[PATCH] CacheFS - general filesystem cache

[PATCH] CacheFS - general filesystem cache

[David Howells]

Hi Linus, Andrew,

I've packaged my generic filesystem cache filesystem into patches and also
produced patches for my AFS filesystem to use it. Work is also in progress to
alter the NFS client use this interface too, and I think the ISO9660
filesystem could also benefit.

I've put a tarball of them on my Red Hat webpage because one of them is rather
large. If you grab:

	http://people.redhat.com/~dhowells/cachefs/cachefs-patches.tar.bz2

You'll find the following patches contained therein:

    (1) cachefs-nullfileptr-2681mm4.diff

	Fix a bug introduced into do_generic_mapping_read() by which a file
	pointer becomes required. I had arranged things so that it was
	optional so that I could call it directly on an inode.

    (2) cachefs-rastateinit-2681mm4.diff

	Revert the change made to file_ra_state_init() that removed the symbol
	export that I'd put in, and also make it fully initialise the state
	again.

	This was also done so that do_generic_mapping_read() could be called
	directly on an inode.

    (3) cachefs-fs-2681mm4.diff

	The cachefs filesystem source and interface header file.

    (4) cachefs-doc-2681mm4.diff

	Documentation about using cachefs.

    (5) cachefs-wppageop-2681mm4.diff

	Add a notification operation to the address space that gets invoked
	when a read-only protected page is about to be made writable. I've
	shown this to Rik van Riel who seems happy with it.

	Using a combination of (5) and (6), I can have cachefs write a netfs
	page into the cache in the background whilst the netfs readpage
	completes and passes the now unlocked page back to userspace.

	Previously, the netfs readpage function had to be synchronous with
	respect to writing the page to disc. Now it can use this callback and
	prepare_write() to synchronise upon the page being modified in the
	page cache before it gets DMA'd to cachefs's disc block.

	David Woodhouse is also interested in using this to do lazy
	reservations in JFFS2 shared writable mapping support.

    (6) cachefs-fspagesync-2681mm4.diff

	This adds another page flag for intra-filesystem synchronisation, to
	operate in a manner similar to PG_writeback.

	I've made my AFS client use this to indicate that a page has now been
	written to the backing cache and can now be safely modified.

    (7) cachefs-afs-2681mm4.diff

	This changes my AFS client so that it can make use of cachefs.

Signed-Off-By: David Howells <dhowells@redhat.com>

I've also put a utility on my webpage for dumping the cachefs update journal
contents:

	http://people.redhat.com/~dhowells/cachefs/dump-journal.c

It just needs the fs/cachefs/cachefs-int.h header file placing in the same
directory to compile.

David

Re: [PATCH] CacheFS - general filesystem cache

[Linus Torvalds]

On Fri, 27 Aug 2004, David Howells wrote:
> 
>     (4) cachefs-doc-2681mm4.diff
> 
> 	Documentation about using cachefs.

Heh:

      Three levels of indirection are currently supported:

       - single indirection
       - double indirection

somebody has trouble counting.

More seriously, I'd _really_ love to see something like a "swapfs", ie 
tmpfs with cachefs as a backing store. It would be a lot more useful for 
testing that AFS+cachefs, and would hopefully also act as a example of how 
to use it _without_ having to worry about AFS.

Is that possible?

		Linus

Re: [PATCH] CacheFS - general filesystem cache

[Jeff Garzik]

David Howells wrote:
> Hi Linus, Andrew,
> 
> I've packaged my generic filesystem cache filesystem into patches and also
> produced patches for my AFS filesystem to use it. Work is also in progress to
> alter the NFS client use this interface too, and I think the ISO9660
> filesystem could also benefit.

<and the audience goes wild>  whee :)

IMHO cachefs is a big deal particularly now that the NFSv4 delegation 
code has been merged.

For those unfamiliar with the changes from NFSv3 -> NFSv4, one of the 
many nice features of v4 is that it has sane caching.  A file can be 
"delegated" to the client, such that, the client has complete control 
over the file including caching.  Sorta like a lease.  Great for 
cachefs, IOW.  :)


>     (7) cachefs-afs-2681mm4.diff
> 
> 	This changes my AFS client so that it can make use of cachefs.

