Postgres performance on Veritas VxVM

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Hi,

I'm running Postgres 8.3.7 on Solaris 10 x64, using VxFS/VxVM 5.0 MP3.
I'm trying to determine the best storage configuration for my workload,
so I ran some tests of Postgres performance with various combinations of
VM and filesystem. What I noticed was that when all other factors were
the same, running on VxVM was nearly twice as slow as running on a plain
disk.

My configuration was a VxFS filesystem mounted at /sql, 'noatime,cio',
and another mounted at /sql/pg_xlog,
'noatime,cio,mincache=direct,convosync=direct'. This forced direct I/O
for the WAL. Without VxVM, these filesystems were on plain disk slices.
With VxVM, I added the single disk to a diskgroup and created two
volumes on it for the filesystems, layout=concat, with the default
options.

The backend storage is a 14-disk RAID10 using an internal RAID card (Sun
StorageTek STK-RAID-INT), with 256MB battery-backed write cache. The
system has 32GB RAM and nothing apart from Postgres was running on it
during the test. The same postgresql.conf was used for both tests; I
configured 256MB of shared_buffers and 20 checkpoint_segments.

The benchmark I'm using is a database import tool called "osm2pgsql",
with a subset of our actual workload. Without VxVM, the import
completed in 2 hours, 34 minutes. With VxVM, it took 5 hours 7 minutes.

Has anyone else tested Postgres on VxVM and noticed such a large
performance hit?

- river.
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