On Fri, May 17, 2013 at 8:29 AM, Merlin Moncure <mmonc...@gmail.com> wrote: > On Fri, May 17, 2013 at 4:47 AM, Andres Freund <and...@2ndquadrant.com> wrote: >> On 2013-05-15 16:46:33 -0500, Jon Nelson wrote: >>> > * Is wal file creation performance actually relevant? Is the performance >>> > of a system running on fallocate()d wal files any different? >>> >>> In my limited testing, I noticed a drop of approx. 100ms per WAL file. >>> I do not have a good idea for how to really stress the WAL-file >>> creation area without calling pg_start_backup and pg_stop_backup over >>> and over (with archiving enabled). >> >> My point is that wal file creation usually isn't all that performance >> sensitive. Once the cluster has enough WAL files it will usually recycle >> them and thus never allocate new ones. So for this to be really >> beneficial it would be interesting to show different performance during >> normal running. You could also check out of how many extents a wal file >> is made out of with fallocate in comparison to the old style method >> (filefrag will give you that for most filesystems). > > But why does it have to be *really* beneficial? We're already making > optional posix_fxxx calls and fallocate seems to do exactly what we > would want in this context. Even if the 100ms drop doesn't show up > all that often, I'd still take it just for the defragmentation > benefits and the patch is fairly tiny.
Here is sample output of filefrag on a somewhat busy database from our testing environment that exactly duplicates our production workloads.. It does a lot of batch processing at night and a mix of 80%oltp 20% olap during the day. This is on ext3. Interestingly, on ext4 servers I never saw more than 2 extents per file (but those servers are mostly not as busy). [root@rpisatysw001 pg_xlog]# filefrag * 00000001000006D200000064: 490 extents found, perfection would be 1 extent 00000001000006D200000065: 33 extents found, perfection would be 1 extent 00000001000006D200000066: 43 extents found, perfection would be 1 extent 00000001000006D200000067: 71 extents found, perfection would be 1 extent 00000001000006D200000068: 43 extents found, perfection would be 1 extent 00000001000006D200000069: 156 extents found, perfection would be 1 extent 00000001000006D20000006A: 52 extents found, perfection would be 1 extent 00000001000006D20000006B: 108 extents found, perfection would be 1 extent merlin -- Sent via pgsql-hackers mailing list (email@example.com) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers