General Filesystem Caching

Overview

This facility is a general purpose cache for network filesystems, though it could be used for caching other things such as ISO9660 filesystems too.

FS-Cache mediates between cache backends (such as CacheFiles) and network filesystems:

+---------+
|         |                                    +--------------+
|   NFS   |--+                                 |              |
|         |  |                             +-->|   CacheFS    |
+---------+  |               +----------+  |   |  /dev/hda5   |
             |               |          |  |   +--------------+
+---------+  +-------------->|          |  |
|         |      +-------+   |          |--+
|   AFS   |----->|       |   | FS-Cache |
|         |      | netfs |-->|          |--+
+---------+  +-->|  lib  |   |          |  |
             |   |       |   |          |  |   +--------------+
+---------+  |   +-------+   +----------+  |   |              |
|         |  |                             +-->|  CacheFiles  |
|   9P    |--+                                 |  /var/cache  |
|         |                                    +--------------+
+---------+

Or to look at it another way, FS-Cache is a module that provides a caching facility to a network filesystem such that the cache is transparent to the user:

+---------+
|         |
| Server  |
|         |
+---------+
     |                  NETWORK
~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
     |
     |           +----------+
     V           |          |
+---------+      |          |
|         |      |          |
|   NFS   |----->| FS-Cache |
|         |      |          |--+
+---------+      |          |  |   +--------------+   +--------------+
     |           |          |  |   |              |   |              |
     V           +----------+  +-->|  CacheFiles  |-->|  Ext3        |
+---------+                        |  /var/cache  |   |  /dev/sda6   |
|         |                        +--------------+   +--------------+
|   VFS   |                                ^                     ^
|         |                                |                     |
+---------+                                +--------------+      |
     |                  KERNEL SPACE                      |      |
~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
     |                  USER SPACE                        |      |
     V                                                    |      |
+---------+                                           +--------------+
|         |                                           |              |
| Process |                                           | cachefilesd  |
|         |                                           |              |
+---------+                                           +--------------+

FS-Cache does not follow the idea of completely loading every netfs file opened in its entirety into a cache before permitting it to be accessed and then serving the pages out of that cache rather than the netfs inode because:

  1. It must be practical to operate without a cache.
  2. The size of any accessible file must not be limited to the size of the cache.
  3. The combined size of all opened files (this includes mapped libraries) must not be limited to the size of the cache.
  4. The user should not be forced to download an entire file just to do a one-off access of a small portion of it (such as might be done with the “file” program).

It instead serves the cache out in chunks as and when requested by the netfs using it.

FS-Cache provides the following facilities:

  • More than one cache can be used at once. Caches can be selected explicitly by use of tags.

  • Caches can be added / removed at any time, even whilst being accessed.

  • The netfs is provided with an interface that allows either party to withdraw caching facilities from a file (required for (2)).

  • The interface to the netfs returns as few errors as possible, preferring rather to let the netfs remain oblivious.

  • There are three types of cookie: cache, volume and data file cookies. Cache cookies represent the cache as a whole and are not normally visible to the netfs; the netfs gets a volume cookie to represent a collection of files (typically something that a netfs would get for a superblock); and data file cookies are used to cache data (something that would be got for an inode).

  • Volumes are matched using a key. This is a printable string that is used to encode all the information that might be needed to distinguish one superblock, say, from another. This would be a compound of things like cell name or server address, volume name or share path. It must be a valid pathname.

  • Cookies are matched using a key. This is a binary blob and is used to represent the object within a volume (so the volume key need not form part of the blob). This might include things like an inode number and uniquifier or a file handle.

  • Cookie resources are set up and pinned by marking the cookie in-use. This prevents the backing resources from being culled. Timed garbage collection is employed to eliminate cookies that haven’t been used for a short while, thereby reducing resource overload. This is intended to be used when a file is opened or closed.

    A cookie can be marked in-use multiple times simultaneously; each mark must be unused.

  • Begin/end access functions are provided to delay cache withdrawal for the duration of an operation and prevent structs from being freed whilst we’re looking at them.

