[Gluster-devel] Feature proposal - FS-Cache support in FUSE

[Anand Avati]

On Mon, Sep 1, 2014 at 6:07 AM, Vimal A R <arvimal at yahoo.in> wrote:

> Hello fuse-devel / fs-cache / gluster-devel lists,
>
> I would like to propose the idea of implementing FS-Cache support in the
> fuse kernel module, which I am planning to do as part of my UG university
> course. This proposal is by no means final, since I have just started to
> look into this.
>
> There are several user-space filesystems which are based on the FUSE
> kernel module. As of now, if I understand correct, the only networked
> filesystems having FS-Cache support are NFS and AFS.
>
> Implementing support hooks for fs-cache in the fuse module would provide
> networked filesystems such as GlusterFS the benefit of  a client-side
> caching mechanism, which should decrease the access times.
>

If you are planning to test this with GlusterFS, note that one of the first
challenges would be to have persistent filehandles in FUSE. While GlusterFS
has a notion of a persistent handle (GFID, 128bit) which is constant across
clients and remounts, the FUSE kernel module is presented a transient LONG
(64/32 bit) which is specific to the mount instance (actually, the address
of the userspace inode_t within glusterfs process - allows for constant
time filehandle resolution).

This would be a challenge with any FUSE based filesystem which has
persistent filehandles larger than 64bit.

Thanks



> When enabled, FS-Cache would maintain a virtual indexing tree to cache the
> data or object-types per network FS. Indices in the tree are used by
> FS-Cache to find objects faster. The tree or index structure under the main
> network FS index depends on the filesystem. Cookies are used to represent
> the indices, the pages etc..
>
> The tree structure would be as following:
>
> a) The virtual index tree maintained by fs-cache would look like:
>
> * FS-Cache master index -> The network-filesystem indice (NFS/AFS etc..)
> -> per-share indices -> File-handle indices -> Page indices
>
> b) In case of FUSE-based filesystems, the tree would be similar to :
>
> * FS-Cache master index -> FUSE indice -> Per FS indices -> file-handle
> indices -> page indices.
>
> c) In case of FUSE based filesystems as GlusterFS, the tree would as :
>
> * FS-Cache master index -> FUSE indice (fuse.glusterfs) -> GlusterFS
> volume ID (a UUID exists for each volume) - > GlusterFS file-handle indices
> (based on the GFID of a file) -> page indices.
>
> The idea is to enable FUSE to work with the FS-Cache network filesystem
> API, which is documented at '
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/caching/netfs-api.txt
> '.
>
> The implementation of FS-Cache support in NFS can be taken as a guideline
> to understand and start off.
>
> I will reply to this mail with any other updates that would come up whilst
> pursuing this further. I request any sort of feedback/suggestions, ideas,
> any pitfalls etc.. that can help in taking this further.
>
> Thank you,
>
> Vimal
>
> References:
> *
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/caching/fscache.txt
> *
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/caching/netfs-api.txt
> *
> https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/filesystems/caching/object.txt
> * http://people.redhat.com/dhowells/fscache/FS-Cache.pdf
> * http://people.redhat.com/steved/fscache/docs/HOWTO.txt
> * https://en.wikipedia.org/wiki/CacheFS
> * https://lwn.net/Articles/160122/
> * http://www.linux-mag.com/id/7378/
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