On Wed, May 20, 2009 at 11:05:27AM +0200, Miklos Szeredi wrote: > On Tue, 19 May 2009, Valerie Aurora wrote: > > As Jan said, readdir() of read-only unioned file systems works with a > > tmpfs top layer. If you think about it, this is the exact equivalent > > of the version of union mounts which used the in-kernel caching > > approach - except that it's better, because it reuses existing code > > and caches between readdir() calls. Cool, huh? > > Yeah... OTOH tmpfs is probably a way too heavyweight solution for > cases where memory is short, and union mounts would typically be used > on such systems. (Sorry for the delay - I've been on vacation.) Hm, my intuition is that a tmpfs mount would be fairly lightweight in terms of memory - the main overhead over the barebones solution would be one superblock and vfsmount struct per mount. What am I missing? > The big reason why kernel impementation of readdir is hard is that > unswappable kernel memory needs to be used for caching directory > contents while the directory is open. Well, tmpfs does the same, > dentries and inodes are _not_ swappable, and they gobble up memory. That's a good point. It seemed to me that it wouldn't be too difficult to make those entries evictable - drop a reference count and set the ->d_release to mark the directory as needing rebuilding. What do you think? > So where's the advantage over implementing a thin deduplicating and > caching layer for union mounts? > > Thanks, > Miklos -VAL -- 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
