Vinum RAID5 speed question

[Greg 'groggy' Lehey]

On Friday, 26 September 2003 at 13:22:09 +0200, Martin Brecher wrote:
> Hi, -
>
> I plan on setting up a vinum RAID5 array of three 120GB IDE disks. The
> disks will be attached to two Promise 100 TX2 controllers, on which two
> other disks are residing, too.
>
> The following questions have come to my mind:
>
> 1. I thought about getting disks with 8MB cache. Does the bigger cache
> size affect performance in a RAID5 scenario?

Possibly.

> 2. Currently I'm running a RAID5 with 4 older 5400rpm disks in that
> machine, and write speeds are around 3MB/s. I guess that several people
> here are running vinum RAID5 with current IDE disks, so what throughput
> can I expect from the new planned setup?

Difficult to say.  Possibly a little more.

> 3. The volume is meant for storing all kinds of files with all kinds of
> sizes. My current setup has a blocksize of 489kb. From other people's
> experience, what blocksize might bring the best results for such general
> purpose?

That probably bases on a recommendation I made some years ago, before
people pointed out the errors of my ways.  You'll get fractionally
(and possibly not measurably) better performance if the stripe size is
a multiple of the block size.  Since that's 16 kB nowadays, a stripe
size of 496 kB might be better.
 
> 4. Any other advice would be welcome, of course. :-)

Well, the obvious one is that RAID-5 is not intended for writing :-)

Greg
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