On Fri, May 25, 2012 at 04:46:51PM -0400, Mike Snitzer wrote: > I'd love to see the merge_bvec stuff go away but it does serve a > purpose: filesystems benefit from accurately building up much larger > bios (based on underlying device limits). XFS has leveraged this for > some time and ext4 adopted this (commit bd2d0210cf) because of the > performance advantage. That commit only talks about skipping buffer heads, from the patch description I don't see how merge_bvec_fn would have anything to do with what it's after. > So if you don't have a mechanism for the filesystem's IO to have > accurate understanding of the limits of the device the filesystem is > built on (merge_bvec was the mechanism) and are leaning on late > splitting does filesystem performance suffer? So is the issue that it may take longer for an IO to complete, or is it CPU utilization/scalability? If it's the former, we've got a real problem. If it's the latter - it might be a problem in the interim (I don't expect generic_make_request() to be splitting bios in the common case long term), but I doubt it's going to be much of an issue. > Would be nice to see before and after XFS and ext4 benchmarks against a > RAID device (level 5 or 6). I'm especially interested to get Dave > Chinner's and Ted's insight here. Yeah. I can't remember who it was, but Ted knows someone who was able to benchmark on a 48 core system. I don't think we need numbers from a 48 core machine for these patches, but whatever workloads they were testing that were problematic CPU wise would be useful to test. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
