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hardare config question

From: Erik Myllymaki <erik(dot)myllymaki(at)aviawest(dot)com>
To: pgsql-performance(at)postgresql(dot)org
Subject: hardare config question
Date: 2006-04-28 15:37:57
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-performance
This is a question that I also posted on Dell hardware forums, and I realize it 
probably belongs there more than here. But I am thinking someone might have 
some anecdotal information that could help me and this post may help someone 
else down the road.

My PowerEdge 1800 (dual 3ghz Xeon, 3GB ram) came with a single SATA drive. I 
installed and developed a modest database application (Postgresql, Windows 2003 
Server, Python) on it and was pretty impressed with the performance.

I wanted to add some more space, speed and reliability so I bought a used 3ware 
9500s SATA raid card. I bought three more of the same drives (Seagate 
ST3808110as) and configured them in RAID 5 (3 in RAID one as hot spare). I 
reinstalled the OS and software (my efforts to ghost were not 
fruitful...another story), and the first thing I did was run a sql script to 
make tables, indexes, sequences etc for my app and import about 20MB of data.

When I had this installed on a single SATA drive running from the PE1800's 
on-board SATA interface, this operation took anywhere from 65-80 seconds.

With my new RAID card and drives, this operation took 272 seconds!?

I did a quick search and found a few posts about how RAID 5 with Databases is a 
poor choice and one in particular about Postgres and how you could expect 
performance to be halved with RAID 5 over a single drive or RAID 1 (until you 
get above 6 disks in your RAID 5 array). So, a poorly planned configuration on 
my part.

I scrubbed the RAID config, made two RAID 1 containers (also read about how 
moving database logs to a different partition than the database data is optimal 
for speed and reliability). I installed the OS on the first RAID 1 volume, the 
Postgresql apps and data on the other, and used Junction from sysinternals to 
put the pg_xlogs back on the OS partition (does Postgresql have an easier way 
to do this on Windows?).

Well, things didn't improve noticeably - 265 seconds.

Next step, turn on the 3ware RAID card's write cache (even though I have no 
Battery Backup Unit on the RAID card and am warned about possible data loss in 
the event of power loss).

This helped - down to  172 seconds.

Is this loss in performance just the normal overhead involved when adding a 
raid card - writes now having to go to two drives instead of one? Or maybe is 
the SATA interface on the Dell 1800s motherboard faster than the interface on 
the 3ware raid card (SATA II ?).

Thanks for any help you guidance you can provide.


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