Date: Fri, 21 Nov 2003 10:08:42 +0000
From: Adam Witney <awitney@sghms.ac.uk>
Subject: Re: Hardware advice
In-reply-to: <Pine.LNX.4.33.0305301046160.31612-100000@css120.ihs.com>
To: "scott.marlowe" <scott.marlowe@ihs.com>
Cc: pgsql-performance <pgsql-performance@postgresql.org>
Message-id: <BBE394AA.28E1D%awitney@sghms.ac.uk>
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On 30/5/03 6:17 pm, "scott.marlowe" <scott.marlowe@ihs.com> wrote:

> On Fri, 30 May 2003, Adam Witney wrote:
> 
>> Hi scott, 
>> 
>> Thanks for the info
>> 
>>> You might wanna do something like go to all 146 gig drives, put a mirror
>>> set on the first 20 or so gigs for the OS, and then use the remainder
>>> (5x120gig or so ) to make your RAID5.  The more drives in a RAID5 the
>>> better, generally, up to about 8 or 12 as the optimal for most setups.
>> 
>> I am not quite sure I understand what you mean here... Do you mean take 20Gb
>> from each of the 5 drives to setup a 20Gb RAID 1 device? Or just from the
>> first 2 drives?
> 
> You could do it either way, since the linux kernel supports more than 2
> drives in a mirror.  But, this costs on writes, so don't do it for things
> like /var or the pg_xlog directory.
> 
> There are a few ways you could arrange 5 146 gig drives.
> 
> One might be to make the first 20 gig on each drive part of a mirror set
> where the first two drives are the live mirror, and the next three are hot
> spares.  Then you could setup your RAID5 to have 4 live drives and 1 hot
> spare.
> 
> Hot spares are nice to have because they provide for the shortest period
> of time during which your machine is running with a degraded RAID array.
> 
> note that in linux you can set the kernel parameter
> dev.raid.speed_limit_max and dev.raid.speed_limit_min to control the
> rebuild bandwidth used so that when a disk dies you can set a compromise
> between fast rebuilds, and lowering the demands on the I/O subsystem
> during a rebuild.  The max limit default is 100k / second, which is quite
> slow.  On a machine with Ultra320 gear, you could set that to 10 ot 20
> megs a second and still not saturate your SCSI buss.
> 
> Now that I think of it, you could probably set it up so that you have a
> mirror set for the OS, one for pg_xlog, and then use the rest of the
> drives as RAID5.  Then grab space on the fifth drive to make a hot spare
> for both the pg_xlog and the OS drive.
> 
> Drive 0
> [OS RAID1 20 Gig D0][big data drive RAID5 106 Gig D0]
> Drive 1
> [OS RAID1 20 Gig D1][big data drive RAID5 106 Gig D1]
> Drive 2
> [pg_xlog RAID1 20 gig D0][big data drive RAID5 106 Gig D2]
> Drive 3
> [pg_xlog RAID1 20 gig D1][big data drive RAID5 106 Gig D3]
> Drive 4
> [OS hot spare 20 gig][g_clog hot spare 20 gig][big data drive RAID5 106
> Gig hot spare]
> 
> That would give you ~ 300 gigs storage.
> 
> Of course, there will likely be slightly less performance than you might
> get from dedicated RAID arrays for each RAID1/RAID5 set, but my guess is
> that by having 4 (or 5 if you don't want a hot spare) drives in the RAID5
> it'll still be faster than a dedicated 3 drive RAID array.
> 

Hi Scott,

Just following up a post from a few months back... I have now purchased the
hardware, do you have a recommended/preferred Linux distro that is easy to
configure for software RAID?

Thanks again

Adam


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