Re: Questions about RAID 6
On Wed April 28 2010 15:10:32 Stan Hoeppner wrote:
> Mike Bird put forth on 4/28/2010 1:48 PM:
> > I've designed commercial database managers and OLTP systems.
>
> Are you saying you've put production OLTP databases on N-way software RAID
> 1 sets?
No. I've used N-way RAID-1 for general servers - mail, web, samba, etc.
Nevertheless N-way RAID-1 would be a reasonable basis for a small OLTP
database as the overwhelming majority of OLTP disk transfers are reads.
> > If CPU usage had ever become a factor in anything I had designed
> > I would have been fired. If they're not I/O bound they're useless.
>
> That's an odd point to make given that we're discussing N-way RAID 1. By
> using N-way RAID 1, you're making the system I/O bound before you even
> create the db.
You had claimed that "on a loaded system, such as a transactional database
server or busy ftp upload server, such a RAID setup will bring the system to
its knees in short order as the CPU overhead for each 'real' disk I/O is now
increased 4x and the physical I/O bandwidth is increased 4x".
Your claim is irrelevant as neither CPU utilisation nor I/O bandwith are
of concern in such systems. They are seek-bound.
> Given the way most database engines do locking, you'll get zero additional
> seek benefit on reads, and you'll take a 4x hit on writes. I don't know
> how you could possibly argue otherwise.
Linux can overlap seeks on multiple spindles, as can most operating
systems of the last fifty years.
--Mike Bird
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