[PATCH 00/43] Permit filesystem local caching [ver #46]

From: David Howells
Date: Wed Apr 01 2009 - 19:03:55 EST

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These patches add local caching for network filesystems such as NFS. To give a
really quick overview of the way the facility works:

+---------+
| |
| NFS |--+
| | |
+---------+ | +----------+
| | |
+---------+ +-->| |
| | | |
| AFS |----->| FS-Cache |
| | | |--+
+---------+ +-->| | |
| | | | +--------------+ +--------------+
+---------+ | +----------+ | | | | |
| | | +-->| CacheFiles |-->| Ext3 |
| ISOFS |--+ | /var/cache | | /dev/sda6 |
| | +--------------+ +--------------+
+---------+

(1) NFS, say, asks FS-Cache to store/retrieve data for it;

(2) FS-Cache asks the cache backend, in this case CacheFiles to honour the
operation;

(3) CacheFiles 'opens' a file in a mounted filesystem, say Ext3, and does read
and write operations of a sort on it;

(4) Ext3 decides how the cache data is laid out on disk - CacheFiles just
attempts to use one sparse file per netfs inode.

(5) If NFS asks for data from the cache, but the file has a hole in it, NFS
falls back to asking the server. The data obtained from the server is
then written over the hole in the file.

To look at it another way:

