> > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
