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New hardware thoughts

From: Ben Suffolk <ben(at)vanilla(dot)net>
To: pgsql-performance(at)postgresql(dot)org
Subject: New hardware thoughts
Date: 2006-10-20 07:49:22
Message-ID: 0D8823B6-2A2D-40BA-B207-9DA0D50EDFB6@vanilla.net (view raw or flat)
Thread:
Lists: pgsql-performance
Hello all,

I am currently working out the best type of machine for a high volume  
pgsql database that I going to need for a project. I will be  
purchasing a new server specifically for the database, and it won't  
be running any other applications. I will be using FreeBSD 6.1 Stable.

I think it may be beneficial if I give a brief overview of the types  
of database access. There are several groups of tables and associated  
accesses to them.

The first can be thought of as users details and configuration  
tables. They will have low read and write access (say around 10 - 20  
a min). SIzed at around 1/2 Million rows.

The second part is logging, this will be used occasionally for reads  
when reports are run, but I will probably back that off to more  
aggregated data tables, so can probably think of this as a write only  
tables. Several table will each have around 200-300 inserts a second.  
The can be archived on a regular basis to keep the size down, may be  
once a day, or once a week. Not sure yet.

The third part will be transactional and will have around 50  
transaction a second. A transaction is made up of a query followed by  
an update, followed by approx 3 inserts. In addition some of these  
tables will be read out of the transactions at approx once per second.

There will be around 50 simultaneous connections.

I hope that overview is a) enough and b) useful background to this  
discussion.

I have some thoughts but I need them validating / discussing. If I  
had the money I could buy the hardware and sped time testing  
different options, thing is I need to get this pretty much right on  
the hardware front first time. I'll almost certainly be buying Dell  
kit, but could go for HP as an alternative.

Processor : I understand that pgsql is not CPU intensive, but that  
each connection uses its own process. The HW has an option of upto 4  
dual core xeon processors. My thoughts would be that more lower spec  
processors would be better than fewer higher spec ones. But the  
question is 4 (8 cores) wasted because there will be so much blocking  
on I/O. Is 2 (4 cores) processors enough. I was thinking 2 x 2.6G  
dual core Xeons would be enough.

Memory : I know this is very important for pgsql, and the more you  
have the more of the tables can reside in memory. I was thinking of  
around 8 - 12G, but the machine can hold a lot more. Thing is memory  
is still quite expensive, and so I don't to over spec it if its not  
going to get used.

Disk : Ok so this is the main bottleneck of the system. And the thing  
I know least about, so need the most help with. I understand you get  
good improvements if you keep the transaction log on a different disk  
from the database, and that raid 5 is not as good as people think  
unless you have lots of disks.

My option in disks is either 5 x 15K rpm disks or 8 x 10K rpm disks  
(all SAS), or if I pick a different server I can have 6 x 15K rpm or  
8 x 10K rpm (again SAS). In each case controlled by a PERC 5/i (which  
I think is an LSI Mega Raid SAS 8408E card).

So the question here is will more disks at a slower speed be better  
than fewer disks as a higher speed?

Assuming I was going to have a mirrored pair for the O/S and  
transaction logs that would leave me with 3 or 4 15K rpm for the  
database, 3 would mean raid 5 (not great at 3 disks), 4 would give me  
raid 10 option if I wanted it.  Or I could have raid 5 across all 5/6  
disks and not separate the transaction and database onto different  
disks. Better performance from raid 5 with more disks, but does  
having the transaction logs and database on the same disks  
counteract / worsen the performance?

If I had the 8 10K disks, I could have 2 as a mirrored pair for O/S  
Transaction, and still have 6 for raid 5. But the disks are slower.

Anybody have any good thoughts on my disk predicament, and which  
options will serve me better.

Your thoughts are much appreciated.

Regards

Ben







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