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Based on kernel version 3.13. Page generated on 2014-01-20 22:02 EST.

1				  ==========================
2				  General Filesystem Caching
3				  ==========================
4	
5	========
6	OVERVIEW
7	========
8	
9	This facility is a general purpose cache for network filesystems, though it
10	could be used for caching other things such as ISO9660 filesystems too.
11	
12	FS-Cache mediates between cache backends (such as CacheFS) and network
13	filesystems:
14	
15		+---------+
16		|         |                        +--------------+
17		|   NFS   |--+                     |              |
18		|         |  |                 +-->|   CacheFS    |
19		+---------+  |   +----------+  |   |  /dev/hda5   |
20		             |   |          |  |   +--------------+
21		+---------+  +-->|          |  |
22		|         |      |          |--+
23		|   AFS   |----->| FS-Cache |
24		|         |      |          |--+
25		+---------+  +-->|          |  |
26		             |   |          |  |   +--------------+
27		+---------+  |   +----------+  |   |              |
28		|         |  |                 +-->|  CacheFiles  |
29		|  ISOFS  |--+                     |  /var/cache  |
30		|         |                        +--------------+
31		+---------+
32	
33	Or to look at it another way, FS-Cache is a module that provides a caching
34	facility to a network filesystem such that the cache is transparent to the
35	user:
36	
37		+---------+
38		|         |
39		| Server  |
40		|         |
41		+---------+
42		     |                  NETWORK
43		~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
44		     |
45		     |           +----------+
46		     V           |          |
47		+---------+      |          |
48		|         |      |          |
49		|   NFS   |----->| FS-Cache |
50		|         |      |          |--+
51		+---------+      |          |  |   +--------------+   +--------------+
52		     |           |          |  |   |              |   |              |
53		     V           +----------+  +-->|  CacheFiles  |-->|  Ext3        |
54		+---------+                        |  /var/cache  |   |  /dev/sda6   |
55		|         |                        +--------------+   +--------------+
56		|   VFS   |                                ^                     ^
57		|         |                                |                     |
58		+---------+                                +--------------+      |
59		     |                  KERNEL SPACE                      |      |
60		~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
61		     |                  USER SPACE                        |      |
62		     V                                                    |      |
63		+---------+                                           +--------------+
64		|         |                                           |              |
65		| Process |                                           | cachefilesd  |
66		|         |                                           |              |
67		+---------+                                           +--------------+
68	
69	
70	FS-Cache does not follow the idea of completely loading every netfs file
71	opened in its entirety into a cache before permitting it to be accessed and
72	then serving the pages out of that cache rather than the netfs inode because:
73	
74	 (1) It must be practical to operate without a cache.
75	
76	 (2) The size of any accessible file must not be limited to the size of the
77	     cache.
78	
79	 (3) The combined size of all opened files (this includes mapped libraries)
80	     must not be limited to the size of the cache.
81	
82	 (4) The user should not be forced to download an entire file just to do a
83	     one-off access of a small portion of it (such as might be done with the
84	     "file" program).
85	
86	It instead serves the cache out in PAGE_SIZE chunks as and when requested by
87	the netfs('s) using it.
88	
89	
90	FS-Cache provides the following facilities:
91	
92	 (1) More than one cache can be used at once.  Caches can be selected
93	     explicitly by use of tags.
94	
95	 (2) Caches can be added / removed at any time.
96	
97	 (3) The netfs is provided with an interface that allows either party to
98	     withdraw caching facilities from a file (required for (2)).
99	
100	 (4) The interface to the netfs returns as few errors as possible, preferring
101	     rather to let the netfs remain oblivious.
102	
103	 (5) Cookies are used to represent indices, files and other objects to the
104	     netfs.  The simplest cookie is just a NULL pointer - indicating nothing
105	     cached there.
106	
107	 (6) The netfs is allowed to propose - dynamically - any index hierarchy it
108	     desires, though it must be aware that the index search function is
109	     recursive, stack space is limited, and indices can only be children of
110	     indices.
111	
112	 (7) Data I/O is done direct to and from the netfs's pages.  The netfs
113	     indicates that page A is at index B of the data-file represented by cookie
114	     C, and that it should be read or written.  The cache backend may or may
115	     not start I/O on that page, but if it does, a netfs callback will be
116	     invoked to indicate completion.  The I/O may be either synchronous or
117	     asynchronous.
118	
119	 (8) Cookies can be "retired" upon release.  At this point FS-Cache will mark
120	     them as obsolete and the index hierarchy rooted at that point will get
121	     recycled.
122	
123	 (9) The netfs provides a "match" function for index searches.  In addition to
124	     saying whether a match was made or not, this can also specify that an
125	     entry should be updated or deleted.
