1 ==========================
2 General Filesystem Caching
3 ==========================
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.
12 FS-Cache mediates between cache backends (such as CacheFS) and network
19 +---------+ | +----------+ | | /dev/hda5 |
20 | | | | +--------------+
23 | AFS |----->| FS-Cache |
26 | | | | +--------------+
27 +---------+ | +----------+ | | |
28 | | | +-->| CacheFiles |
29 | ISOFS |--+ | /var/cache |
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
43 ~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
49 | NFS |----->| FS-Cache |
51 +---------+ | | | +--------------+ +--------------+
53 V +----------+ +-->| CacheFiles |-->| Ext3 |
54 +---------+ | /var/cache | | /dev/sda6 |
55 | | +--------------+ +--------------+
58 +---------+ +--------------+ |
60 ~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
63 +---------+ +--------------+
65 | Process | | cachefilesd |
67 +---------+ +--------------+
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:
74 (1) It must be practical to operate without a cache.
76 (2) The size of any accessible file must not be limited to the size of the
79 (3) The combined size of all opened files (this includes mapped libraries)
80 must not be limited to the size of the cache.
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
86 It instead serves the cache out in PAGE_SIZE chunks as and when requested by
87 the netfs('s) using it.
90 FS-Cache provides the following facilities:
92 (1) More than one cache can be used at once. Caches can be selected
93 explicitly by use of tags.
95 (2) Caches can be added / removed at any time.
97 (3) The netfs is provided with an interface that allows either party to
98 withdraw caching facilities from a file (required for (2)).
100 (4) The interface to the netfs returns as few errors as possible, preferring
101 rather to let the netfs remain oblivious.
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
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
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
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
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.
127 (10) As much as possible is done asynchronously.
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
136 +------------------------------------+
140 +--------------------------+ +-----------+
142 homedir mirror afs.org redhat.com
144 +------------+ +---------------+ +----------+
146 00001 00002 00007 00125 vol00001 vol00002
148 +---+---+ +-----+ +---+ +------+------+ +-----+----+
149 | | | | | | | | | | | | |
150 PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak
160 In the example above, you can see two netfs's being backed: NFS and AFS. These
161 have different index hierarchies:
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.
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
175 The very top index is the FS-Cache master index in which individual netfs's
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
183 The netfs API to FS-Cache can be found in:
185 Documentation/filesystems/caching/netfs-api.txt
187 The cache backend API to FS-Cache can be found in:
189 Documentation/filesystems/caching/backend-api.txt
191 A description of the internal representations and object state machine can be
194 Documentation/filesystems/caching/object.txt
197 =======================
198 STATISTICAL INFORMATION
199 =======================
201 If FS-Cache is compiled with the following options enabled:
203 CONFIG_FSCACHE_STATS=y
204 CONFIG_FSCACHE_HISTOGRAM=y
206 then it will gather certain statistics and display them through a number of
209 (*) /proc/fs/fscache/stats
211 This shows counts of a number of events that can happen in FS-Cache:
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 Updates n=N Number of update cookie requests seen
239 nul=N Number of upd reqs given a NULL parent
240 run=N Number of upd reqs granted CPU time
241 Relinqs n=N Number of relinquish cookie requests seen
242 nul=N Number of rlq reqs given a NULL parent
243 wcr=N Number of rlq reqs waited on completion of creation
244 AttrChg n=N Number of attribute changed requests seen
245 ok=N Number of attr changed requests queued
246 nbf=N Number of attr changed rejected -ENOBUFS
247 oom=N Number of attr changed failed -ENOMEM
248 run=N Number of attr changed ops given CPU time
249 Allocs n=N Number of allocation requests seen
250 ok=N Number of successful alloc reqs
251 wt=N Number of alloc reqs that waited on lookup completion
252 nbf=N Number of alloc reqs rejected -ENOBUFS
253 int=N Number of alloc reqs aborted -ERESTARTSYS
254 ops=N Number of alloc reqs submitted
255 owt=N Number of alloc reqs waited for CPU time
256 Retrvls n=N Number of retrieval (read) requests seen
257 ok=N Number of successful retr reqs
258 wt=N Number of retr reqs that waited on lookup completion
259 nod=N Number of retr reqs returned -ENODATA
260 nbf=N Number of retr reqs rejected -ENOBUFS
261 int=N Number of retr reqs aborted -ERESTARTSYS
262 oom=N Number of retr reqs failed -ENOMEM
263 ops=N Number of retr reqs submitted
264 owt=N Number of retr reqs waited for CPU time
265 Stores n=N Number of storage (write) requests seen
266 ok=N Number of successful store reqs
267 agn=N Number of store reqs on a page already pending storage
268 nbf=N Number of store reqs rejected -ENOBUFS
269 oom=N Number of store reqs failed -ENOMEM
270 ops=N Number of store reqs submitted
271 run=N Number of store reqs granted CPU time
272 Ops pend=N Number of times async ops added to pending queues
273 run=N Number of times async ops given CPU time
274 enq=N Number of times async ops queued for processing
275 can=N Number of async ops cancelled
276 dfr=N Number of async ops queued for deferred release
277 rel=N Number of async ops released
278 gc=N Number of deferred-release async ops garbage collected
279 CacheOp alo=N Number of in-progress alloc_object() cache ops
280 luo=N Number of in-progress lookup_object() cache ops
281 luc=N Number of in-progress lookup_complete() cache ops
282 gro=N Number of in-progress grab_object() cache ops
283 upo=N Number of in-progress update_object() cache ops
284 dro=N Number of in-progress drop_object() cache ops
285 pto=N Number of in-progress put_object() cache ops
286 syn=N Number of in-progress sync_cache() cache ops
287 atc=N Number of in-progress attr_changed() cache ops
288 rap=N Number of in-progress read_or_alloc_page() cache ops
289 ras=N Number of in-progress read_or_alloc_pages() cache ops
290 alp=N Number of in-progress allocate_page() cache ops
291 als=N Number of in-progress allocate_pages() cache ops
292 wrp=N Number of in-progress write_page() cache ops
293 ucp=N Number of in-progress uncache_page() cache ops
294 dsp=N Number of in-progress dissociate_pages() cache ops
297 (*) /proc/fs/fscache/histogram
299 cat /proc/fs/fscache/histogram
300 JIFS SECS OBJ INST OP RUNS OBJ RUNS RETRV DLY RETRIEVLS
301 ===== ===== ========= ========= ========= ========= =========
303 This shows the breakdown of the number of times each amount of time
304 between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The
305 columns are as follows:
