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gerrit-public.fairphone.software / kernel / msm-5.4 / 2d6fff637037395cc946ef910a880b5fa67b5370 / . / Documentation / filesystems / caching / fscache.txt
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David Howells2d6fff62009-04-03 16:42:36 +0100[diff] [blame^]1 ==========================
2 General Filesystem Caching
3 ==========================
4
5========
6OVERVIEW
7========
8
9This facility is a general purpose cache for network filesystems, though it
10could be used for caching other things such as ISO9660 filesystems too.
11
12FS-Cache mediates between cache backends (such as CacheFS) and network
13filesystems:
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
33Or to look at it another way, FS-Cache is a module that provides a caching
34facility to a network filesystem such that the cache is transparent to the
35user:
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
70FS-Cache does not follow the idea of completely loading every netfs file
71opened in its entirety into a cache before permitting it to be accessed and
72then 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
86It instead serves the cache out in PAGE_SIZE chunks as and when requested by
87the netfs('s) using it.
88
89
90FS-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
130FS-Cache maintains a virtual indexing tree in which all indices, files, objects
131and pages are kept. Bits of this tree may actually reside in one or more
132caches.
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 | | | | | | | | | | | | |
150PG0 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
160In the example above, you can see two netfs's being backed: NFS and AFS. These
161have 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
175The very top index is the FS-Cache master index in which individual netfs's
176have entries.
177
178Any index object may reside in more than one cache, provided it only has index
179children. Any index with non-index object children will be assumed to only
180reside in one cache.
181
182
183The netfs API to FS-Cache can be found in:
184
185 Documentation/filesystems/caching/netfs-api.txt
186
187The cache backend API to FS-Cache can be found in:
188
189 Documentation/filesystems/caching/backend-api.txt
190
191
192=======================
193STATISTICAL INFORMATION
194=======================
195
196If FS-Cache is compiled with the following options enabled:
197
198 CONFIG_FSCACHE_PROC=y (implied by the following two)
199 CONFIG_FSCACHE_STATS=y
200 CONFIG_FSCACHE_HISTOGRAM=y
201
202then it will gather certain statistics and display them through a number of
203proc files.
204
205 (*) /proc/fs/fscache/stats
206
207 This shows counts of a number of events that can happen in FS-Cache:
208
209 CLASS EVENT MEANING
210 ======= ======= =======================================================
211 Cookies idx=N Number of index cookies allocated
212 dat=N Number of data storage cookies allocated
213 spc=N Number of special cookies allocated
214 Objects alc=N Number of objects allocated
215 nal=N Number of object allocation failures
216 avl=N Number of objects that reached the available state
217 ded=N Number of objects that reached the dead state
218 ChkAux non=N Number of objects that didn't have a coherency check
219 ok=N Number of objects that passed a coherency check
220 upd=N Number of objects that needed a coherency data update
221 obs=N Number of objects that were declared obsolete
222 Pages mrk=N Number of pages marked as being cached
223 unc=N Number of uncache page requests seen
224 Acquire n=N Number of acquire cookie requests seen
225 nul=N Number of acq reqs given a NULL parent
226 noc=N Number of acq reqs rejected due to no cache available
227 ok=N Number of acq reqs succeeded
228 nbf=N Number of acq reqs rejected due to error
229 oom=N Number of acq reqs failed on ENOMEM
230 Lookups n=N Number of lookup calls made on cache backends
231 neg=N Number of negative lookups made
232 pos=N Number of positive lookups made
233 crt=N Number of objects created by lookup
234 Updates n=N Number of update cookie requests seen
235 nul=N Number of upd reqs given a NULL parent
236 run=N Number of upd reqs granted CPU time
237 Relinqs n=N Number of relinquish cookie requests seen
238 nul=N Number of rlq reqs given a NULL parent
239 wcr=N Number of rlq reqs waited on completion of creation
240 AttrChg n=N Number of attribute changed requests seen
241 ok=N Number of attr changed requests queued
242 nbf=N Number of attr changed rejected -ENOBUFS
243 oom=N Number of attr changed failed -ENOMEM
244 run=N Number of attr changed ops given CPU time
245 Allocs n=N Number of allocation requests seen
246 ok=N Number of successful alloc reqs
247 wt=N Number of alloc reqs that waited on lookup completion
248 nbf=N Number of alloc reqs rejected -ENOBUFS
249 ops=N Number of alloc reqs submitted
250 owt=N Number of alloc reqs waited for CPU time
251 Retrvls n=N Number of retrieval (read) requests seen
252 ok=N Number of successful retr reqs
253 wt=N Number of retr reqs that waited on lookup completion
254 nod=N Number of retr reqs returned -ENODATA
255 nbf=N Number of retr reqs rejected -ENOBUFS
256 int=N Number of retr reqs aborted -ERESTARTSYS
257 oom=N Number of retr reqs failed -ENOMEM
258 ops=N Number of retr reqs submitted
259 owt=N Number of retr reqs waited for CPU time
260 Stores n=N Number of storage (write) requests seen
261 ok=N Number of successful store reqs
262 agn=N Number of store reqs on a page already pending storage
263 nbf=N Number of store reqs rejected -ENOBUFS
264 oom=N Number of store reqs failed -ENOMEM
265 ops=N Number of store reqs submitted
266 run=N Number of store reqs granted CPU time
267 Ops pend=N Number of times async ops added to pending queues
268 run=N Number of times async ops given CPU time
269 enq=N Number of times async ops queued for processing
270 dfr=N Number of async ops queued for deferred release
271 rel=N Number of async ops released
272 gc=N Number of deferred-release async ops garbage collected
273
274
275 (*) /proc/fs/fscache/histogram
276
277 cat /proc/fs/fscache/histogram
278 +HZ +TIME OBJ INST OP RUNS OBJ RUNS RETRV DLY RETRIEVLS
279 ===== ===== ========= ========= ========= ========= =========
280
281 This shows the breakdown of the number of times each amount of time
282 between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The
283 columns are as follows:
284
285 COLUMN TIME MEASUREMENT
286 ======= =======================================================
287 OBJ INST Length of time to instantiate an object
288 OP RUNS Length of time a call to process an operation took
289 OBJ RUNS Length of time a call to process an object event took
290 RETRV DLY Time between an requesting a read and lookup completing
291 RETRIEVLS Time between beginning and end of a retrieval
292
293 Each row shows the number of events that took a particular range of times.
294 Each step is 1 jiffy in size. The +HZ column indicates the particular
295 jiffy range covered, and the +TIME field the equivalent number of seconds.
296
297
298=========
299DEBUGGING
300=========
301
302The FS-Cache facility can have runtime debugging enabled by adjusting the value
303in:
304
305 /sys/module/fscache/parameters/debug
306
307This is a bitmask of debugging streams to enable:
308
309 BIT VALUE STREAM POINT
310 ======= ======= =============================== =======================
311 0 1 Cache management Function entry trace
312 1 2 Function exit trace
313 2 4 General
314 3 8 Cookie management Function entry trace
315 4 16 Function exit trace
316 5 32 General
317 6 64 Page handling Function entry trace
318 7 128 Function exit trace
319 8 256 General
320 9 512 Operation management Function entry trace
321 10 1024 Function exit trace
322 11 2048 General
323
324The appropriate set of values should be OR'd together and the result written to
325the control file. For example:
326
327 echo $((1|8|64)) >/sys/module/fscache/parameters/debug
328
329will turn on all function entry debugging.
330