126	
127	(10) As much as possible is done asynchronously.
128	
129	
130	FS-Cache maintains a virtual indexing tree in which all indices, files, objects
131	and pages are kept.  Bits of this tree may actually reside in one or more
132	caches.
133	
134	                                           FSDEF
135	                                             |
136	                        +------------------------------------+
137	                        |                                    |
138	                       NFS                                  AFS
139	                        |                                    |
140	           +--------------------------+                +-----------+
141	           |                          |                |           |
142	        homedir                     mirror          afs.org   redhat.com
143	           |                          |                            |
144	     +------------+           +---------------+              +----------+
145	     |            |           |               |              |          |
146	   00001        00002       00007           00125        vol00001   vol00002
147	     |            |           |               |                         |
148	 +---+---+     +-----+      +---+      +------+------+            +-----+----+
149	 |   |   |     |     |      |   |      |      |      |            |     |    |
150	PG0 PG1 PG2   PG0  XATTR   PG0 PG1   DIRENT DIRENT DIRENT        R/W   R/O  Bak
151	                     |                                            |
152	                    PG0                                       +-------+
153	                                                              |       |
154	                                                            00001   00003
155	                                                              |
156	                                                          +---+---+
157	                                                          |   |   |
158	                                                         PG0 PG1 PG2
159	
160	In the example above, you can see two netfs's being backed: NFS and AFS.  These
161	have different index hierarchies:
162	
163	 (*) The NFS primary index contains per-server indices.  Each server index is
164	     indexed by NFS file handles to get data file objects.  Each data file
165	     objects can have an array of pages, but may also have further child
166	     objects, such as extended attributes and directory entries.  Extended
167	     attribute objects themselves have page-array contents.
168	
169	 (*) The AFS primary index contains per-cell indices.  Each cell index contains
170	     per-logical-volume indices.  Each of volume index contains up to three
171	     indices for the read-write, read-only and backup mirrors of those volumes.
172	     Each of these contains vnode data file objects, each of which contains an
173	     array of pages.
174	
175	The very top index is the FS-Cache master index in which individual netfs's
176	have entries.
177	
178	Any index object may reside in more than one cache, provided it only has index
179	children.  Any index with non-index object children will be assumed to only
180	reside in one cache.
181	
182	
183	The netfs API to FS-Cache can be found in:
184	
185		Documentation/filesystems/caching/netfs-api.txt
186	
187	The cache backend API to FS-Cache can be found in:
188	
189		Documentation/filesystems/caching/backend-api.txt
190	
191	A description of the internal representations and object state machine can be
192	found in:
193	
194		Documentation/filesystems/caching/object.txt
195	
196	
197	=======================
198	STATISTICAL INFORMATION
199	=======================
200	
201	If FS-Cache is compiled with the following options enabled:
202	
203		CONFIG_FSCACHE_STATS=y
204		CONFIG_FSCACHE_HISTOGRAM=y
205	
206	then it will gather certain statistics and display them through a number of
207	proc files.
208	
209	 (*) /proc/fs/fscache/stats
210	
211	     This shows counts of a number of events that can happen in FS-Cache:
212	
213		CLASS	EVENT	MEANING
214		=======	=======	=======================================================
215		Cookies	idx=N	Number of index cookies allocated
216			dat=N	Number of data storage cookies allocated
217			spc=N	Number of special cookies allocated
218		Objects	alc=N	Number of objects allocated
219			nal=N	Number of object allocation failures
220			avl=N	Number of objects that reached the available state
221			ded=N	Number of objects that reached the dead state
222		ChkAux	non=N	Number of objects that didn't have a coherency check
223			ok=N	Number of objects that passed a coherency check
224			upd=N	Number of objects that needed a coherency data update
225			obs=N	Number of objects that were declared obsolete
226		Pages	mrk=N	Number of pages marked as being cached
227			unc=N	Number of uncache page requests seen
228		Acquire	n=N	Number of acquire cookie requests seen
229			nul=N	Number of acq reqs given a NULL parent
230			noc=N	Number of acq reqs rejected due to no cache available
231			ok=N	Number of acq reqs succeeded
232			nbf=N	Number of acq reqs rejected due to error
