On Tue, May 31, 2011 at 12:18:11PM +0200, Bernd Schubert wrote: > > Out of interest, did anyone ever benchmark if dirindex provides any > advantages to readdir? And did those benchmarks include the > disadvantages of the present implementation (non-linear inode > numbers from readdir, so disk seeks on stat() (e.g. from 'ls -l') or > 'rm -fr $dir')? The problem is that seekdir/telldir is terminally broken (and so is NFSv2 for using a such a tiny cookie) in that it fundamentally assumes a linear data structure. If you're going to use any kind of tree-based data structure, a 32-bit "offset" for seekdir/telldir just doesn't cut it. We actually play games where we memoize the low 32-bits of the hash and keep track of which cookies we hand out via seekdir/telldir so that things mostly work --- except for NFSv2, where with the 32-bit cookie, you're just hosed. The reason why we have to iterate over the directory in hash tree order is because if we have a leaf node split, half the directories entries get copied to another directory entry, given the promises made by seekdir() and telldir() about directory entries appearing exactly once during a readdir() stream, even if you hold the fd open for weeks or days, mean that you really have to iterate over things in hash order. I'd have to look, since it's been too many years, but as I recall the problem was that there is a common path for NFSv2 and NFSv3/v4, so we don't know whether we can hand back a 32-bit cookie or a 64-bit cookie, so we're always handing the NFS server a 32-bit "offset", even though ew could do better. Actually, if we had an interface where we could give you a 128-bit "offset" into the directory, we could probably eliminate the duplicate cookie problem entirely. We just send 64-bits worth of hash, plus the first two bytes of the of file name. > 3) Disable dirindexing for readdirs That won't work, since it will break POSIX compliance. Once again, we're tied by the decisions made decades ago... - Ted -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
