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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 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	
124	================================================================================
125	DEBUGFS ENTRIES
126	================================================================================
127	
128	/sys/kernel/debug/f2fs/ contains information about all the partitions mounted as
129	f2fs. Each file shows the whole f2fs information.
130	
131	/sys/kernel/debug/f2fs/status includes:
132	 - major file system information managed by f2fs currently
133	 - average SIT information about whole segments
134	 - current memory footprint consumed by f2fs.
135	
136	================================================================================
137	SYSFS ENTRIES
138	================================================================================
139	
140	Information about mounted f2f2 file systems can be found in
141	/sys/fs/f2fs.  Each mounted filesystem will have a directory in
142	/sys/fs/f2fs based on its device name (i.e., /sys/fs/f2fs/sda).
143	The files in each per-device directory are shown in table below.
144	
145	Files in /sys/fs/f2fs/<devname>
146	(see also Documentation/ABI/testing/sysfs-fs-f2fs)
147	..............................................................................
148	 File                         Content
149	
150	 gc_max_sleep_time            This tuning parameter controls the maximum sleep
151	                              time for the garbage collection thread. Time is
152	                              in milliseconds.
153	
154	 gc_min_sleep_time            This tuning parameter controls the minimum sleep
155	                              time for the garbage collection thread. Time is
156	                              in milliseconds.
157	
158	 gc_no_gc_sleep_time          This tuning parameter controls the default sleep
159	                              time for the garbage collection thread. Time is
160	                              in milliseconds.
161	
162	 gc_idle                      This parameter controls the selection of victim
163	                              policy for garbage collection. Setting gc_idle = 0
164	                              (default) will disable this option. Setting
165	                              gc_idle = 1 will select the Cost Benefit approach
166	                              & setting gc_idle = 2 will select the greedy aproach.
167	
168	 reclaim_segments             This parameter controls the number of prefree
169	                              segments to be reclaimed. If the number of prefree
170				      segments is larger than this number, f2fs tries to
171				      conduct checkpoint to reclaim the prefree segments
172				      to free segments. By default, 100 segments, 200MB.
173	
174	================================================================================
175	USAGE
176	================================================================================
177	
178	1. Download userland tools and compile them.
179	
180	2. Skip, if f2fs was compiled statically inside kernel.
181	   Otherwise, insert the f2fs.ko module.
182	 # insmod f2fs.ko
183	
184	3. Create a directory trying to mount
185	 # mkdir /mnt/f2fs
186	
187	4. Format the block device, and then mount as f2fs
188	 # mkfs.f2fs -l label /dev/block_device
189	 # mount -t f2fs /dev/block_device /mnt/f2fs
190	
191	mkfs.f2fs
192	---------
193	The mkfs.f2fs is for the use of formatting a partition as the f2fs filesystem,
194	which builds a basic on-disk layout.
195	
196	The options consist of:
197	-l [label]   : Give a volume label, up to 512 unicode name.
198	-a [0 or 1]  : Split start location of each area for heap-based allocation.
199	               1 is set by default, which performs this.
200	-o [int]     : Set overprovision ratio in percent over volume size.
201	               5 is set by default.
202	-s [int]     : Set the number of segments per section.
203	               1 is set by default.
204	-z [int]     : Set the number of sections per zone.
205	               1 is set by default.
206	-e [str]     : Set basic extension list. e.g. "mp3,gif,mov"
207	-t [0 or 1]  : Disable discard command or not.
208	               1 is set by default, which conducts discard.
209	
210	fsck.f2fs
211	---------
212	The fsck.f2fs is a tool to check the consistency of an f2fs-formatted
213	partition, which examines whether the filesystem metadata and user-made data
214	are cross-referenced correctly or not.
215	Note that, initial version of the tool does not fix any inconsistency.
216	
217	The options consist of:
218	  -d debug level [default:0]
219	
220	dump.f2fs
221	---------
222	The dump.f2fs shows the information of specific inode and dumps SSA and SIT to
223	file. Each file is dump_ssa and dump_sit.
