==========================
			  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  |
	|         |                        +--------------+
	+---------+


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.


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