2d6fff6370
Add the API for a generic facility (FS-Cache) by which filesystems (such as AFS
or NFS) may call on local caching capabilities without having to know anything
about how the cache works, or even if there is a cache:
+---------+
| | +--------------+
| NFS |--+ | |
| | | +-->| CacheFS |
+---------+ | +----------+ | | /dev/hda5 |
| | | | +--------------+
+---------+ +-->| | |
| | | |--+
| AFS |----->| FS-Cache |
| | | |--+
+---------+ +-->| | |
| | | | +--------------+
+---------+ | +----------+ | | |
| | | +-->| CacheFiles |
| ISOFS |--+ | /var/cache |
| | +--------------+
+---------+
General documentation and documentation of the netfs specific API are provided
in addition to the header files.
As this patch stands, it is possible to build a filesystem against the facility
and attempt to use it. All that will happen is that all requests will be
immediately denied as if no cache is present.
Further patches will implement the core of the facility. The facility will
transfer requests from networking filesystems to appropriate caches if
possible, or else gracefully deny them.
If this facility is disabled in the kernel configuration, then all its
operations will trivially reduce to nothing during compilation.
WHY NOT I_MAPPING?
==================
I have added my own API to implement caching rather than using i_mapping to do
this for a number of reasons. These have been discussed a lot on the LKML and
CacheFS mailing lists, but to summarise the basics:
(1) Most filesystems don't do hole reportage. Holes in files are treated as
blocks of zeros and can't be distinguished otherwise, making it difficult
to distinguish blocks that have been read from the network and cached from
those that haven't.
(2) The backing inode must be fully populated before being exposed to
userspace through the main inode because the VM/VFS goes directly to the
backing inode and does not interrogate the front inode's VM ops.
Therefore:
(a) The backing inode must fit entirely within the cache.
(b) All backed files currently open must fit entirely within the cache at
the same time.
(c) A working set of files in total larger than the cache may not be
cached.
(d) A file may not grow larger than the available space in the cache.
(e) A file that's open and cached, and remotely grows larger than the
cache is potentially stuffed.
(3) Writes go to the backing filesystem, and can only be transferred to the
network when the file is closed.
(4) There's no record of what changes have been made, so the whole file must
be written back.
(5) The pages belong to the backing filesystem, and all metadata associated
with that page are relevant only to the backing filesystem, and not
anything stacked atop it.
OVERVIEW
========
FS-Cache provides (or will provide) the following facilities:
(1) Caches can be added / removed at any time, even whilst in use.
(2) Adds a facility by which tags can be used to refer to caches, even if
they're not available yet.
(3) More than one cache can be used at once. Caches can be selected
explicitly by use of tags.
(4) The netfs is provided with an interface that allows either party to
withdraw caching facilities from a file (required for (1)).
(5) A netfs may annotate cache objects that belongs to it. This permits the
storage of coherency maintenance data.
(6) Cache objects will be pinnable and space reservations will be possible.
(7) The interface to the netfs returns as few errors as possible, preferring
rather to let the netfs remain oblivious.
(8) 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.
(9) 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.
(10) Indices can be used to group files together to reduce key size and to make
group invalidation easier. The use of indices may make lookup quicker,
but that's cache dependent.
(11) Data I/O is effectively done directly 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.
(12) 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.
(13) 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 index 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, two netfs's can be seen to be backed: NFS and AFS. These
have different index hierarchies:
(*) The NFS primary index will probably contain 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 FS-Cache overview can be found in:
Documentation/filesystems/caching/fscache.txt
The netfs API to FS-Cache can be found in:
Documentation/filesystems/caching/netfs-api.txt
Signed-off-by: David Howells <dhowells@redhat.com>
Acked-by: Steve Dickson <steved@redhat.com>
Acked-by: Trond Myklebust <Trond.Myklebust@netapp.com>
Acked-by: Al Viro <viro@zeniv.linux.org.uk>
Tested-by: Daire Byrne <Daire.Byrne@framestore.com>
331 lines
14 KiB
Plaintext
331 lines
14 KiB
Plaintext
==========================
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General Filesystem Caching
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==========================
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========
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OVERVIEW
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========
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This facility is a general purpose cache for network filesystems, though it
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could be used for caching other things such as ISO9660 filesystems too.