224	
225	The dump.f2fs is used to debug on-disk data structures of the f2fs filesystem.
226	It shows on-disk inode information reconized by a given inode number, and is
227	able to dump all the SSA and SIT entries into predefined files, ./dump_ssa and
228	./dump_sit respectively.
229	
230	The options consist of:
231	  -d debug level [default:0]
232	  -i inode no (hex)
233	  -s [SIT dump segno from #1~#2 (decimal), for all 0~-1]
234	  -a [SSA dump segno from #1~#2 (decimal), for all 0~-1]
235	
236	Examples:
237	# dump.f2fs -i [ino] /dev/sdx
238	# dump.f2fs -s 0~-1 /dev/sdx (SIT dump)
239	# dump.f2fs -a 0~-1 /dev/sdx (SSA dump)
240	
241	================================================================================
242	DESIGN
243	================================================================================
244	
245	On-disk Layout
246	--------------
247	
248	F2FS divides the whole volume into a number of segments, each of which is fixed
249	to 2MB in size. A section is composed of consecutive segments, and a zone
250	consists of a set of sections. By default, section and zone sizes are set to one
251	segment size identically, but users can easily modify the sizes by mkfs.
252	
253	F2FS splits the entire volume into six areas, and all the areas except superblock
254	consists of multiple segments as described below.
255	
256	                                            align with the zone size <-|
257	                 |-> align with the segment size
258	     _________________________________________________________________________
259	    |            |            |   Segment   |    Node     |   Segment  |      |
260	    | Superblock | Checkpoint |    Info.    |   Address   |   Summary  | Main |
261	    |    (SB)    |   (CP)     | Table (SIT) | Table (NAT) | Area (SSA) |      |
262	    |____________|_____2______|______N______|______N______|______N_____|__N___|
263	                                                                       .      .
264	                                                             .                .
265	                                                 .                            .
266	                                    ._________________________________________.
267	                                    |_Segment_|_..._|_Segment_|_..._|_Segment_|
268	                                    .           .
269	                                    ._________._________
270	                                    |_section_|__...__|_
271	                                    .            .
272			                    .________.
273		                            |__zone__|
274	
275	- Superblock (SB)
276	 : It is located at the beginning of the partition, and there exist two copies
277	   to avoid file system crash. It contains basic partition information and some
278	   default parameters of f2fs.
279	
280	- Checkpoint (CP)
281	 : It contains file system information, bitmaps for valid NAT/SIT sets, orphan
282	   inode lists, and summary entries of current active segments.
283	
284	- Segment Information Table (SIT)
285	 : It contains segment information such as valid block count and bitmap for the
286	   validity of all the blocks.
287	
288	- Node Address Table (NAT)
289	 : It is composed of a block address table for all the node blocks stored in
290	   Main area.
291	
292	- Segment Summary Area (SSA)
293	 : It contains summary entries which contains the owner information of all the
294	   data and node blocks stored in Main area.
295	
296	- Main Area
297	 : It contains file and directory data including their indices.
298	
299	In order to avoid misalignment between file system and flash-based storage, F2FS
300	aligns the start block address of CP with the segment size. Also, it aligns the
301	start block address of Main area with the zone size by reserving some segments
302	in SSA area.
303	
304	Reference the following survey for additional technical details.
305	https://wiki.linaro.org/WorkingGroups/Kernel/Projects/FlashCardSurvey
306	
307	File System Metadata Structure
308	------------------------------
309	
310	F2FS adopts the checkpointing scheme to maintain file system consistency. At
311	mount time, F2FS first tries to find the last valid checkpoint data by scanning
312	CP area. In order to reduce the scanning time, F2FS uses only two copies of CP.
313	One of them always indicates the last valid data, which is called as shadow copy
314	mechanism. In addition to CP, NAT and SIT also adopt the shadow copy mechanism.
315	
316	For file system consistency, each CP points to which NAT and SIT copies are
317	valid, as shown as below.
