linux/Documentation/filesystems/caching/fscache.rst

.. SPDX-License-Identifier: GPL-2.0

==========================
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:

	Documentation/filesystems/caching/netfs-api.rst

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

	Documentation/filesystems/caching/backend-api.rst


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
	=======	===============================================================


Cookie List
===========

FS-Cache provides a list of cookies:

	/proc/fs/fscache/cookies

This will look something like::

	# head /proc/fs/fscache/cookies
	COOKIE   VOLUME   REF ACT ACC S FL DEF
	======== ======== === === === = == ================
	00000435 00000001   1   0  -1 - 08 0000000201d080070000000000000000, 0000000000000000
	00000436 00000001   1   0  -1 - 00 0000005601d080080000000000000000, 0000000000000051
	00000437 00000001   1   0  -1 - 08 00023b3001d0823f0000000000000000, 0000000000000000
	00000438 00000001   1   0  -1 - 08 0000005801d0807b0000000000000000, 0000000000000000
	00000439 00000001   1   0  -1 - 08 00023b3201d080a10000000000000000, 0000000000000000
	0000043a 00000001   1   0  -1 - 08 00023b3401d080a30000000000000000, 0000000000000000
	0000043b 00000001   1   0  -1 - 08 00023b3601d080b30000000000000000, 0000000000000000
	0000043c 00000001   1   0  -1 - 08 00023b3801d080b40000000000000000, 0000000000000000

where the columns are:

	=======	===============================================================
	COLUMN	DESCRIPTION
	=======	===============================================================
	COOKIE	The cookie debug ID (also appears in traces)
	VOLUME	The parent volume cookie debug ID
	REF	Number of references on the volume cookie
	ACT	Number of times the cookie is marked for in use
	ACC	Number of access pins in the cookie
	S	State of the cookie
	FL	Flags on the cookie
	DEF	Key, auxiliary data
	=======	===============================================================


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.