On Wed, Mar 14, 2012 at 08:50:02AM -0400, Ted Ts'o wrote: > On Wed, Mar 14, 2012 at 09:12:02AM +0100, Lukas Czerner wrote: > > I kind of like the idea about having the separate btree with inode > > numbers for the directory reading, just because it does not affect > > allocation policy nor the write performance which is a good thing. Also > > it has been done before in btrfs and it works very well for them. The > > only downside (aside from format change) is the extra step when removing > > the directory entry, but the positives outperform the negatives. > > Well, there will be extra (journaled!) block reads and writes involved > when adding or removing directory entries. So the performance on > workloads that do a large number of directory adds and removed will > have to be impacted. By how much is not obvious, but it will be > something which is measurable. Most of the difference in btrfs rm times comes from our double indexing. We store the directory name 3 times, once for each dir index and once in the inode backref. But the inode backref doesn't generate seeks the same way the extra directory index does, so it is probably only 10% of the cost. We also delete file data crcs during rm, so if you're benchmarking things to see how much it will hurt, benchmark btrfs on zero length files. In terms of other costs, most users of readdir will eventually wander off and stat or open the files in the dir. With your current indexes, the stat reads the same directory blocks that readdir used. With the double index, the stat reads the name index instead of the readdir index. For the common case of normal directories, today's ext4 will have the directory blocks in cache when stat is called. Btrfs has to do extra reads to pull in the name index. Josef worked around this by prefilling a dentry during readdir with details on where to find the inode. Either way, I'd definitely recommend sitting down with the xfs directory indexes and look at how they manage locality without the double indexing. It's faster than btrfs and without the penalty. It's also worth mentioning that on an SSD, there is no benefit at all to the extra index. So when the industry finally floods the markets with hybrid PCM shingled drives, this might all be moot. -chris -- 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
