linux/Documentation/filesystems/caching/fscache.txt

			  ==========================
			  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.txt

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

	Documentation/filesystems/caching/backend-api.txt


=======================
STATISTICAL INFORMATION
=======================

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

	CONFIG_FSCACHE_PROC=y (implied by the following two)
	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
	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
		ops=N	Number of alloc reqs submitted
		owt=N	Number of alloc reqs waited for CPU time
	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
	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
	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
		dfr=N	Number of async ops queued for deferred release
		rel=N	Number of async ops released
		gc=N	Number of deferred-release async ops garbage collected


 (*) /proc/fs/fscache/histogram

	cat /proc/fs/fscache/histogram
	+HZ   +TIME 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 +HZ column indicates the particular
     jiffy range covered, and the +TIME field the equivalent number of seconds.


=========
DEBUGGING
=========

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.