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SubjectRe: [PATCH 0/7] OMFS filesystem version 3
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> > There are also plans to add some sort of cache coherency protocol,
> > where the filesystem can asynchronously call back to fuse to
> > invalidate data or metadata.
>
> Great!
>
> I suggest adding another option (as well) where the filesystem can ask
> fuse to send it synchronous validation requests - some things require
> that. (In my own work, the choice of A->B async invalidation and B->A
> synchronous validation is heuristic: some usage patterns benefit from
> one, some from the other.)

Yes, that makes sense. I expect this could be done by extending the
existing requests with a flag saying the result is already cached.
And then the filesystem can either reply with a special "cached data
is valid" code, or it can reply normally with the fresh data.

> > > Otherwise I don't see how the kernel could coherently cache file pages
> > > for some kinds of FUSE filesystems. (E.g. sshfs, for example: every
> > > operation must surely invoke a user space request or involve granting
> > > a caching right to the kernel, to keep accesses coherent with other
> > > users of the same remote files).
> > >
> > > Ergo, either its not coherent, or there is some coherency protocol,
> > > which does require _some_ work in the user space implementation.
> >
> > Sshfs is not coherent (but neither is NFS), it just has timeouts for
> > caches and invalidation based on modification time.
>
> Fwiw, I think NFS version 4 is coherent (it uses leases), and older
> NFS should be coherent when you use fcntl file locks (it's not very
> efficient though).
>
> I have been bitten a few times by timeout based caches in the past
> (NFS and SMB (pre-oplock)). Simple things like editing a file, then
> running "ssh compiler-box make" from the editor quietly building
> incorrect code - and even subsequent make commands don't fix it. Or
> when I edit a file, then tell someone I've changed the file - and then
> they edit the file, and my edits are lost. Very annoying. Nobody
> should build those kind of caches into new software. :-)

Oh well, you can turn off caching if it bothers you :) OTOH it would
be rather hard (and probably against the point) to try to extend the
sftp protocol to handle cache coherency. Sshfs is not meant to be a
normal filesystem (although some people are trying to use it for home
directories and such), just a simple way to access remote files.

Miklos


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