Skip site navigation (1) Skip section navigation (2)

Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on

From: Mike Nielsen <miken(at)bigpond(dot)net(dot)au>
To: Bjoern Metzdorf <bm(at)turtle-entertainment(dot)de>
Cc: Postgresql performance <pgsql-performance(at)postgresql(dot)org>
Subject: Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on
Date: 2002-11-21 20:03:46
Message-ID: 1037909025.21282.510.camel@CPE-144-132-182-167.nsw.bigpond.net.au (view raw or flat)
Thread:
Lists: pgsql-adminpgsql-performance
Bjoern,

You may find that hoping for a doubling of performance by using RAID 1
is a little on the optimistic side.

Except on very long sequential reads, media transfer rates are unlikely
to be the limiting factor in disk throughput.  Seek and rotational
latencies are the cost factor in random I/O, and with RAID 1, the
performance gain comes from reducing the mean latency --  on a single
request, one disk will be closer to the data than the other.  If the
software that's handling the RAID 1 will schedule concurrent requests,
you lose the advantage of reducing mean latency in this fashion, but you
can get some improvement in throughput by overlapping some latency
periods.

While not wanting to argue against intelligent I/O design, memory is
cheap these days, and usually gives big bang-for-buck in improving
response times.

As to the specifics of how one level or another of Linux implements RAID
1, I'm afraid I can't shed much light at the moment.

Regards,

Mike
On Fri, 2002-11-22 at 06:24, Bjoern Metzdorf wrote:

> > A mirrored 2x36 setup will probably yield a marginal hit on writes (vs a
> > single disk) and an improvement on reads due to having two drives to read
> > from and will (based on the Scientific Wild Ass Guess method and knowing
> 
> slightly offtopic:
> 
> Does anyone one if linux software raid 1 supports this method (reading from
> both disks, thus doubling performance)?
> 
> Regards,
> Bjoern
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 4: Don't 'kill -9' the postmaster

Michael Nielsen

ph: 0411-097-023 email: miken(at)bigpond(dot)net(dot)au


Mike Nielsen

________________________________________________________________________

In response to

pgsql-performance by date

Next:From: scott.marloweDate: 2002-11-21 20:17:05
Subject: Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on
Previous:From: Tom LaneDate: 2002-11-21 19:41:39
Subject: Re: Query Analysis

pgsql-admin by date

Next:From: scott.marloweDate: 2002-11-21 20:17:05
Subject: Re: [ADMIN] H/W RAID 5 on slower disks versus no raid on
Previous:From: Tom LaneDate: 2002-11-21 19:45:11
Subject: Re: Query performanc issue - too many table?

Privacy Policy | About PostgreSQL
Copyright © 1996-2014 The PostgreSQL Global Development Group