307 COLUMN TIME MEASUREMENT
308 ======= =======================================================
309 OBJ INST Length of time to instantiate an object
310 OP RUNS Length of time a call to process an operation took
311 OBJ RUNS Length of time a call to process an object event took
312 RETRV DLY Time between an requesting a read and lookup completing
313 RETRIEVLS Time between beginning and end of a retrieval
315 Each row shows the number of events that took a particular range of times.
316 Each step is 1 jiffy in size. The JIFS column indicates the particular
317 jiffy range covered, and the SECS field the equivalent number of seconds.
324 If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a
325 list of all the objects currently allocated and allow them to be viewed
328 /proc/fs/fscache/objects
330 This will look something like:
332 [root@andromeda ~]# head /proc/fs/fscache/objects
333 OBJECT PARENT STAT CHLDN OPS OOP IPR EX READS EM EV F S | NETFS_COOKIE_DEF TY FL NETFS_DATA OBJECT_KEY, AUX_DATA
334 ======== ======== ==== ===== === === === == ===== == == = = | ================ == == ================ ================
335 17e4b 2 ACTV 0 0 0 0 0 0 7b 4 0 8 | NFS.fh DT 0 ffff88001dd82820 010006017edcf8bbc93b43298fdfbe71e50b57b13a172c0117f38472, e567634700000000000000000000000063f2404a000000000000000000000000c9030000000000000000000063f2404a
336 1693a 2 ACTV 0 0 0 0 0 0 7b 4 0 8 | NFS.fh DT 0 ffff88002db23380 010006017edcf8bbc93b43298fdfbe71e50b57b1e0162c01a2df0ea6, 420ebc4a000000000000000000000000420ebc4a0000000000000000000000000e1801000000000000000000420ebc4a
338 where the first set of columns before the '|' describe the object:
341 ======= ===============================================================
342 OBJECT Object debugging ID (appears as OBJ%x in some debug messages)
343 PARENT Debugging ID of parent object
345 CHLDN Number of child objects of this object
346 OPS Number of outstanding operations on this object
347 OOP Number of outstanding child object management operations
349 EX Number of outstanding exclusive operations
350 READS Number of outstanding read operations
351 EM Object's event mask
352 EV Events raised on this object
354 S Object slow-work work item flags
356 and the second set of columns describe the object's cookie, if present:
359 =============== =======================================================
360 NETFS_COOKIE_DEF Name of netfs cookie definition
361 TY Cookie type (IX - index, DT - data, hex - special)
363 NETFS_DATA Netfs private data stored in the cookie
364 OBJECT_KEY Object key } 1 column, with separating comma
365 AUX_DATA Object aux data } presence may be configured
367 The data shown may be filtered by attaching the a key to an appropriate keyring
368 before viewing the file. Something like:
370 keyctl add user fscache:objlist <restrictions> @s
372 where <restrictions> are a selection of the following letters:
374 K Show hexdump of object key (don't show if not given)
375 A Show hexdump of object aux data (don't show if not given)
377 and the following paired letters:
379 C Show objects that have a cookie
380 c Show objects that don't have a cookie
381 B Show objects that are busy
382 b Show objects that aren't busy
383 W Show objects that have pending writes
384 w Show objects that don't have pending writes
385 R Show objects that have outstanding reads
386 r Show objects that don't have outstanding reads
387 S Show objects that have slow work queued
388 s Show objects that don't have slow work queued
390 If neither side of a letter pair is given, then both are implied. For example:
392 keyctl add user fscache:objlist KB @s
394 shows objects that are busy, and lists their object keys, but does not dump
395 their auxiliary data. It also implies "CcWwRrSs", but as 'B' is given, 'b' is
398 By default all objects and all fields will be shown.
405 If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime
406 debugging enabled by adjusting the value in:
408 /sys/module/fscache/parameters/debug
410 This is a bitmask of debugging streams to enable:
412 BIT VALUE STREAM POINT
413 ======= ======= =============================== =======================
414 0 1 Cache management Function entry trace
415 1 2 Function exit trace
417 3 8 Cookie management Function entry trace
418 4 16 Function exit trace
420 6 64 Page handling Function entry trace
421 7 128 Function exit trace
423 9 512 Operation management Function entry trace
424 10 1024 Function exit trace
427 The appropriate set of values should be OR'd together and the result written to
428 the control file. For example:
430 echo $((1|8|64)) >/sys/module/fscache/parameters/debug
432 will turn on all function entry debugging.