> Check fscache (or cachefs), right now it has the 'problem' that it > considers all cached pages as purgeable, but David Howell's did express > some interest in adding the ability to pin pages until the fs has > written back the data or at least he will probably consider accepting > patches to that extent. OK. It's not clear to me who is the 'owner' of a page. Is it the page cache or cachefs? So what exectly happens when I update a page in fscache? Is it copied to an cachefs page and then later written back to disk, or is it immediately sent to disk? > Effectively both clean and dirty pages would get backed on disk by > fscache, this might be advantageous especially if FUSE has to pull the > data off of some slow medium or from the network. That sounds good, though I think it would be better if caching could be made optional. Many userspace filesystems are not network filesystems, and often recreating the data is cheaper than reading back from a disk cache. > If the dirty pages can > get pinned, then a write would work as follows, > > writepage > update (and pin) page in the fscache > queue (async) write request to userspace > > Now as far as the kernel is concerned once writepage returns the page is > clean and can be dropped. Userspace can then use a downcall to pull the > page out of the fscache and unpin it. I don't know if fscache/cachefs > has a filesystem like interface which would allow access to the page > directly with open/seek/read operations without going back through the > FUSE kernel module. It sounds to me cleaner if cached data is always accessed through FUSE, so I don't think such an interface is neccessary, nor that it would improve performance. Thanks, Miklos
