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From: Alex Turner <armtuk@gmail.com>
To: Jacques Caron <jc@directinfos.com>
Cc: PFC <lists@boutiquenumerique.com>
Cc: mudfoot@rawbw.com
Cc: pgsql-performance@postgresql.org
Subject: Re: Multiple disks: RAID 5 or PG Cluster
Date: Sun, 19 Jun 2005 00:58:47 -0400
Message-ID: <33c6269f050618215872479c8d@mail.gmail.com> (raw)
In-Reply-To: <6.2.0.14.0.20050618181239.0446d7a8@pop.interactivemediafactory.net>
References: <7a1914222f313b7c5e88bedb8a662f12@implements.be>
	<1119040721.42b334d1c3ec9@webmail.rawbw.com>
	<op.sskpeoazth1vuj@localhost>
	<6.2.0.14.0.20050618181239.0446d7a8@pop.interactivemediafactory.net>

Of course these numbers are not true as soon as you exceed the stripe size 
for a read operation, which is often only 128k. Typically a stripe of 
mirrors will not read from seperate halves of the mirrors either, so RAID 10 
is only N/2 best case in my experience, Raid 0+1 is a mirror of stripes and 
will read from independant halves, but gives worse redundancy.

Alex Turner
NetEconomist

On 6/18/05, Jacques Caron <jc@directinfos.com> wrote:
> 
> Hi,
> 
> At 18:00 18/06/2005, PFC wrote:
> > I don't know what I'm talking about, but wouldn't mirorring be
> > faster
> >than striping for random reads like you often get on a database ? (ie. 
> the
> >reads can be dispatched to any disk) ? (or course, not for writes, but if
> >you won't use fsync, random writes should be reduced no ?)
> 
> Roughly, for random reads, the performance (in terms of operations/s)
> compared to a single disk setup, with N being the number of drives, is:
> 
> RAID 0 (striping):
> - read = N
> - write = N
> - capacity = N
> - redundancy = 0
> 
> RAID 1 (mirroring, N=2):
> - read = N
> - write = 1
> - capacity = 1
> - redundancy = 1
> 
> RAID 5 (striping + parity, N>=3)
> - read = N-1
> - write = 1/2
> - capacity = N-1
> - redundancy = 1
> 
> RAID 10 (mirroring + striping, N=2n, N>=4)
> - read = N
> - write = N/2
> - capacity = N/2
> - redundancy < N/2
> 
> So depending on your app, i.e. your read/write ratio, how much data can be
> cached, whether the data is important or not, how much data you have, etc,
> one or the other option might be better.
> 
> Jacques.
> 
> 
> 
> ---------------------------(end of broadcast)---------------------------
> TIP 9: the planner will ignore your desire to choose an index scan if your
> joining column's datatypes do not match
>

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  Subject: Re: Multiple disks: RAID 5 or PG Cluster
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