  • Data I/O is done by asynchronous DIO to/from a buffer described by the netfs using an iov_iter.

  • An invalidation facility is available to discard data from the cache and to deal with I/O that’s in progress that is accessing old data.

  • Cookies can be “retired” upon release, thereby causing the object to be removed from the cache.

The netfs API to FS-Cache can be found in:

The cache backend API to FS-Cache can be found in:

Statistical Information

If FS-Cache is compiled with the following options enabled:

CONFIG_FSCACHE_STATS=y

then it will gather certain statistics and display them through:

/proc/fs/fscache/stats

This shows counts of a number of events that can happen in FS-Cache:

CLASS EVENT MEANING
Cookies n=N Number of data storage cookies allocated
v=N Number of volume index cookies allocated
vcol=N Number of volume index key collisions
voom=N Number of OOM events when allocating volume cookies
Acquire n=N Number of acquire cookie requests seen
ok=N Number of acq reqs succeeded
oom=N Number of acq reqs failed on ENOMEM
LRU n=N Number of cookies currently on the LRU
exp=N Number of cookies expired off of the LRU
rmv=N Number of cookies removed from the LRU
drp=N Number of LRU’d cookies relinquished/withdrawn
at=N Time till next LRU cull (jiffies)
Invals n=N Number of invalidations
Updates n=N Number of update cookie requests seen
rsz=N Number of resize requests
rsn=N Number of skipped resize requests
Relinqs n=N Number of relinquish cookie requests seen
rtr=N Number of rlq reqs with retire=true
drop=N Number of cookies no longer blocking re-acquisition
NoSpace nwr=N Number of write requests refused due to lack of space
ncr=N Number of create requests refused due to lack of space
cull=N Number of objects culled to make space
IO rd=N Number of read operations in the cache
wr=N Number of write operations in the cache

Netfslib will also add some stats counters of its own.

Cache List

FS-Cache provides a list of cache cookies:

/proc/fs/fscache/cookies

This will look something like:

# cat /proc/fs/fscache/caches
CACHE    REF   VOLS  OBJS  ACCES S NAME
======== ===== ===== ===== ===== = ===============
00000001     2     1  2123     1 A default

where the columns are:

COLUMN DESCRIPTION
CACHE Cache cookie debug ID (also appears in traces)
REF Number of references on the cache cookie
VOLS Number of volumes cookies in this cache
OBJS Number of cache objects in use
ACCES Number of accesses pinning the cache
S State
NAME Name of the cache.

The state can be (-) Inactive, (P)reparing, (A)ctive, (E)rror or (W)ithdrawing.

Volume List

FS-Cache provides a list of volume cookies:

/proc/fs/fscache/volumes

This will look something like:

VOLUME   REF   nCOOK ACC FL CACHE           KEY
======== ===== ===== === == =============== ================
00000001    55    54   1 00 default         afs,example.com,100058

where the columns are:

COLUMN DESCRIPTION
VOLUME The volume cookie debug ID (also appears in traces)
REF Number of references on the volume cookie
nCOOK Number of cookies in the volume
ACC Number of accesses pinning the cache
FL Flags on the volume cookie
CACHE Name of the cache or “-“
KEY The indexing key for the volume

Debugging

If CONFIG_FSCACHE_DEBUG is enabled, the FS-Cache facility can have runtime debugging enabled by adjusting the value in:

/sys/module/fscache/parameters/debug

This is a bitmask of debugging streams to enable:

BIT VALUE STREAM POINT
0 1 Cache management Function entry trace
1 2   Function exit trace
2 4   General
3 8 Cookie management Function entry trace
4 16   Function exit trace
5 32   General
6-8     (Not used)
9 512 I/O operation management Function entry trace
10 1024   Function exit trace
11 2048   General

The appropriate set of values should be OR’d together and the result written to the control file. For example:

echo $((1|8|512)) >/sys/module/fscache/parameters/debug

will turn on all function entry debugging.