General Filesystem Caching


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 CacheFS) and network filesystems:

|         |                        +--------------+
|   NFS   |--+                     |              |
|         |  |                 +-->|   CacheFS    |
+---------+  |   +----------+  |   |  /dev/hda5   |
             |   |          |  |   +--------------+
+---------+  +-->|          |  |
|         |      |          |--+
|   AFS   |----->| FS-Cache |
|         |      |          |--+
+---------+  +-->|          |  |
             |   |          |  |   +--------------+
+---------+  |   +----------+  |   |              |
|         |  |                 +-->|  CacheFiles  |
|  ISOFS  |--+                     |  /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 PAGE_SIZE chunks as and when requested by the netfs(‘s) using it.

FS-Cache provides the following facilities:

  1. More than one cache can be used at once. Caches can be selected explicitly by use of tags.
  2. Caches can be added / removed at any time.
  3. The netfs is provided with an interface that allows either party to withdraw caching facilities from a file (required for (2)).
  4. The interface to the netfs returns as few errors as possible, preferring rather to let the netfs remain oblivious.
  5. Cookies are used to represent indices, files and other objects to the netfs. The simplest cookie is just a NULL pointer - indicating nothing cached there.
  6. The netfs is allowed to propose - dynamically - any index hierarchy it desires, though it must be aware that the index search function is recursive, stack space is limited, and indices can only be children of indices.
  7. Data I/O is done direct to and from the netfs’s pages. The netfs indicates that page A is at index B of the data-file represented by cookie C, and that it should be read or written. The cache backend may or may not start I/O on that page, but if it does, a netfs callback will be invoked to indicate completion. The I/O may be either synchronous or asynchronous.
  8. Cookies can be “retired” upon release. At this point FS-Cache will mark them as obsolete and the index hierarchy rooted at that point will get recycled.
  9. The netfs provides a “match” function for index searches. In addition to saying whether a match was made or not, this can also specify that an entry should be updated or deleted.
  1. As much as possible is done asynchronously.

FS-Cache maintains a virtual indexing tree in which all indices, files, objects and pages are kept. Bits of this tree may actually reside in one or more caches:

                        |                                    |
                       NFS                                  AFS
                        |                                    |
           +--------------------------+                +-----------+
           |                          |                |           |
        homedir                     mirror
           |                          |                            |
     +------------+           +---------------+              +----------+
     |            |           |               |              |          |
   00001        00002       00007           00125        vol00001   vol00002
     |            |           |               |                         |
 +---+---+     +-----+      +---+      +------+------+            +-----+----+
 |   |   |     |     |      |   |      |      |      |            |     |    |
                     |                                            |
                    PG0                                       +-------+
                                                              |       |
                                                            00001   00003
                                                          |   |   |
                                                         PG0 PG1 PG2

In the example above, you can see two netfs’s being backed: NFS and AFS. These have different index hierarchies:

  • The NFS primary index contains per-server indices. Each server index is indexed by NFS file handles to get data file objects. Each data file objects can have an array of pages, but may also have further child objects, such as extended attributes and directory entries. Extended attribute objects themselves have page-array contents.
  • The AFS primary index contains per-cell indices. Each cell index contains per-logical-volume indices. Each of volume index contains up to three indices for the read-write, read-only and backup mirrors of those volumes. Each of these contains vnode data file objects, each of which contains an array of pages.

The very top index is the FS-Cache master index in which individual netfs’s have entries.

Any index object may reside in more than one cache, provided it only has index children. Any index with non-index object children will be assumed to only reside in one cache.

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

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

A description of the internal representations and object state machine can be found in:

Statistical Information

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


then it will gather certain statistics and display them through a number of proc files.


