Re: Software vs Hardware RAID 10?
On Thu, Aug 30, 2007 at 10:37:50AM +0200, Goswin von Brederlow wrote:
> If you write a single block then you have to read the 2 parity blocks
> for the stripe, update them and write 3 blocks. So you have 5 times
> the traffic on the bus. For anything doing single block writes this is
> quite the killer. I would imagine a database like oracle to just die
> with software raid6.
I especially avoided non-stripe-aligned writes because if they make up
any noticable portion of your I/O load then your RAID is probably poorly
configured and you loose anyway. For serious database use IMHO only
RAID1/RAID10 should be used over as much physical disks as possible, and
if your workload is read-intensive then even RAID10 with many copies
(not just 2) may be desirable.
> With 6 disks software raid1 and raid10 on some layouts will only read
> from 3 disks. Raid6 on the other hand reads from 4 disks. So you get
> an extra boost.
Ok, that is true. However, for SW RAID6, if the read is not
stripe-aligned it will go through the stripe cache, meaning the data
will be copied in memory one more time than for RAID1/RAID10, meaning
more pressure on the CPU cache. And for any writes RAID6 loses.
Gabor
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