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         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                 ops=N   Number of alloc reqs submitted
 254                 owt=N   Number of alloc reqs waited for CPU time
 255         Retrvls n=N     Number of retrieval (read) requests seen
 256                 ok=N    Number of successful retr reqs
 257                 wt=N    Number of retr reqs that waited on lookup completion
 258                 nod=N   Number of retr reqs returned -ENODATA
 259                 nbf=N   Number of retr reqs rejected -ENOBUFS
 260                 int=N   Number of retr reqs aborted -ERESTARTSYS
 261                 oom=N   Number of retr reqs failed -ENOMEM
 262                 ops=N   Number of retr reqs submitted
 263                 owt=N   Number of retr reqs waited for CPU time
 264         Stores  n=N     Number of storage (write) requests seen
 265                 ok=N    Number of successful store reqs
 266                 agn=N   Number of store reqs on a page already pending storage
 267                 nbf=N   Number of store reqs rejected -ENOBUFS
 268                 oom=N   Number of store reqs failed -ENOMEM
 269                 ops=N   Number of store reqs submitted
 270                 run=N   Number of store reqs granted CPU time
 271         Ops     pend=N  Number of times async ops added to pending queues
 272                 run=N   Number of times async ops given CPU time
 273                 enq=N   Number of times async ops queued for processing
 274                 dfr=N   Number of async ops queued for deferred release
 275                 rel=N   Number of async ops released
 276                 gc=N    Number of deferred-release async ops garbage collected
 277
 278
 279  (*) /proc/fs/fscache/histogram
 280
 281         cat /proc/fs/fscache/histogram
 282         JIFS  SECS  OBJ INST  OP RUNS   OBJ RUNS  RETRV DLY RETRIEVLS
 283         ===== ===== ========= ========= ========= ========= =========
 284
 285      This shows the breakdown of the number of times each amount of time
 286      between 0 jiffies and HZ-1 jiffies a variety of tasks took to run.  The
 287      columns are as follows:
 288
 289         COLUMN          TIME MEASUREMENT
 290         =======         =======================================================
 291         OBJ INST        Length of time to instantiate an object
 292         OP RUNS         Length of time a call to process an operation took
 293         OBJ RUNS        Length of time a call to process an object event took
 294         RETRV DLY       Time between an requesting a read and lookup completing
 295         RETRIEVLS       Time between beginning and end of a retrieval
 296
 297      Each row shows the number of events that took a particular range of times.
 298      Each step is 1 jiffy in size.  The JIFS column indicates the particular
 299      jiffy range covered, and the SECS field the equivalent number of seconds.
 300
 301
 302 =========
 303 DEBUGGING
 304 =========
 305
 306 If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime
 307 debugging enabled by adjusting the value in:
 308
 309         /sys/module/fscache/parameters/debug
 310
 311 This is a bitmask of debugging streams to enable:
 312
 313         BIT     VALUE   STREAM                          POINT
 314         ======= ======= =============================== =======================
 315         0       1       Cache management                Function entry trace
 316         1       2                                       Function exit trace
 317         2       4                                       General
 318         3       8       Cookie management               Function entry trace
 319         4       16                                      Function exit trace
 320         5       32                                      General
 321         6       64      Page handling                   Function entry trace
 322         7       128                                     Function exit trace
 323         8       256                                     General
 324         9       512     Operation management            Function entry trace
 325         10      1024                                    Function exit trace
 326         11      2048                                    General
 327
 328 The appropriate set of values should be OR'd together and the result written to
 329 the control file.  For example:
 330
 331         echo $((1|8|64)) >/sys/module/fscache/parameters/debug
 332
 333 will turn on all function entry debugging.