	F2FS adopts the checkpointing scheme to maintain file system consistency. At
352	mount time, F2FS first tries to find the last valid checkpoint data by scanning
353	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
354	One of them always indicates the last valid data, which is called as shadow copy
355	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
356	
357	For file system consistency, each CP points to which NAT and SIT copies are
358	valid, as shown as below.
359	
360	  +--------+----------+---------+
361	  |   CP   |    SIT   |   NAT   |
362	  +--------+----------+---------+
363	  .         .          .          .
364	  .            .              .              .
365	  .               .                 .                 .
366	  +-------+-------+--------+--------+--------+--------+
367	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
368	  +-------+-------+--------+--------+--------+--------+
369	     |             ^                          ^
370	     |             |                          |
371	     `----------------------------------------'
372	
373	Index Structure
374	---------------
375	
376	The key data structure to manage the data locations is a "node". Similar to
377	traditional file structures, F2FS has three types of node: inode, direct node,
378	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
379	indices, two direct node pointers, two indirect node pointers, and one double
380	indirect node pointer as described below. One direct node block contains 1018
381	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
382	one inode block (i.e., a file) covers:
383	
384	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
385	
386	   Inode block (4KB)
387	     |- data (923)
388	     |- direct node (2)
389	     |          `- data (1018)
390	     |- indirect node (2)
391	     |            `- direct node (1018)
392	     |                       `- data (1018)
393	     `- double indirect node (1)
394	                         `- indirect node (1018)
395				              `- direct node (1018)
396		                                         `- data (1018)
397	
398	Note that, all the node blocks are mapped by NAT which means the location of
399	each node is translated by the NAT table. In the consideration of the wandering
400	tree problem, F2FS is able to cut off the propagation of node updates caused by
401	leaf data writes.
402	
403	Directory Structure
404	-------------------
405	
406	A directory entry occupies 11 bytes, which consists of the following attributes.
407	
408	- hash		hash value of the file name
409	- ino		inode number
410	- len		the length of file name
411	- type		file type such as directory, symlink, etc
412	
413	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
414	used to represent whether each dentry is valid or not. A dentry block occupies
415	4KB with the following composition.
416	
417	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
418		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
419	
420	                         [Bucket]
421	             +--------------------------------+
422	             |dentry block 1 | dentry block 2 |
423	             +--------------------------------+
424	             .               .
425	       .                             .
426	  .       [Dentry Block Structure: 4KB]       .
427	  +--------+----------+----------+------------+
428	  | bitmap | reserved | dentries | file names |
429	  +--------+----------+----------+------------+
430	  [Dentry Block: 4KB] .   .
431			 .               .
432	            .                          .
433	            +------+------+-----+------+
434	            | hash | ino  | len | type |
435	            +------+------+-----+------+
436	            [Dentry Structure: 11 bytes]
437	
438	F2FS implements multi-level hash tables for directory structure. Each level has
439	a hash table with dedicated number of hash buckets as shown below. Note that
440	"A(2B)" means a bucket includes 2 data blocks.
441	
442	----------------------
443	A : bucket
444	B : block
445	N : MAX_DIR_HASH_DEPTH
446	----------------------
447	
448	level #0   | A(2B)
449	           |
450	level #1   | A(2B) - A(2B)
451	           |
452	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
453	     .     |   .       .       .       .
454	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
455	     .     |   .       .       .       .
456	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
457	
458	The number of blocks and buckets are determined by,
459	
460	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
461	  # of blocks in level #n = |
462	                            `- 4, Otherwise
463	
464	                             ,- 2^ (n + dir_level),
465				     |            if n < MAX_DIR_HASH_DEPTH / 2,
466	  # of buckets in level #n = |
467	                             `- 2^((MAX_DIR_HASH_DEPTH / 2 + dir_level) - 1),
468				                  Otherwise
469	
470	When F2FS finds a file name in a directory, at first a hash value of the file
471	name is calculated. Then, F2FS scans the hash table in level #0 to find the
472	dentry consisting of the file name and its inode number. If not found, F2FS
473	scans the next hash table in level #1. In this way, F2FS scans hash tables in
474	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
475	one bucket determined by the following equation, which shows O(log(# of files))
476	complexity.
477	
478	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
479	
480	In the case of file creation, F2FS finds empty consecutive slots that cover the
481	file name. F2FS searches the empty slots in the hash tables of whole levels from
482	1 to N in the same way as the lookup operation.
483	
484	The following figure shows an example of two cases holding children.
485	       --------------> Dir <--------------
486	       |                                 |
487	    child                             child
488	
489	    child - child                     [hole] - child
490	
491	    child - child - child             [hole] - [hole] - child
492	
493	   Case 1:                           Case 2:
494	   Number of children = 6,           Number of children = 3,
495	   File size = 7                     File size = 7
496	
497	Default Block Allocation
498	------------------------
499	
500	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
501	and Hot/Warm/Cold data.
502	
503	- Hot node	contains direct node blocks of directories.
504	- Warm node	contains direct node blocks except hot node blocks.
505	- Cold node	contains indirect node blocks
506	- Hot data	contains dentry blocks
507	- Warm data	contains data blocks except hot and cold data blocks
508	- Cold data	contains multimedia data or migrated data blocks
509	
510	LFS has two schemes for free space management: threaded log and copy-and-compac-
511	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
512	for devices showing very good sequential write performance, since free segments
513	are served all the time for writing new data. However, it suffers from cleaning
514	overhead under high utilization. Contrarily, the threaded log scheme suffers
515	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
516	scheme where the copy-and-compaction scheme is adopted by default, but the
517	policy is dynamically changed to the threaded log scheme according to the file
518	system status.
519	
520	In order to align F2FS with underlying flash-based storage, F2FS allocates a
521	segment in a unit of section. F2FS expects that the section size would be the
522	same as the unit size of garbage collection in FTL. Furthermore, with respect
523	to the mapping granularity in FTL, F2FS allocates each section of the active
524	logs from different zones as much as possible, since FTL can write the data in
525	the active logs into one allocation unit according to its mapping granularity.
526	
527	Cleaning process
528	----------------
529	
530	F2FS does cleaning both on demand and in the background. On-demand cleaning is
531	triggered when there are not enough free segments to serve VFS calls. Background
532	cleaner is operated by a kernel thread, and triggers the cleaning job when the
533	system is idle.
534	
535	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
536	In the greedy algorithm, F2FS selects a victim segment having the smallest number
537	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
538	according to the segment age and the number of valid blocks in order to address
539	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
540	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
541	algorithm.
542	
543	In order to identify whether the data in the victim segment are valid or not,
544	F2FS manages a bitmap. Each bit represents the validity of a block, and the
545	bitmap is composed of a bit stream covering whole blocks in main area.
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