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FS-Cache mediates between cache backends (such as CacheFS) and network
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filesystems:
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+---------+
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| | +--------------+
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| NFS |--+ | |
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|
| | | +-->| CacheFS |
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+---------+ | +----------+ | | /dev/hda5 |
|
|
| | | | +--------------+
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+---------+ +-->| | |
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|
| | | |--+
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| AFS |----->| FS-Cache |
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|
| | | |--+
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+---------+ +-->| | |
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| | | | +--------------+
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+---------+ | +----------+ | | |
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| | | +-->| CacheFiles |
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| ISOFS |--+ | /var/cache |
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| | +--------------+
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+---------+
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Or to look at it another way, FS-Cache is a module that provides a caching
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facility to a network filesystem such that the cache is transparent to the
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user:
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+---------+
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| |
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| Server |
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| |
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+---------+
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| NETWORK
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~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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| +----------+
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V | |
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+---------+ | |
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| | | |
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| NFS |----->| FS-Cache |
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|
| | | |--+
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+---------+ | | | +--------------+ +--------------+
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| | | | | | | |
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V +----------+ +-->| CacheFiles |-->| Ext3 |
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+---------+ | /var/cache | | /dev/sda6 |
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| | +--------------+ +--------------+
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| VFS | ^ ^
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| | | |
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+---------+ +--------------+ |
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| KERNEL SPACE | |
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~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
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| USER SPACE | |
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V | |
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+---------+ +--------------+
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| | | |
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| Process | | cachefilesd |
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| | | |
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+---------+ +--------------+
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FS-Cache does not follow the idea of completely loading every netfs file
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opened in its entirety into a cache before permitting it to be accessed and
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then serving the pages out of that cache rather than the netfs inode because:
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(1) It must be practical to operate without a cache.
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(2) The size of any accessible file must not be limited to the size of the
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cache.
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(3) The combined size of all opened files (this includes mapped libraries)
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must not be limited to the size of the cache.
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(4) The user should not be forced to download an entire file just to do a
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one-off access of a small portion of it (such as might be done with the
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"file" program).
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It instead serves the cache out in PAGE_SIZE chunks as and when requested by
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the netfs('s) using it.
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FS-Cache provides the following facilities:
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(1) More than one cache can be used at once. Caches can be selected
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explicitly by use of tags.
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(2) Caches can be added / removed at any time.
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(3) The netfs is provided with an interface that allows either party to
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withdraw caching facilities from a file (required for (2)).
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(4) The interface to the netfs returns as few errors as possible, preferring
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rather to let the netfs remain oblivious.
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(5) Cookies are used to represent indices, files and other objects to the
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netfs. The simplest cookie is just a NULL pointer - indicating nothing
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cached there.
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(6) The netfs is allowed to propose - dynamically - any index hierarchy it
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desires, though it must be aware that the index search function is
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recursive, stack space is limited, and indices can only be children of
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indices.
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(7) Data I/O is done direct to and from the netfs's pages. The netfs
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indicates that page A is at index B of the data-file represented by cookie
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C, and that it should be read or written. The cache backend may or may
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not start I/O on that page, but if it does, a netfs callback will be
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invoked to indicate completion. The I/O may be either synchronous or
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asynchronous.
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(8) Cookies can be "retired" upon release. At this point FS-Cache will mark
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them as obsolete and the index hierarchy rooted at that point will get
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recycled.
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(9) The netfs provides a "match" function for index searches. In addition to
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saying whether a match was made or not, this can also specify that an
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entry should be updated or deleted.
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(10) As much as possible is done asynchronously.
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FS-Cache maintains a virtual indexing tree in which all indices, files, objects
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and pages are kept. Bits of this tree may actually reside in one or more
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caches.
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FSDEF
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+------------------------------------+
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| |
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NFS AFS
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| |
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+--------------------------+ +-----------+
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| | | |
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homedir mirror afs.org redhat.com
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| | |
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+------------+ +---------------+ +----------+
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00001 00002 00007 00125 vol00001 vol00002
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+---+---+ +-----+ +---+ +------+------+ +-----+----+
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| | | | | | | | | | | | |
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PG0 PG1 PG2 PG0 XATTR PG0 PG1 DIRENT DIRENT DIRENT R/W R/O Bak
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| |
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PG0 +-------+
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00001 00003
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+---+---+
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| | |
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PG0 PG1 PG2
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In the example above, you can see two netfs's being backed: NFS and AFS. These
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have different index hierarchies:
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(*) The NFS primary index contains per-server indices. Each server index is
|
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indexed by NFS file handles to get data file objects. Each data file
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objects can have an array of pages, but may also have further child
|
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objects, such as extended attributes and directory entries. Extended
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attribute objects themselves have page-array contents.
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(*) The AFS primary index contains per-cell indices. Each cell index contains
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per-logical-volume indices. Each of volume index contains up to three
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indices for the read-write, read-only and backup mirrors of those volumes.
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Each of these contains vnode data file objects, each of which contains an
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array of pages.
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The very top index is the FS-Cache master index in which individual netfs's
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have entries.
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Any index object may reside in more than one cache, provided it only has index
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children. Any index with non-index object children will be assumed to only
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reside in one cache.
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The netfs API to FS-Cache can be found in:
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Documentation/filesystems/caching/netfs-api.txt
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The cache backend API to FS-Cache can be found in:
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Documentation/filesystems/caching/backend-api.txt
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=======================
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STATISTICAL INFORMATION
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=======================
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If FS-Cache is compiled with the following options enabled:
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CONFIG_FSCACHE_PROC=y (implied by the following two)
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CONFIG_FSCACHE_STATS=y
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CONFIG_FSCACHE_HISTOGRAM=y
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then it will gather certain statistics and display them through a number of
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proc files.
