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

Wed Sep 3 07:25:57 UTC 2014

David / Dan / Anand,

Thank you for all the suggestions. I will make a note of the points, and will get back here if I have anything to report or doubts.

Thanks a lot,

Vimal

On Tuesday, 2 September 2014 8:50 PM, Anand Avati <avati at gluster.org> wrote:

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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