Re: [Xen-users] performace of disks

["Rob MacGregor" <rob.macgregor@xxxxxxxxx>]

On Thu, Jul 24, 2008 at 19:49, Victor Hugo dos Santos
<listas.vhs@xxxxxxxxx> wrote:
> On Thu, Jul 24, 2008 at 1:39 PM, Fernando Jiménez Solano
> <fernandojs@xxxxxxxxxxxxxx> wrote:
>
> mmmm.. mmmm.. and mmmm
> but.. all virtual disks aren't one big image for Dom0 ???
> in other words, this type of data in the virtual disks is relevant ??

The problem is that RAID5 (and by extension RAID50) is relatively poor
for writes, but good for reads.  Why?  Because to write to one disk
you have to read from all the others (except one) in the RAID5 group
to compute the parity, to write to that first disk, and then write the
parity to the last disk - this means that each write involves every
other disk (a minimum of 3).  RAID50 reduces the impact because each
group will be smaller, for the same total number of disks.

RAID1 (and by extension RAID10) is good for writes as each write only
involves 2 disks - no reads and only 2 writes.

On the other hand, for the same number of disks RAID5 (and RAID50) can
be faster on sustained reads.  This is because in any disk group the
reads can be spread across more disks, reducing delays.

Others have covered the impact of stripe sizes.

-- 
 Please keep list traffic on the list.

Rob MacGregor
 Whoever fights monsters should see to it that in the process he
 doesn't become a monster. Friedrich Nietzsche

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