233			oom=N	Number of acq reqs failed on ENOMEM
234		Lookups	n=N	Number of lookup calls made on cache backends
235			neg=N	Number of negative lookups made
236			pos=N	Number of positive lookups made
237			crt=N	Number of objects created by lookup
238			tmo=N	Number of lookups timed out and requeued
239		Updates	n=N	Number of update cookie requests seen
240			nul=N	Number of upd reqs given a NULL parent
241			run=N	Number of upd reqs granted CPU time
242		Relinqs	n=N	Number of relinquish cookie requests seen
243			nul=N	Number of rlq reqs given a NULL parent
244			wcr=N	Number of rlq reqs waited on completion of creation
245		AttrChg	n=N	Number of attribute changed requests seen
246			ok=N	Number of attr changed requests queued
247			nbf=N	Number of attr changed rejected -ENOBUFS
248			oom=N	Number of attr changed failed -ENOMEM
249			run=N	Number of attr changed ops given CPU time
250		Allocs	n=N	Number of allocation requests seen
251			ok=N	Number of successful alloc reqs
252			wt=N	Number of alloc reqs that waited on lookup completion
253			nbf=N	Number of alloc reqs rejected -ENOBUFS
254			int=N	Number of alloc reqs aborted -ERESTARTSYS
255			ops=N	Number of alloc reqs submitted
256			owt=N	Number of alloc reqs waited for CPU time
257			abt=N	Number of alloc reqs aborted due to object death
258		Retrvls	n=N	Number of retrieval (read) requests seen
259			ok=N	Number of successful retr reqs
260			wt=N	Number of retr reqs that waited on lookup completion
261			nod=N	Number of retr reqs returned -ENODATA
262			nbf=N	Number of retr reqs rejected -ENOBUFS
263			int=N	Number of retr reqs aborted -ERESTARTSYS
264			oom=N	Number of retr reqs failed -ENOMEM
265			ops=N	Number of retr reqs submitted
266			owt=N	Number of retr reqs waited for CPU time
267			abt=N	Number of retr reqs aborted due to object death
268		Stores	n=N	Number of storage (write) requests seen
269			ok=N	Number of successful store reqs
270			agn=N	Number of store reqs on a page already pending storage
271			nbf=N	Number of store reqs rejected -ENOBUFS
272			oom=N	Number of store reqs failed -ENOMEM
273			ops=N	Number of store reqs submitted
274			run=N	Number of store reqs granted CPU time
275			pgs=N	Number of pages given store req processing time
276			rxd=N	Number of store reqs deleted from tracking tree
277			olm=N	Number of store reqs over store limit
278		VmScan	nos=N	Number of release reqs against pages with no pending store
279			gon=N	Number of release reqs against pages stored by time lock granted
280			bsy=N	Number of release reqs ignored due to in-progress store
281			can=N	Number of page stores cancelled due to release req
282		Ops	pend=N	Number of times async ops added to pending queues
283			run=N	Number of times async ops given CPU time
284			enq=N	Number of times async ops queued for processing
285			can=N	Number of async ops cancelled
286			rej=N	Number of async ops rejected due to object lookup/create failure
287			dfr=N	Number of async ops queued for deferred release
288			rel=N	Number of async ops released
289			gc=N	Number of deferred-release async ops garbage collected
290		CacheOp	alo=N	Number of in-progress alloc_object() cache ops
291			luo=N	Number of in-progress lookup_object() cache ops
292			luc=N	Number of in-progress lookup_complete() cache ops
293			gro=N	Number of in-progress grab_object() cache ops
294			upo=N	Number of in-progress update_object() cache ops
295			dro=N	Number of in-progress drop_object() cache ops
296			pto=N	Number of in-progress put_object() cache ops
297			syn=N	Number of in-progress sync_cache() cache ops
298			atc=N	Number of in-progress attr_changed() cache ops
299			rap=N	Number of in-progress read_or_alloc_page() cache ops
300			ras=N	Number of in-progress read_or_alloc_pages() cache ops
301			alp=N	Number of in-progress allocate_page() cache ops
302			als=N	Number of in-progress allocate_pages() cache ops
303			wrp=N	Number of in-progress write_page() cache ops
304			ucp=N	Number of in-progress uncache_page() cache ops
305			dsp=N	Number of in-progress dissociate_pages() cache ops
306	
307	
308	 (*) /proc/fs/fscache/histogram
309	
310		cat /proc/fs/fscache/histogram
311		JIFS  SECS  OBJ INST  OP RUNS   OBJ RUNS  RETRV DLY RETRIEVLS
312		===== ===== ========= ========= ========= ========= =========
313	
314	     This shows the breakdown of the number of times each amount of time
315	     between 0 jiffies and HZ-1 jiffies a variety of tasks took to run.  The
316	     columns are as follows:
317	
318		COLUMN		TIME MEASUREMENT
319		=======		=======================================================
320		OBJ INST	Length of time to instantiate an object
321		OP RUNS		Length of time a call to process an operation took
322		OBJ RUNS	Length of time a call to process an object event took
323		RETRV DLY	Time between an requesting a read and lookup completing
324		RETRIEVLS	Time between beginning and end of a retrieval
325	
326	     Each row shows the number of events that took a particular range of times.
327	     Each step is 1 jiffy in size.  The JIFS column indicates the particular
328	     jiffy range covered, and the SECS field the equivalent number of seconds.