Yay, a user :)

	Jeff

Re: [PATCH] CacheFS - general filesystem cache

[David Howells]

> 
> Heh:
> 
>       Three levels of indirection are currently supported:
> 
>        - single indirection
>        - double indirection
> 
> somebody has trouble counting.

Not really. I struck triple indirection off the list because I decided not to
go that far yet. I haven't yet persuaded emacs to automatically update counting
words in this situation:-)

> More seriously, I'd _really_ love to see something like a "swapfs", ie 
> tmpfs with cachefs as a backing store. It would be a lot more useful for 
> testing that AFS+cachefs, and would hopefully also act as a example of how 
> to use it _without_ having to worry about AFS.
> 
> Is that possible?

Ramfs with on-disc caching, perhaps? :-)

I'd have to provide cache file pinning to prevent them being automatically
recycled, but I want to do that anyway for other reasons.

It'd also be nice to be able to use it as emergency swap, but I think that
might be a little tricky, though if you pin a file, give it a maximum size, and
swap it on you could probably do that.

David

Re: [PATCH] CacheFS - general filesystem cache

[Christoph Hellwig]

On Fri, Aug 27, 2004 at 01:23:03AM +0100, David Howells wrote:
> 
> Hi Linus, Andrew,
> 
> I've packaged my generic filesystem cache filesystem into patches and also
> produced patches for my AFS filesystem to use it. Work is also in progress to
> alter the NFS client use this interface too, and I think the ISO9660
> filesystem could also benefit.
> 
> I've put a tarball of them on my Red Hat webpage because one of them is rather
> large. If you grab:
> 
> 	http://people.redhat.com/~dhowells/cachefs/cachefs-patches.tar.bz2

Umm, a tarball is the worst possible distribution method for patches.
Either inline them in the mail or if they are too large provide a direct
url to the patch.

Re: [PATCH] CacheFS - general filesystem cache

[Martin J. Bligh]

>> I've packaged my generic filesystem cache filesystem into patches and also
>> produced patches for my AFS filesystem to use it. Work is also in progress to
>> alter the NFS client use this interface too, and I think the ISO9660
>> filesystem could also benefit.
>> 
>> I've put a tarball of them on my Red Hat webpage because one of them is rather
>> large. If you grab:
>> 
>> 	http://people.redhat.com/~dhowells/cachefs/cachefs-patches.tar.bz2
> 
> Umm, a tarball is the worst possible distribution method for patches.

Nah, they have rpms as well ... those are even worse ;-)

M.

Re: [PATCH] CacheFS - general filesystem cache

[David Howells]

Troy Benjegerdes <hozer@hozed.org> wrote:
> So is it even remotely feasable to be able to use a cachefs filesystem
> when the 'backing store' (say NFSv4) filesystem is not accessible?

Well, in cachefs's current state:

 (1) That's mostly netfs policy. CacheFS is mainly for serving pages to the
     netfs upon demand. The netfs has to create a cachefs index tree suitable
     for it to be able to find its files. I'm currently arranging things in my
     AFS client and in the NFS client such that these correspond to server
     concepts such as cell address, volume ID, file ID; server IP address,
     file handle. I'm not arranging things as directories and
     files.

 (2) The user can just add or remove caches with mount/umount, even when live;
     so you could find that your inodes that were being backed in the cache
     are suddenly unavailable. Caches could be locked in the mount tree as
     long as a netfs is using them for disconnected mode operation.

 (3) CacheFS does not enforce any complete-file reading which you'd require to
     have disconnected operation. It only cares about pages. This is netfs
     policy.

 (4) CacheFS does not provide pinning facilities, other than active cache
     inodes don't get eaten because there's too little space and we need to
     recycle something.

I'm planning on adding pinning facilities for use with kAFS. I'd like to add
simple disconnected operation facilities too.

> 
> Obviously if the file's not already cached, you lose.

The file, and potentially the nest of directories in which it resides.

David