DRBD and Postgres: how to improve the perfomance?

Hello.
We have made some performance tests with DRBD and Postgresql 8.2.3. We
have two identical servers in a cluster (Dell 2950) with a partition of
100 GB managed by DRBD: once we checked Postgres keeping his data folder
in a local partition, the second time we moved the data folder in the
shared partition. The two servers are connected point to point using a
cross cable to reduce their latency.
The partition is mounted with the option noatime in order to not update
the inode access time in case of read access.
We used pgbench for the testings, creating a dabase of about 3GB with a
scale of 200. After we perfomed 10 tests for each configuration,
simulating the usage of 100 clients with 500 transactions each.

DRBD configuration:
--------------------------------------------------------------------------------------------
resource drbd0 {

protocol C;
incon-degr-cmd "halt -f";

on db-node1 {
device /dev/drbd0;
disk /dev/sda2;
address 10.0.0.201:7788;
meta-disk internal;
}

on db-node2 {
device /dev/drbd0;
disk /dev/sda2;
address 10.0.0.202:7788;
meta-disk internal;
}
syncer {
rate 700000K;
}
}
--------------------------------------------------------------------------------------------

Pgbench

--------------------------------------------------------------------------------------------
pgbench -i pgbench -s 200
pgbench -c 100 -t 500 pgbench
--------------------------------------------------------------------------------------------

The results were that the TPS (transaction per second) with Postgres
running in the local partition is almost double than the one with the DRDB:

Postgres in shared DRBD partition: 60.863324 TPS
Postgres in local partition: 122.016138 TPS

Obviously, working with the database in DRBD, we had two writes instead
of only one but we are a bit disappointed about the low results. We
would like to know if there is any way to improve the performance in
order to have a 3/4 rate instead of the 1/2 one.

We would really appreciate it if you could give us some feedback.

Thank you in advance,
Maila Fatticcioni

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Maila Fatticcioni
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Mediterranean Broadband Infrastructure s.r.l.
ITALY
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