+---------+
| |
| Server |
| |
+---------+
| NETWORK
~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
| +----------+
V | |
+---------+ | |
| | | |
| NFS |----->| FS-Cache |
| | | |--+
+---------+ | | | +--------------+ +--------------+
| | | | | | | |
V +----------+ +-->| CacheFiles |-->| Ext3 |
+---------+ | /var/cache | | /dev/sda6 |
| | +--------------+ +--------------+
| VFS | ^ ^
| | | |
+---------+ +--------------+ |
| KERNEL SPACE | |
~~~~~|~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|~~~~~~|~~~~
| USER SPACE | |
V | |
+---------+ +--------------+
| | | |
| Process | | cachefilesd |
| | | |
+---------+ +--------------+

FS-Cache attempts to provide a caching facility to a network filesystem such
that it's transparent to the users of that network filesystem.

The patches can roughly be broken down into a number of sets:

(*) 01-slow-work.diff
(*) 02-slow-work-dynamic.diff
(*) 03-slow-work-config.diff
(*) 04-slow-work-doc.diff

A thread pool for (very) slow work items, such as batches of lookup,
mkdir, create and [sg]etxattr calls.

It doesn't make sense to use an ordinary work queue because I want more
than one thread, but I don't want to be limited to 1 thread per CPU.
The work items in question take a long time, most of which is spent
sleeping on I/O. For the duration the worker thread is unable to do
anything else.

These four patches add the basic facility, make it dynamic, make it
configurable and document it.

(*) 05-release-page.diff

Call the page release function after a failed readahead.

(*) 06-fscache-page-flags.diff

Add two extra page flags that FS-Cache then uses to keep track of two
bits of per-cached-page information:

(1) This page is known by the cache, and that the cache must be
informed if the page is going to go away. It's an indication to
the netfs that the cache has an interest in this page, where an
interest may be a pointer to it, resources allocated or reserved
for it, or I/O in progress upon it.

(2) This page is being written to disk by the cache, and that it
cannot be released until completion. Ideally it shouldn't be
changed until completion either so as to maintain the known state
of the cache. This cannot be unified with PG_writeback as the
page may be being written to both the server and the cache at the
same time or at different times.

To avoid using extra page bits, I could, for example, set up a radix tree
per data storage object to keep track of both these bits, however this
would mean that the netfs would have to do a call, spinlock, conditional
jumps, etc to find out either state.

On the other hand, if we can spare two page flags, those are sufficient.

Note that the cache doesn't necessarily need to be able to find the netfs
pages, but may have to allocate/pin resources for backing them.

Further note that PG_private may not be used as I want to be able to use
caching with ISOFS eventually, and PG_private is owned by the block
buffer code.

These bits can be otherwise used by any filesystem that doesn't want to
use FS-Cache.

(*) 07-fscache-netfs-api.diff
(*) 08-fscache-backend-api.diff
(*) 09-fscache-kconfig.diff
(*) 10-fscache-proc.diff
(*) 11-fscache-fsdef.diff
(*) 12-fscache-tag-handling.diff
(*) 13-fscache-cache-handling.diff
(*) 14-fscache-cookie-jar.diff
(*) 15-fscache-netfs-reg.diff
(*) 16-fscache-bits.diff
(*) 17-fscache-object.diff
(*) 18-fscache-cookie.diff
(*) 19-fscache-operation.diff
(*) 20-fscache-io.diff

Patches to provide a local caching facility for network filesystems.

FS-Cache is a layer that takes requests from any one of a number of
netfs's and passes them to an appropriate cache, if there is one.
FS-Cache makes operations requested by the netfs transparently
asynchronous where possible.

FS-Cache also protects the netfs against (a) there being no cache, (b)
the cache suffering a fatal I/O error and (c) the cache being removed;
and protects the cache against (d) the netfs uncaching pages that the
cache is using and (e) conflicting operations from the netfs, some of
which may be queued for asynchronous processing.

Five documents in text file format that describe the FS-Cache interface
are added by these patches:

Documentation/filesystems/caching/fscache.txt gives an overview of the
facility and describes the statistical data it makes available.

Documentation/filesystems/caching/netfs-api.txt describes the API by
which a network filesystem would make use of the FS-Cache facility.

Documentation/filesystems/caching/backend-api.txt describes the API that
a cache backend must implement to provide caching services through
FS-Cache.

Documentation/filesystems/caching/object.txt describes the object
management state machine used.

Documentation/filesystems/caching/operations.txt describes the operation
scheduling facility provided and used by FS-Cache that can also be used
by cache backend modules.

The patches provide the following components of the FS-Cache facility:

(07) The netfs API header file and documentation. A netfs can actually
be built and run against just this patch. It won't actually do
anything without the later patches, though, but it will compile.

(08) The cache backend API header file and documentation.

(09) The main selector configuration option, the main module load/unload
hooks and the debugging code declarations. With this patch applied,
it is possible to enable caching in a client netfs, though it won't
actually do anything.

(10) The /proc files for statistics presentation, plus the internal
interfaces for driving it.

(11) The top level index definition.

(12) Cache reference tag handling.

(13) Cache (un)registration and error handling.

(14) Cookie (de)allocator and initialisation.

(15) Netfs (un)registration handling. This is partly usable. Without a
later patch, however, a cookie will be leaked from unregistration.

(16) Bit waiting utility functions.

(17) The object management state machine implementation and
documentation.

(18) Implementation of the cookie management part of the netfs API. With
this, it's possible for netfs's to actually be granted cookies and
to release them without error.

(19) Cache I/O operation scheduler and documentation.

(20) Implementation of the data I/O part of the netfs API.

(*) 21-cachefiles-ext3-f_mapping.diff
(*) 22-cachefiles-write.diff
(*) 23-cachefiles-monitor.diff
(*) 24-cachefiles-export.diff
(*) 25-cachefiles.diff

Patches to provide a local cache in a directory of an already mounted
filesystem.

The latter patch adds a document in text file format that describes the
CacheFiles cache backend and gives instructions on how it is set up and
used. This will be Documentation/filesystems/caching/cachefiles.txt when
the patch is applied.

(*) 26-afs-fscache.diff

Patches to provide AFS with local caching.

(*) 27-nfs-comment.diff
(*) 28-nfs-fscache-option.diff
(*) 29-nfs-fscache-kconfig.diff
(*) 30-nfs-fscache-top-index.diff
(*) 31-nfs-fscache-server-obj.diff
(*) 32-nfs-fscache-super-obj.diff
(*) 33-nfs-fscache-inode-obj.diff
(*) 34-nfs-fscache-use-inode.diff
(*) 35-nfs-fscache-invalidate-pages.diff
(*) 36-nfs-fscache-iostats.diff
(*) 37-nfs-fscache-page-management.diff
(*) 38-nfs-fscache-read-context.diff
(*) 39-nfs-fscache-read-fallback.diff
(*) 40-nfs-fscache-read-from-cache.diff
(*) 41-nfs-fscache-store-to-cache.diff
(*) 42-nfs-fscache-display.diff
(*) 43-nfs-fscache-mount.diff

Patches to provide NFS with local caching.

I've renamed the files in fs/fscache/ from fsc-XXX to XXX, and similarly the
files called cf-XXX to XXX in fs/cachefiles/.

I've also fixed some minor conflicts with NFS changes that have gone in to
Linus's tree.

I've been testing these patches by throwing batches of eight parallel "tar cf"
commands across three different 350MB NFS-based kernel trees (3 tars on first
tree, 3 on second, 2 on third), sometimes with one or more of the trees
preloaded into the cache. The complete working data set does not fit into the
RAM of my test machine, so even three tars that can be entirely satisfied from
the cache may have to reread everything from disk.

I've also been running tars against AFS mounted kernel trees.

--
A tarball of the patches is available at:

http://people.redhat.com/~dhowells/fscache/patches/nfs+fscache-46.tar.bz2

This builds on top of Linus's current kernel.

The code can also be obtained by GIT from:

git://git.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-2.6-fscache.git
http://www.kernel.org/pub/scm/linux/kernel/git/dhowells/linux-2.6-fscache.git

tagged as 'fscache-46'.

To use this version of CacheFiles, the cachefilesd-0.9 is also required. It
is available as an SRPM:

http://people.redhat.com/~dhowells/fscache/cachefilesd-0.9-1.fc7.src.rpm

Or as individual bits:

http://people.redhat.com/~dhowells/fscache/cachefilesd-0.9.tar.bz2
http://people.redhat.com/~dhowells/fscache/cachefilesd.fc
http://people.redhat.com/~dhowells/fscache/cachefilesd.if
http://people.redhat.com/~dhowells/fscache/cachefilesd.te
http://people.redhat.com/~dhowells/fscache/cachefilesd.spec

The .fc, .if and .te files are for manipulating SELinux.

David
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