318	
319	  +--------+----------+---------+
320	  |   CP   |    SIT   |   NAT   |
321	  +--------+----------+---------+
322	  .         .          .          .
323	  .            .              .              .
324	  .               .                 .                 .
325	  +-------+-------+--------+--------+--------+--------+
326	  | CP #0 | CP #1 | SIT #0 | SIT #1 | NAT #0 | NAT #1 |
327	  +-------+-------+--------+--------+--------+--------+
328	     |             ^                          ^
329	     |             |                          |
330	     `----------------------------------------'
331	
332	Index Structure
333	---------------
334	
335	The key data structure to manage the data locations is a "node". Similar to
336	traditional file structures, F2FS has three types of node: inode, direct node,
337	indirect node. F2FS assigns 4KB to an inode block which contains 923 data block
338	indices, two direct node pointers, two indirect node pointers, and one double
339	indirect node pointer as described below. One direct node block contains 1018
340	data blocks, and one indirect node block contains also 1018 node blocks. Thus,
341	one inode block (i.e., a file) covers:
342	
343	  4KB * (923 + 2 * 1018 + 2 * 1018 * 1018 + 1018 * 1018 * 1018) := 3.94TB.
344	
345	   Inode block (4KB)
346	     |- data (923)
347	     |- direct node (2)
348	     |          `- data (1018)
349	     |- indirect node (2)
350	     |            `- direct node (1018)
351	     |                       `- data (1018)
352	     `- double indirect node (1)
353	                         `- indirect node (1018)
354				              `- direct node (1018)
355		                                         `- data (1018)
356	
357	Note that, all the node blocks are mapped by NAT which means the location of
358	each node is translated by the NAT table. In the consideration of the wandering
359	tree problem, F2FS is able to cut off the propagation of node updates caused by
360	leaf data writes.
361	
362	Directory Structure
363	-------------------
364	
365	A directory entry occupies 11 bytes, which consists of the following attributes.
366	
367	- hash		hash value of the file name
368	- ino		inode number
369	- len		the length of file name
370	- type		file type such as directory, symlink, etc
371	
372	A dentry block consists of 214 dentry slots and file names. Therein a bitmap is
373	used to represent whether each dentry is valid or not. A dentry block occupies
374	4KB with the following composition.
375	
376	  Dentry Block(4 K) = bitmap (27 bytes) + reserved (3 bytes) +
377		              dentries(11 * 214 bytes) + file name (8 * 214 bytes)
378	
379	                         [Bucket]
380	             +--------------------------------+
381	             |dentry block 1 | dentry block 2 |
382	             +--------------------------------+
383	             .               .
384	       .                             .
385	  .       [Dentry Block Structure: 4KB]       .
386	  +--------+----------+----------+------------+
387	  | bitmap | reserved | dentries | file names |
388	  +--------+----------+----------+------------+
389	  [Dentry Block: 4KB] .   .
390			 .               .
391	            .                          .
392	            +------+------+-----+------+
393	            | hash | ino  | len | type |
394	            +------+------+-----+------+
395	            [Dentry Structure: 11 bytes]
396	
397	F2FS implements multi-level hash tables for directory structure. Each level has
398	a hash table with dedicated number of hash buckets as shown below. Note that
399	"A(2B)" means a bucket includes 2 data blocks.
400	
401	----------------------
402	A : bucket
403	B : block
404	N : MAX_DIR_HASH_DEPTH
405	----------------------
406	
407	level #0   | A(2B)
408	           |
409	level #1   | A(2B) - A(2B)
410	           |
411	level #2   | A(2B) - A(2B) - A(2B) - A(2B)
412	     .     |   .       .       .       .
413	level #N/2 | A(2B) - A(2B) - A(2B) - A(2B) - A(2B) - ... - A(2B)
414	     .     |   .       .       .       .
