Documentation/filesystems/caching/fscache.txt

   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                 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 ===========
 337 OBJECT LIST
 338 ===========
 339
 340 If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a
 341 list of all the objects currently allocated and allow them to be viewed
 342 through:
 343
 344         /proc/fs/fscache/objects
 345
 346 This 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
 354 where 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
 372 and 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
 383 The data shown may be filtered by attaching the a key to an appropriate keyring
 384 before viewing the file.  Something like:
 385
 386                 keyctl add user fscache:objlist <restrictions> @s
 387
 388 where <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
 393 and 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
 406 If neither side of a letter pair is given, then both are implied.  For example:
 407
 408         keyctl add user fscache:objlist KB @s
 409
 410 shows objects that are busy, and lists their object keys, but does not dump
 411 their auxiliary data.  It also implies "CcWwRrSs", but as 'B' is given, 'b' is
 412 not implied.
 413
 414 By default all objects and all fields will be shown.
 415
 416
 417 =========
 418 DEBUGGING
 419 =========
 420
 421 If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime
 422 debugging enabled by adjusting the value in:
 423
 424         /sys/module/fscache/parameters/debug
 425
 426 This 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
 443 The appropriate set of values should be OR'd together and the result written to
 444 the control file.  For example:
 445
 446         echo $((1|8|64)) >/sys/module/fscache/parameters/debug
 447
 448 will turn on all function entry debugging.