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 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.

(10) 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::

                                            FSDEF
                                              |
                         +------------------------------------+
                         |                                    |
                        NFS                                  AFS
                         |                                    |
            +--------------------------+                +-----------+
            |                          |                |           |
         homedir                     mirror          afs.org   redhat.com
            |                          |                            |
      +------------+           +---------------+              +----------+
      |            |           |               |              |          |
    00001        00002       00007           00125        vol00001   vol00002
      |            |           |               |                         |
  +---+---+     +-----+      +---+      +------+------+            +-----+----+
  |   |   |     |     |      |   |      |      |      |            |     |    |
 PG0 PG1 PG2   PG0  XATTR   PG0 PG1   DIRENT DIRENT DIRENT        R/W   R/O  Bak
                      |                                            |
                     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:

	Documentation/filesystems/caching/netfs-api.rst

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

	Documentation/filesystems/caching/backend-api.rst

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

	Documentation/filesystems/caching/object.rst


Statistical Information
=======================

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

	CONFIG_FSCACHE_STATS=y
	CONFIG_FSCACHE_HISTOGRAM=y

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

/proc/fs/fscache/stats
----------------------

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

+--------------+-------+-------------------------------------------------------+
|CLASS         |EVENT  |MEANING                                                |
+==============+=======+=======================================================+
|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  |Number of pages marked as being cached                 |
|              |unc=N  |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                               |
+--------------+-------+-------------------------------------------------------+



/proc/fs/fscache/histogram
--------------------------

     ::

	cat /proc/fs/fscache/histogram
	JIFS  SECS  OBJ INST  OP RUNS   OBJ RUNS  RETRV DLY RETRIEVLS
	===== ===== ========= ========= ========= ========= =========

     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:

	=========	=======================================================
	COLUMN		TIME MEASUREMENT
	=========	=======================================================
	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::

	/proc/fs/fscache/objects

This will look something like::

	[root@andromeda ~]# head /proc/fs/fscache/objects
	OBJECT   PARENT   STAT CHLDN OPS OOP IPR EX READS EM EV F S | NETFS_COOKIE_DEF TY FL NETFS_DATA       OBJECT_KEY, AUX_DATA
	======== ======== ==== ===== === === === == ===== == == = = | ================ == == ================ ================
	   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:

	=======	===============================================================
	COLUMN	DESCRIPTION
	=======	===============================================================
	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
	IPR
	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:

	================ ======================================================
	COLUMN		 DESCRIPTION
	================ ======================================================
	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.


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	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.