415	level #N   | A(4B) - A(4B) - A(4B) - A(4B) - A(4B) - ... - A(4B)
416	
417	The number of blocks and buckets are determined by,
418	
419	                            ,- 2, if n < MAX_DIR_HASH_DEPTH / 2,
420	  # of blocks in level #n = |
421	                            `- 4, Otherwise
422	
423	                             ,- 2^n, if n < MAX_DIR_HASH_DEPTH / 2,
424	  # of buckets in level #n = |
425	                             `- 2^((MAX_DIR_HASH_DEPTH / 2) - 1), Otherwise
426	
427	When F2FS finds a file name in a directory, at first a hash value of the file
428	name is calculated. Then, F2FS scans the hash table in level #0 to find the
429	dentry consisting of the file name and its inode number. If not found, F2FS
430	scans the next hash table in level #1. In this way, F2FS scans hash tables in
431	each levels incrementally from 1 to N. In each levels F2FS needs to scan only
432	one bucket determined by the following equation, which shows O(log(# of files))
433	complexity.
434	
435	  bucket number to scan in level #n = (hash value) % (# of buckets in level #n)
436	
437	In the case of file creation, F2FS finds empty consecutive slots that cover the
438	file name. F2FS searches the empty slots in the hash tables of whole levels from
439	1 to N in the same way as the lookup operation.
440	
441	The following figure shows an example of two cases holding children.
442	       --------------> Dir <--------------
443	       |                                 |
444	    child                             child
445	
446	    child - child                     [hole] - child
447	
448	    child - child - child             [hole] - [hole] - child
449	
450	   Case 1:                           Case 2:
451	   Number of children = 6,           Number of children = 3,
452	   File size = 7                     File size = 7
453	
454	Default Block Allocation
455	------------------------
456	
457	At runtime, F2FS manages six active logs inside "Main" area: Hot/Warm/Cold node
458	and Hot/Warm/Cold data.
459	
460	- Hot node	contains direct node blocks of directories.
461	- Warm node	contains direct node blocks except hot node blocks.
462	- Cold node	contains indirect node blocks
463	- Hot data	contains dentry blocks
464	- Warm data	contains data blocks except hot and cold data blocks
465	- Cold data	contains multimedia data or migrated data blocks
466	
467	LFS has two schemes for free space management: threaded log and copy-and-compac-
468	tion. The copy-and-compaction scheme which is known as cleaning, is well-suited
469	for devices showing very good sequential write performance, since free segments
470	are served all the time for writing new data. However, it suffers from cleaning
471	overhead under high utilization. Contrarily, the threaded log scheme suffers
472	from random writes, but no cleaning process is needed. F2FS adopts a hybrid
473	scheme where the copy-and-compaction scheme is adopted by default, but the
474	policy is dynamically changed to the threaded log scheme according to the file
475	system status.
476	
477	In order to align F2FS with underlying flash-based storage, F2FS allocates a
478	segment in a unit of section. F2FS expects that the section size would be the
479	same as the unit size of garbage collection in FTL. Furthermore, with respect
480	to the mapping granularity in FTL, F2FS allocates each section of the active
481	logs from different zones as much as possible, since FTL can write the data in
482	the active logs into one allocation unit according to its mapping granularity.
483	
484	Cleaning process
485	----------------
486	
487	F2FS does cleaning both on demand and in the background. On-demand cleaning is
488	triggered when there are not enough free segments to serve VFS calls. Background
489	cleaner is operated by a kernel thread, and triggers the cleaning job when the
490	system is idle.
491	
492	F2FS supports two victim selection policies: greedy and cost-benefit algorithms.
493	In the greedy algorithm, F2FS selects a victim segment having the smallest number
494	of valid blocks. In the cost-benefit algorithm, F2FS selects a victim segment
495	according to the segment age and the number of valid blocks in order to address
496	log block thrashing problem in the greedy algorithm. F2FS adopts the greedy
497	algorithm for on-demand cleaner, while background cleaner adopts cost-benefit
498	algorithm.
499	
500	In order to identify whether the data in the victim segment are valid or not,
501	F2FS manages a bitmap. Each bit represents the validity of a block, and the
502	bitmap is composed of a bit stream covering whole blocks in main area.
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