Subject: Re: PowerEdge 2950 questions
From: Mark Lewis <mark.lewis@mir3.com>
To: Merlin Moncure <mmoncure@gmail.com>
Cc: Jeff Davis <pgsql@j-davis.com>, Bucky Jordan <bjordan@lumeta.com>,
	pgsql-performance@postgresql.org
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Organization: MIR3, Inc.
Date: Thu, 24 Aug 2006 12:28:42 -0700
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> it's worse than that.  if you need to read something that is not in
> the o/s cache, all the disks except for one need to be sent to a
> physical location in order to get the data.  Thats the basic rule with
> striping: it optimizes for sequential i/o in expense of random i/o.
> There are some optimizations that can help, but not much.  caching by
> the controller can increase performance on writes because it can
> optimize the movement across the disks by instituting a delay between
> the write request and the actual write.
> 
> raid 1 (or 1+x) is the opposite.  It allows the drive heads to move
> independantly on reads when combined with some smart algorithms.
> writes however must involve all the disk heads however.  Many
> controllers do not to seem to optimze raid 1 properly although linux
> software raid seems to.
> 
> A 4 disk raid 1, for example, could deliver four times the seek
> performance which would make it feel much faster than a 4 disk raid 0
> under certain conditions.

I understand random mid-sized seeks (seek to x and read 512k) being slow
on RAID5, but if the read size is small enough not to cross a stripe
boundary, this could be optimized to only one seek on one drive.  Do
most controllers just not do this, or is there some other reason that
I'm not thinking of that would force all disks to seek?

-- Mark