329	
330	
331	===========
332	OBJECT LIST
333	===========
334	
335	If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a
336	list of all the objects currently allocated and allow them to be viewed
337	through:
338	
339		/proc/fs/fscache/objects
340	
341	This will look something like:
342	
343		[root@andromeda ~]# head /proc/fs/fscache/objects
344		OBJECT   PARENT   STAT CHLDN OPS OOP IPR EX READS EM EV F S | NETFS_COOKIE_DEF TY FL NETFS_DATA       OBJECT_KEY, AUX_DATA
345		======== ======== ==== ===== === === === == ===== == == = = | ================ == == ================ ================
346		   17e4b        2 ACTV     0   0   0   0  0     0 7b  4 0 0 | NFS.fh           DT  0 ffff88001dd82820 010006017edcf8bbc93b43298fdfbe71e50b57b13a172c0117f38472, e567634700000000000000000000000063f2404a000000000000000000000000c9030000000000000000000063f2404a
347		   1693a        2 ACTV     0   0   0   0  0     0 7b  4 0 0 | NFS.fh           DT  0 ffff88002db23380 010006017edcf8bbc93b43298fdfbe71e50b57b1e0162c01a2df0ea6, 420ebc4a000000000000000000000000420ebc4a0000000000000000000000000e1801000000000000000000420ebc4a
348	
349	where the first set of columns before the '|' describe the object:
350	
351		COLUMN	DESCRIPTION
352		=======	===============================================================
353		OBJECT	Object debugging ID (appears as OBJ%x in some debug messages)
354		PARENT	Debugging ID of parent object
355		STAT	Object state
356		CHLDN	Number of child objects of this object
357		OPS	Number of outstanding operations on this object
358		OOP	Number of outstanding child object management operations
359		IPR
360		EX	Number of outstanding exclusive operations
361		READS	Number of outstanding read operations
362		EM	Object's event mask
363		EV	Events raised on this object
364		F	Object flags
365		S	Object work item busy state mask (1:pending 2:running)
366	
367	and the second set of columns describe the object's cookie, if present:
368	
369		COLUMN		DESCRIPTION
370		===============	=======================================================
371		NETFS_COOKIE_DEF Name of netfs cookie definition
372		TY		Cookie type (IX - index, DT - data, hex - special)
373		FL		Cookie flags
374		NETFS_DATA	Netfs private data stored in the cookie
375		OBJECT_KEY	Object key	} 1 column, with separating comma
376		AUX_DATA	Object aux data	} presence may be configured
377	
378	The data shown may be filtered by attaching the a key to an appropriate keyring
379	before viewing the file.  Something like:
380	
381			keyctl add user fscache:objlist <restrictions> @s
382	
383	where <restrictions> are a selection of the following letters:
384	
385		K	Show hexdump of object key (don't show if not given)
386		A	Show hexdump of object aux data (don't show if not given)
387	
388	and the following paired letters:
389	
390		C	Show objects that have a cookie
391		c	Show objects that don't have a cookie
392		B	Show objects that are busy
393		b	Show objects that aren't busy
394		W	Show objects that have pending writes
395		w	Show objects that don't have pending writes
396		R	Show objects that have outstanding reads
397		r	Show objects that don't have outstanding reads
398		S	Show objects that have work queued
399		s	Show objects that don't have work queued
400	
401	If neither side of a letter pair is given, then both are implied.  For example:
402	
403		keyctl add user fscache:objlist KB @s
404	
405	shows objects that are busy, and lists their object keys, but does not dump
406	their auxiliary data.  It also implies "CcWwRrSs", but as 'B' is given, 'b' is
407	not implied.
408	
409	By default all objects and all fields will be shown.
410	
411	
412	=========
413	DEBUGGING
414	=========
415	
416	If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime
417	debugging enabled by adjusting the value in:
418	
419		/sys/module/fscache/parameters/debug
420	
421	This is a bitmask of debugging streams to enable:
422	
423		BIT	VALUE	STREAM				POINT
424		=======	=======	===============================	=======================
425		0	1	Cache management		Function entry trace
426		1	2					Function exit trace
427		2	4					General
428		3	8	Cookie management		Function entry trace
429		4	16					Function exit trace
430		5	32					General
431		6	64	Page handling			Function entry trace
432		7	128					Function exit trace
433		8	256					General
434		9	512	Operation management		Function entry trace
435		10	1024					Function exit trace
436		11	2048					General
437	
438	The appropriate set of values should be OR'd together and the result written to
439	the control file.  For example:
440	
441		echo $((1|8|64)) >/sys/module/fscache/parameters/debug
442	
443	will turn on all function entry debugging.
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