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Re: OFA Architecture with RAID

From: Jonathan Lewis <jonathan_at_jlcomp.demon.co.uk>
Date: Sun, 28 Feb 1999 12:26:45 -0000
Message-ID: <920204831.8334.0.nnrp-07.9e984b29@news.demon.co.uk>


An interesting point on RAID-5 is that some physical implementations have a pretty low overhead for large writes.

The 'write' overhead comes largely from the need to read the parity block, XOR out the previous value for the changed block, XOR in the current value, and then write the parity block back.

Imagine a RAID-5 device on 5 discs, where the stripe size is 16K per disc. If you write (the right) 64K, then some implementations will be able to recognise a full stripe, throw away the old parity block, then calculate and write the new parity block just once. Net overhead is only 25% on writes.

It's worth testing on your own hardware, with your version of the O/S and Oracle, but it is possible that the right number of discs and the correct choice of stripe size may give you a data load time which is not affected to dramatically by using Raid5

Jonathan Lewis
Yet another Oracle-related web site: www.jlcomp.demon.co.uk

Steve Phelan wrote in message
<36D92489.D4FFC9_at_toneline.n-o-s-p-a-m.demon.co.uk>...
>In the case of a large datawarehouse style database I find that RAID5
really does have a place.
>Sure, you accept that the weekly (and/or monthly) update is going to be
slower, but you live with
>that. What you get in return is a good degree of fault-tolerance for a
reasonably cheap price.
>
>Of course a two or three way real mirror (RAID0) is the best route (and you
can even break a mirror
>for backups), but that means 2 or 3 times the cost of the disk for you
database, which can be large
>for a large database. I think RAID5 is good in these situations where you
just don't have that sort
>of budget.
Received on Sun Feb 28 1999 - 06:26:45 CST

Original text of this message

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