This shows counts of a number of events that can happen in FS-Cache:
Cookies idx=N Number of index cookies allocated
dat=N Number of data storage cookies allocated
spc=N Number of special cookies allocated
Objects alc=N Number of objects allocated
nal=N Number of object allocation failures
avl=N Number of objects that reached the available state
ded=N Number of objects that reached the dead state
ChkAux non=N Number of objects that didn’t have a coherency check
ok=N Number of objects that passed a coherency check
upd=N Number of objects that needed a coherency data update
obs=N Number of objects that were declared obsolete
Pages mrk=N unc=N Number of pages marked as being cached Number of uncache page requests seen
Acquire n=N Number of acquire cookie requests seen
nul=N Number of acq reqs given a NULL parent
noc=N Number of acq reqs rejected due to no cache available
ok=N Number of acq reqs succeeded
nbf=N Number of acq reqs rejected due to error
oom=N Number of acq reqs failed on ENOMEM
Lookups n=N Number of lookup calls made on cache backends
neg=N Number of negative lookups made
pos=N Number of positive lookups made
crt=N Number of objects created by lookup
tmo=N Number of lookups timed out and requeued
Updates n=N Number of update cookie requests seen
nul=N Number of upd reqs given a NULL parent
run=N Number of upd reqs granted CPU time
Relinqs n=N Number of relinquish cookie requests seen
nul=N Number of rlq reqs given a NULL parent
wcr=N Number of rlq reqs waited on completion of creation
AttrChg n=N Number of attribute changed requests seen
ok=N Number of attr changed requests queued
nbf=N Number of attr changed rejected -ENOBUFS
oom=N Number of attr changed failed -ENOMEM
run=N Number of attr changed ops given CPU time
Allocs n=N Number of allocation requests seen
ok=N Number of successful alloc reqs
wt=N Number of alloc reqs that waited on lookup completion
nbf=N Number of alloc reqs rejected -ENOBUFS
int=N Number of alloc reqs aborted -ERESTARTSYS
ops=N Number of alloc reqs submitted
owt=N Number of alloc reqs waited for CPU time
abt=N Number of alloc reqs aborted due to object death
Retrvls n=N Number of retrieval (read) requests seen
ok=N Number of successful retr reqs
wt=N Number of retr reqs that waited on lookup completion
nod=N Number of retr reqs returned -ENODATA
nbf=N Number of retr reqs rejected -ENOBUFS
int=N Number of retr reqs aborted -ERESTARTSYS
oom=N Number of retr reqs failed -ENOMEM
ops=N Number of retr reqs submitted
owt=N Number of retr reqs waited for CPU time
abt=N Number of retr reqs aborted due to object death
Stores n=N Number of storage (write) requests seen
ok=N Number of successful store reqs
agn=N Number of store reqs on a page already pending storage
nbf=N Number of store reqs rejected -ENOBUFS
oom=N Number of store reqs failed -ENOMEM
ops=N Number of store reqs submitted
run=N Number of store reqs granted CPU time
pgs=N Number of pages given store req processing time
rxd=N Number of store reqs deleted from tracking tree
olm=N Number of store reqs over store limit
VmScan nos=N Number of release reqs against pages with no pending store
gon=N Number of release reqs against pages stored by time lock granted
bsy=N Number of release reqs ignored due to in-progress store
can=N Number of page stores cancelled due to release req
Ops pend=N Number of times async ops added to pending queues
run=N Number of times async ops given CPU time
enq=N Number of times async ops queued for processing
can=N Number of async ops cancelled
rej=N Number of async ops rejected due to object lookup/create failure
ini=N Number of async ops initialised
dfr=N Number of async ops queued for deferred release
rel=N Number of async ops released (should equal ini=N when idle)
gc=N Number of deferred-release async ops garbage collected
CacheOp alo=N Number of in-progress alloc_object() cache ops
luo=N Number of in-progress lookup_object() cache ops
luc=N Number of in-progress lookup_complete() cache ops
gro=N Number of in-progress grab_object() cache ops
upo=N Number of in-progress update_object() cache ops
dro=N Number of in-progress drop_object() cache ops
pto=N Number of in-progress put_object() cache ops
syn=N Number of in-progress sync_cache() cache ops
atc=N Number of in-progress attr_changed() cache ops
rap=N Number of in-progress read_or_alloc_page() cache ops
ras=N Number of in-progress read_or_alloc_pages() cache ops
alp=N Number of in-progress allocate_page() cache ops
als=N Number of in-progress allocate_pages() cache ops
wrp=N Number of in-progress write_page() cache ops
ucp=N Number of in-progress uncache_page() cache ops
dsp=N Number of in-progress dissociate_pages() cache ops
CacheEv nsp=N Number of object lookups/creations rejected due to lack of space
stl=N Number of stale objects deleted
rtr=N Number of objects retired when relinquished
cul=N Number of objects culled


cat /proc/fs/fscache/histogram
===== ===== ========= ========= ========= ========= =========

