--- /dev/null
+ ===============================================
+ CacheFiles: CACHE ON ALREADY MOUNTED FILESYSTEM
+ ===============================================
+
+Contents:
+
+ (*) Overview.
+
+ (*) Requirements.
+
+ (*) Configuration.
+
+ (*) Starting the cache.
+
+ (*) Things to avoid.
+
+
+========
+OVERVIEW
+========
+
+CacheFiles is a caching backend that's meant to use as a cache a directory on
+an already mounted filesystem of a local type (such as Ext3).
+
+CacheFiles uses a userspace daemon to do some of the cache management - such as
+reaping stale nodes and culling. This is called cachefilesd and lives in
+/sbin.
+
+The filesystem and data integrity of the cache are only as good as those of the
+filesystem providing the backing services. Note that CacheFiles does not
+attempt to journal anything since the journalling interfaces of the various
+filesystems are very specific in nature.
+
+CacheFiles creates a proc-file - "/proc/fs/cachefiles" - that is used for
+communication with the daemon. Only one thing may have this open at once, and
+whilst it is open, a cache is at least partially in existence. The daemon
+opens this and sends commands down it to control the cache.
+
+CacheFiles is currently limited to a single cache.
+
+CacheFiles attempts to maintain at least a certain percentage of free space on
+the filesystem, shrinking the cache by culling the objects it contains to make
+space if necessary - see the "Cache Culling" section. This means it can be
+placed on the same medium as a live set of data, and will expand to make use of
+spare space and automatically contract when the set of data requires more
+space.
+
+
+============
+REQUIREMENTS
+============
+
+The use of CacheFiles and its daemon requires the following features to be
+available in the system and in the cache filesystem:
+
+ - dnotify.
+
+ - extended attributes (xattrs).
+
+ - openat() and friends.
+
+ - bmap() support on files in the filesystem (FIBMAP ioctl).
+
+ - The use of bmap() to detect a partial page at the end of the file.
+
+It is strongly recommended that the "dir_index" option is enabled on Ext3
+filesystems being used as a cache.
+
+
+=============
+CONFIGURATION
+=============
+
+The cache is configured by a script in /etc/cachefilesd.conf. These commands
+set up cache ready for use. The following script commands are available:
+
+ (*) brun <N>%
+ (*) bcull <N>%
+ (*) bstop <N>%
+
+ Configure the culling limits. Optional. See the section on culling
+ The defaults are 7%, 5% and 1% respectively.
+
+ (*) dir <path>
+
+ Specify the directory containing the root of the cache. Mandatory.
+
+ (*) tag <name>
+
+ Specify a tag to FS-Cache to use in distinguishing multiple caches.
+ Optional. The default is "CacheFiles".
+
+ (*) debug <mask>
+
+ Specify a numeric bitmask to control debugging in the kernel module.
+ Optional. The default is zero (all off). The following values can be
+ OR'd into the mask to collect various information:
+
+ 1 Turn on trace of function entry (_enter() macros)
+ 2 Turn on trace of function exit (_leave() macros)
+ 4 Turn on trace of internal debug points (_debug())
+
+ This mask can also be set through /proc/sys/fs/cachefiles/debug.
+
+
+==================
+STARTING THE CACHE
+==================
+
+The cache is started by running the daemon. The daemon opens the cache proc
+file, configures the cache and tells it to begin caching. At that point the
+cache binds to fscache and the cache becomes live.
+
+The daemon is run as follows:
+
+ /sbin/cachefilesd [-d]* [-s] [-n] [-f <configfile>]
+
+The flags are:
+
+ (*) -d
+
+ Increase the debugging level. This can be specified multiple times and
+ is cumulative with itself.
+
+ (*) -s
+
+ Send messages to stderr instead of syslog.
+
+ (*) -n
+
+ Don't daemonise and go into background.
+
+ (*) -f <configfile>
+
+ Use an alternative configuration file rather than the default one.
+
+
+===============
+THINGS TO AVOID
+===============
+
+Do not mount other things within the cache as this will cause problems. The
+kernel module contains its own very cut-down path walking facility that ignores
+mountpoints, but the daemon can't avoid them.
+
+Do not create, rename or unlink files and directories in the cache whilst the
+cache is active, as this may cause the state to become uncertain.
