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 191A description of the internal representations and object state machine can be 192found in: 193 194 Documentation/filesystems/caching/object.txt 195 196 197======================= 198STATISTICAL INFORMATION 199======================= 200 201If FS-Cache is compiled with the following options enabled: 202 203 CONFIG_FSCACHE_STATS=y 204 CONFIG_FSCACHE_HISTOGRAM=y 205 206then it will gather certain statistics and display them through a number of 207proc 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=========== 332OBJECT LIST 333=========== 334 335If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a 336list of all the objects currently allocated and allow them to be viewed 337through: 338 339 /proc/fs/fscache/objects 340 341This 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 349where 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 367and 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 378The data shown may be filtered by attaching the a key to an appropriate keyring 379before viewing the file. Something like: 380 381 keyctl add user fscache:objlist <restrictions> @s 382 383where <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 388and 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 401If neither side of a letter pair is given, then both are implied. For example: 402 403 keyctl add user fscache:objlist KB @s 404 405shows objects that are busy, and lists their object keys, but does not dump 406their auxiliary data. It also implies "CcWwRrSs", but as 'B' is given, 'b' is 407not implied. 408 409By default all objects and all fields will be shown. 410 411 412========= 413DEBUGGING 414========= 415 416If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime 417debugging enabled by adjusting the value in: 418 419 /sys/module/fscache/parameters/debug 420 421This 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 438The appropriate set of values should be OR'd together and the result written to 439the control file. For example: 440 441 echo $((1|8|64)) >/sys/module/fscache/parameters/debug 442 443will turn on all function entry debugging. 444