This shows the breakdown of the number of times each amount of time between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The columns are as follows:

OBJ INST Length of time to instantiate an object
OP RUNS Length of time a call to process an operation took
OBJ RUNS Length of time a call to process an object event took
RETRV DLY Time between an requesting a read and lookup completing
RETRIEVLS Time between beginning and end of a retrieval

Each row shows the number of events that took a particular range of times. Each step is 1 jiffy in size. The JIFS column indicates the particular jiffy range covered, and the SECS field the equivalent number of seconds.

Object List

If CONFIG_FSCACHE_OBJECT_LIST is enabled, the FS-Cache facility will maintain a list of all the objects currently allocated and allow them to be viewed through:


This will look something like:

[root@andromeda ~]# head /proc/fs/fscache/objects
======== ======== ==== ===== === === === == ===== == == = = | ================ == == ================ ================
   17e4b        2 ACTV     0   0   0   0  0     0 7b  4 0 0 | NFS.fh           DT  0 ffff88001dd82820 010006017edcf8bbc93b43298fdfbe71e50b57b13a172c0117f38472, e567634700000000000000000000000063f2404a000000000000000000000000c9030000000000000000000063f2404a
   1693a        2 ACTV     0   0   0   0  0     0 7b  4 0 0 | NFS.fh           DT  0 ffff88002db23380 010006017edcf8bbc93b43298fdfbe71e50b57b1e0162c01a2df0ea6, 420ebc4a000000000000000000000000420ebc4a0000000000000000000000000e1801000000000000000000420ebc4a

where the first set of columns before the ‘|’ describe the object:

OBJECT Object debugging ID (appears as OBJ%x in some debug messages)
PARENT Debugging ID of parent object
STAT Object state
CHLDN Number of child objects of this object
OPS Number of outstanding operations on this object
OOP Number of outstanding child object management operations
EX Number of outstanding exclusive operations
READS Number of outstanding read operations
EM Object’s event mask
EV Events raised on this object
F Object flags
S Object work item busy state mask (1:pending 2:running)

and the second set of columns describe the object’s cookie, if present:

NETFS_COOKIE_DEF Name of netfs cookie definition
TY Cookie type (IX - index, DT - data, hex - special)
FL Cookie flags
NETFS_DATA Netfs private data stored in the cookie
OBJECT_KEY Object key } 1 column, with separating comma
AUX_DATA Object aux data } presence may be configured

The data shown may be filtered by attaching the a key to an appropriate keyring before viewing the file. Something like:

keyctl add user fscache:objlist <restrictions> @s

where <restrictions> are a selection of the following letters:

K Show hexdump of object key (don’t show if not given)
A Show hexdump of object aux data (don’t show if not given)

and the following paired letters:

C Show objects that have a cookie
c Show objects that don’t have a cookie
B Show objects that are busy
b Show objects that aren’t busy
W Show objects that have pending writes
w Show objects that don’t have pending writes
R Show objects that have outstanding reads
r Show objects that don’t have outstanding reads
S Show objects that have work queued
s Show objects that don’t have work queued

If neither side of a letter pair is given, then both are implied. For example:

keyctl add user fscache:objlist KB @s

shows objects that are busy, and lists their object keys, but does not dump their auxiliary data. It also implies “CcWwRrSs”, but as ‘B’ is given, ‘b’ is not implied.

By default all objects and all fields will be shown.


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


This is a bitmask of debugging streams to enable:

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 64 Page handling Function entry trace
7 128   Function exit trace
8 256   General
9 512 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|64)) >/sys/module/fscache/parameters/debug

will turn on all function entry debugging.