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(*) /proc/fs/fscache/stats
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This shows counts of a number of events that can happen in FS-Cache:
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CLASS EVENT MEANING
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======= ======= =======================================================
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Cookies idx=N Number of index cookies allocated
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dat=N Number of data storage cookies allocated
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spc=N Number of special cookies allocated
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Objects alc=N Number of objects allocated
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nal=N Number of object allocation failures
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avl=N Number of objects that reached the available state
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ded=N Number of objects that reached the dead state
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ChkAux non=N Number of objects that didn't have a coherency check
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ok=N Number of objects that passed a coherency check
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upd=N Number of objects that needed a coherency data update
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obs=N Number of objects that were declared obsolete
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Pages mrk=N Number of pages marked as being cached
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unc=N Number of uncache page requests seen
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Acquire n=N Number of acquire cookie requests seen
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nul=N Number of acq reqs given a NULL parent
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noc=N Number of acq reqs rejected due to no cache available
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ok=N Number of acq reqs succeeded
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nbf=N Number of acq reqs rejected due to error
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oom=N Number of acq reqs failed on ENOMEM
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Lookups n=N Number of lookup calls made on cache backends
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neg=N Number of negative lookups made
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pos=N Number of positive lookups made
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crt=N Number of objects created by lookup
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Updates n=N Number of update cookie requests seen
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nul=N Number of upd reqs given a NULL parent
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run=N Number of upd reqs granted CPU time
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Relinqs n=N Number of relinquish cookie requests seen
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nul=N Number of rlq reqs given a NULL parent
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wcr=N Number of rlq reqs waited on completion of creation
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AttrChg n=N Number of attribute changed requests seen
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ok=N Number of attr changed requests queued
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nbf=N Number of attr changed rejected -ENOBUFS
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oom=N Number of attr changed failed -ENOMEM
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run=N Number of attr changed ops given CPU time
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Allocs n=N Number of allocation requests seen
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ok=N Number of successful alloc reqs
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wt=N Number of alloc reqs that waited on lookup completion
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nbf=N Number of alloc reqs rejected -ENOBUFS
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ops=N Number of alloc reqs submitted
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owt=N Number of alloc reqs waited for CPU time
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Retrvls n=N Number of retrieval (read) requests seen
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ok=N Number of successful retr reqs
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wt=N Number of retr reqs that waited on lookup completion
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nod=N Number of retr reqs returned -ENODATA
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nbf=N Number of retr reqs rejected -ENOBUFS
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int=N Number of retr reqs aborted -ERESTARTSYS
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oom=N Number of retr reqs failed -ENOMEM
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ops=N Number of retr reqs submitted
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owt=N Number of retr reqs waited for CPU time
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Stores n=N Number of storage (write) requests seen
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ok=N Number of successful store reqs
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agn=N Number of store reqs on a page already pending storage
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nbf=N Number of store reqs rejected -ENOBUFS
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oom=N Number of store reqs failed -ENOMEM
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ops=N Number of store reqs submitted
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run=N Number of store reqs granted CPU time
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Ops pend=N Number of times async ops added to pending queues
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run=N Number of times async ops given CPU time
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enq=N Number of times async ops queued for processing
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dfr=N Number of async ops queued for deferred release
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rel=N Number of async ops released
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gc=N Number of deferred-release async ops garbage collected
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(*) /proc/fs/fscache/histogram
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cat /proc/fs/fscache/histogram
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+HZ +TIME OBJ INST OP RUNS OBJ RUNS RETRV DLY RETRIEVLS
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===== ===== ========= ========= ========= ========= =========
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This shows the breakdown of the number of times each amount of time
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between 0 jiffies and HZ-1 jiffies a variety of tasks took to run. The
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columns are as follows:
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COLUMN TIME MEASUREMENT
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======= =======================================================
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OBJ INST Length of time to instantiate an object
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OP RUNS Length of time a call to process an operation took
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OBJ RUNS Length of time a call to process an object event took
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RETRV DLY Time between an requesting a read and lookup completing
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RETRIEVLS Time between beginning and end of a retrieval
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Each row shows the number of events that took a particular range of times.
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Each step is 1 jiffy in size. The +HZ column indicates the particular
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jiffy range covered, and the +TIME field the equivalent number of seconds.
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=========
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DEBUGGING
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=========
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The FS-Cache facility can have runtime debugging enabled by adjusting the value
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in:
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/sys/module/fscache/parameters/debug
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This is a bitmask of debugging streams to enable:
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BIT VALUE STREAM POINT
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======= ======= =============================== =======================
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0 1 Cache management Function entry trace
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1 2 Function exit trace
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2 4 General
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3 8 Cookie management Function entry trace
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4 16 Function exit trace
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5 32 General
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6 64 Page handling Function entry trace
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7 128 Function exit trace
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8 256 General
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9 512 Operation management Function entry trace
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10 1024 Function exit trace
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11 2048 General
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The appropriate set of values should be OR'd together and the result written to
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the control file. For example:
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echo $((1|8|64)) >/sys/module/fscache/parameters/debug
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will turn on all function entry debugging.
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