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 ini=N Number of async ops initialised 288 dfr=N Number of async ops queued for deferred release 289 rel=N Number of async ops released (should equal ini=N when idle) 290 gc=N Number of deferred-release async ops garbage collected 291 CacheOp alo=N Number of in-progress alloc_object() cache ops 292 luo=N Number of in-progress lookup_object() cache ops 293 luc=N Number of in-progress lookup_complete() cache ops 294 gro=N Number of in-progress grab_object() cache ops 295 upo=N Number of in-progress update_object() cache ops 296 dro=N Number of in-progress drop_object() cache ops 297 pto=N Number of in-progress put_object() cache ops 298 syn=N Number of in-progress sync_cache() cache ops 299 atc=N Number of in-progress attr_changed() cache ops 300 rap=N Number of in-progress read_or_alloc_page() cache ops 301 ras=N Number of in-progress read_or_alloc_pages() cache ops 302 alp=N Number of in-progress allocate_page() cache ops 303 als=N Number of in-progress allocate_pages() cache ops 304 wrp=N Number of in-progress write_page() cache ops 305 ucp=N Number of in-progress uncache_page() cache ops 306 dsp=N Number of in-progress dissociate_pages() cache ops 307 CacheEv nsp=N Number of object lookups/creations rejected due to lack of space 308 stl=N Number of stale objects deleted 309 rtr=N Number of objects retired when relinquished 310 cul=N Number of objects culled 311 312 313 (*) /proc/fs/fscache/histogram 314 315 cat /proc/fs/fscache/histogram 316 JIFS SECS OBJ INST OP RUNS OBJ RUNS RETRV DLY RETRIEVLS 317 ===== ===== ========= ========= ========= ========= ========= 318 319 This shows the breakdown of the number of times each amount of time 320 between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The 321 columns are as follows: 322 323 COLUMN TIME MEASUREMENT 324 ======= ======================================================= 325 OBJ INST Length of time to instantiate an object 326 OP RUNS Length of time a call to process an operation took 327 OBJ RUNS Length of time a call to process an object event took 328 RETRV DLY Time between an requesting a read and lookup completing 329 RETRIEVLS Time between beginning and end of a retrieval 330 331 Each row shows the number of events that took a particular range of times. 332 Each step is 1 jiffy in size. The JIFS column indicates the particular 333 jiffy range covered, and the SECS field the equivalent number of seconds. 334 335 336=========== 337OBJECT LIST 338=========== 339 340If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a 341list of all the objects currently allocated and allow them to be viewed 342through: 343 344 /proc/fs/fscache/objects 345 346This will look something like: 347 348 [root@andromeda ~]# head /proc/fs/fscache/objects 349 OBJECT PARENT STAT CHLDN OPS OOP IPR EX READS EM EV F S | NETFS_COOKIE_DEF TY FL NETFS_DATA OBJECT_KEY, AUX_DATA 350 ======== ======== ==== ===== === === === == ===== == == = = | ================ == == ================ ================ 351 17e4b 2 ACTV 0 0 0 0 0 0 7b 4 0 0 | NFS.fh DT 0 ffff88001dd82820 010006017edcf8bbc93b43298fdfbe71e50b57b13a172c0117f38472, e567634700000000000000000000000063f2404a000000000000000000000000c9030000000000000000000063f2404a 352 1693a 2 ACTV 0 0 0 0 0 0 7b 4 0 0 | NFS.fh DT 0 ffff88002db23380 010006017edcf8bbc93b43298fdfbe71e50b57b1e0162c01a2df0ea6, 420ebc4a000000000000000000000000420ebc4a0000000000000000000000000e1801000000000000000000420ebc4a 353 354where the first set of columns before the '|' describe the object: 355 356 COLUMN DESCRIPTION 357 ======= =============================================================== 358 OBJECT Object debugging ID (appears as OBJ%x in some debug messages) 359 PARENT Debugging ID of parent object 360 STAT Object state 361 CHLDN Number of child objects of this object 362 OPS Number of outstanding operations on this object 363 OOP Number of outstanding child object management operations 364 IPR 365 EX Number of outstanding exclusive operations 366 READS Number of outstanding read operations 367 EM Object's event mask 368 EV Events raised on this object 369 F Object flags 370 S Object work item busy state mask (1:pending 2:running) 371 372and the second set of columns describe the object's cookie, if present: 373 374 COLUMN DESCRIPTION 375 =============== ======================================================= 376 NETFS_COOKIE_DEF Name of netfs cookie definition 377 TY Cookie type (IX - index, DT - data, hex - special) 378 FL Cookie flags 379 NETFS_DATA Netfs private data stored in the cookie 380 OBJECT_KEY Object key } 1 column, with separating comma 381 AUX_DATA Object aux data } presence may be configured 382 383The data shown may be filtered by attaching the a key to an appropriate keyring 384before viewing the file. Something like: 385 386 keyctl add user fscache:objlist <restrictions> @s 387 388where <restrictions> are a selection of the following letters: 389 390 K Show hexdump of object key (don't show if not given) 391 A Show hexdump of object aux data (don't show if not given) 392 393and the following paired letters: 394 395 C Show objects that have a cookie 396 c Show objects that don't have a cookie 397 B Show objects that are busy 398 b Show objects that aren't busy 399 W Show objects that have pending writes 400 w Show objects that don't have pending writes 401 R Show objects that have outstanding reads 402 r Show objects that don't have outstanding reads 403 S Show objects that have work queued 404 s Show objects that don't have work queued 405 406If neither side of a letter pair is given, then both are implied. For example: 407 408 keyctl add user fscache:objlist KB @s 409 410shows objects that are busy, and lists their object keys, but does not dump 411their auxiliary data. It also implies "CcWwRrSs", but as 'B' is given, 'b' is 412not implied. 413 414By default all objects and all fields will be shown. 415 416 417========= 418DEBUGGING 419========= 420 421If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime 422debugging enabled by adjusting the value in: 423 424 /sys/module/fscache/parameters/debug 425 426This is a bitmask of debugging streams to enable: 427 428 BIT VALUE STREAM POINT 429 ======= ======= =============================== ======================= 430 0 1 Cache management Function entry trace 431 1 2 Function exit trace 432 2 4 General 433 3 8 Cookie management Function entry trace 434 4 16 Function exit trace 435 5 32 General 436 6 64 Page handling Function entry trace 437 7 128 Function exit trace 438 8 256 General 439 9 512 Operation management Function entry trace 440 10 1024 Function exit trace 441 11 2048 General 442 443The appropriate set of values should be OR'd together and the result written to 444the control file. For example: 445 446 echo $((1|8|64)) >/sys/module/fscache/parameters/debug 447 448will turn on all function entry debugging. 449