Re: Updated ext4/jbd2 patches based on 2.6.19-rc1

[Alex Tomas <alex@clusterfs.com>]

>>>>> Andrew Morton (AM) writes:

 AM> On Fri, 6 Oct 2006 00:41:03 -0600
 AM> Andreas Dilger <adilger@clusterfs.com> wrote:

 >> The big performance win will come with mballoc and delalloc.  CFS has
 >> been using mballoc for a few years already with Lustre, and IBM + Bull
 >> did a lot of benchmarking on it.  The reason it isn't in the first set of
 >> patches is partly a manageability issue, and partly because it doesn't
 >> directly affect the on-disk format (outside of much better allocation)
 >> so it isn't critical to get into the first round of changes.  I believe
 >> Alex is working on a new set of patches right now.

 AM> Are you sure that these things will improve allocation much?  Reservations
 AM> made a big improvement there.

it depends on underlaying storage and workload. mballoc uses buddy
internally. it's much simpler and cheaper to find free 2^N blocks
compared to bitmap. this is especially important for arrays like
DDN and raid5/6 because they require stripe-aligned/-sized requests
for good throughput. also, last mballoc takes logical block into
account and can preallocate few chunks at different logical offsets
for a file. imagine torrent downloading different pieces from few peers.

thanks, Alex