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Re: Is Raid 5 really that bad for Oracle?

From: Daniel Morgan <damorgan_at_x.washington.edu>
Date: Sun, 01 Aug 2004 22:32:06 -0700
Message-ID: <1091424760.302240@yasure>


joe bayer wrote:

> I am quoting from Jonathan Lewis's book, Practical Oracle 8i, page 206
>
> Raid 5 has an undeservedly bad reputation as far as Oracle database systems
> are concerned. ....
> However, for most small systems, it is almost necessary and perfectly
> acceptable; and for many large systems it is totally adequate.
> ....
> The first point is that many Oracle systems do far less writing than they do
> reading, and the writing is usually in the backgroud anyway, so although the
> write penalty is a notional 100%, this in=s not necessarily all that
> significant and overhead to the total I/O operation and many not impact the
> user directly anyway.
>
> Second, although a single , small random write is likely to sustain a heavy
> penalty, RAID 5 suppliers are aware of the issue and have taken steps to
> reduce the problem. Typically a write that fills a whole stripe does not
> need to re-creates the parity as it writes each disk. It simply discards
> and re-creates a new parity block.
>
> Third, it is beoming common practice to stick a reasonably large
> battery-backed cache on the Raid 5 device.

Depends.

RAID 5 is certainly going to be slower doing writes than any of the RAID versions recommended by Oracle. But is the "slower" significant is quite another matter entirely. With most modern technologies such as NetApp and EMC it is irrelevant as we write to a RAM cache not to disk. But given the low cost of disk I think RAID 5 a waste with little redeeming value.

-- 
Daniel A. Morgan
University of Washington
damorgan_at_x.washington.edu
(replace 'x' with 'u' to respond)
Received on Mon Aug 02 2004 - 00:32:06 CDT

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