+
+Renaming files in the cache might make objects appear to be other objects (the
+filename is part of the lookup key).
+
+Do not change or remove the extended attributes attached to cache files by the
+cache as this will cause the cache state management to get confused.
+
+Do not create files or directories in the cache, lest the cache get confused or
+serve incorrect data.
+
+Do not chmod files in the cache. The module creates things with minimal
+permissions to prevent random users being able to access them directly.
+
+
+=============
+CACHE CULLING
+=============
+
+The cache may need culling occasionally to make space. This involves
+discarding objects from the cache that have been used less recently than
+anything else. Culling is based on the access time of data objects. Empty
+directories are culled if not in use.
+
+Cache culling is done on the basis of the percentage of blocks available in the
+underlying filesystem. There are three "limits":
+
+ (*) brun
+
+ If the amount of available space in the cache rises above this limit, then
+ culling is turned off.
+
+ (*) bcull
+
+ If the amount of available space in the cache falls below this limit, then
+ culling is started.
+
+ (*) bstop
+
+ If the amount of available space in the cache falls below this limit, then
+ no further allocation of disk space is permitted until culling has raised
+ the amount above this limit again.
+
+These must be configured thusly:
+
+ 0 <= bstop < bcull < brun < 100
+
+Note that these are percentages of available space, and do _not_ appear as 100
+minus the percentage displayed by the "df" program.
+
+The userspace daemon scans the cache to build up a table of cullable objects.
+These are then culled in least recently used order. A new scan of the cache is
+started as soon as space is made in the table. Objects will be skipped if
+their atimes have changed or if the kernel module says it is still using them.
+
+
+===============
+CACHE STRUCTURE
+===============
+
+The CacheFiles module will create two directories in the directory it was
+given:
+
+ (*) cache/
+
+ (*) graveyard/
+
+The active cache objects all reside in the first directory. The CacheFiles
+kernel module moves any retired or culled objects that it can't simply unlink
+to the graveyard from which the daemon will actually delete them.
+
+The daemon uses dnotify to monitor the graveyard directory, and will delete
+anything that appears therein.
+
+
+The module represents index objects as directories with the filename "I..." or
+"J...". Note that the "cache/" directory is itself a special index.
+
+Data objects are represented as files if they have no children, or directories
+if they do. Their filenames all begin "D..." or "E...". If represented as a
+directory, data objects will have a file in the directory called "data" that
+actually holds the data.
+
+Special objects are similar to data objects, except their filenames begin
+"S..." or "T...".
+
+
+If an object has children, then it will be represented as a directory.
+Immediately in the representative directory are a collection of directories
+named for hash values of the child object keys with an '@' prepended. Into
+this directory, if possible, will be placed the representations of the child
+objects:
+
+ INDEX INDEX INDEX DATA FILES
+ ========= ========== ================================= ================
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...DB1ry
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...N22ry
+ cache/@4a/I03nfs/@30/Ji000000000000000--fHg8hi8400/@75/Es0g000w...FP1ry
+
+
+If the key is so long that it exceeds NAME_MAX with the decorations added on to
+it, then it will be cut into pieces, the first few of which will be used to
+make a nest of directories, and the last one of which will be the objects
+inside the last directory. The names of the intermediate directories will have
+'+' prepended:
+
+ J1223/@23/+xy...z/+kl...m/Epqr
+
+
+Note that keys are raw data, and not only may they exceed NAME_MAX in size,
+they may also contain things like '/' and NUL characters, and so they may not
+be suitable for turning directly into a filename.
+
+To handle this, CacheFiles will use a suitably printable filename directly and
+"base-64" encode ones that aren't directly suitable. The two versions of
+object filenames indicate the encoding:
+
+ OBJECT TYPE PRINTABLE ENCODED
+ =============== =============== ===============
+ Index "I..." "J..."
+ Data "D..." "E..."
+ Special "S..." "T..."
+
+Intermediate directories are always "@" or "+" as appropriate.
+
+
+Each object in the cache has an extended attribute label that holds the object
+type ID (required to distinguish special objects) and the auxiliary data from
+the netfs. The latter is used to detect stale objects in the cache and update
+or retire them.
+
+
+Note that CacheFiles will erase from the cache any file it doesn't recognise or
+any file of an incorrect type (such as a FIFO file or a device file).
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