From: Luke Lonergan <llonergan@greenplum.com>
To: hubert depesz lubaczewski <depesz@gmail.com>
Cc: pgsql-performance@postgresql.org
Subject: Re: Huge Data sets, simple queries
Date: Mon, 30 Jan 2006 11:48:49 -0800
Message-ID: <C003ADA1.1B4A0%llonergan@greenplum.com> (raw)
In-Reply-To: <9e4684ce0601300953kdb9f812h12820410cc3d5580@mail.gmail.com>
Depesz,
On 1/30/06 9:53 AM, "hubert depesz lubaczewski" <depesz@gmail.com> wrote:
>> double the performance on a reasonable number of drives.
>
> how many is reasonable?
What I mean by that is: given a set of disks N, the read performance of RAID
will be equal to the drive read rate A times the number of drives used for
reading by the RAID algorithm. In the case of RAID5, that number is (N-1),
so the read rate is A x (N-1). In the case of RAID10, that number is N/2,
so the read rate is A x (N/2). So, the ratio of read performance
RAID5/RAID10 is (N-1)/(N/2) = 2 x (N-1)/N. For numbers of drives, this
ratio looks like this:
N RAID5/RAID10
3 1.33
6 1.67
8 1.75
14 1.86
So - I think reasonable would be 6-8, which are common disk configurations.
- Luke
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To: pgsql-performance@postgresql.org
Cc: llonergan@greenplum.com, depesz@gmail.com
Subject: Re: Huge Data sets, simple queries
In-Reply-To: <C003ADA1.1B4A0%llonergan@greenplum.com>
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