Ok - well - I am partially wrong...
If you're stripe size is 64Kb, and you are reading 256k worth of data,
it will be spread across four drives, so you will need to read from
four devices to get your 256k of data (RAID 0 or 5 or 10), but if you
are only reading 64kb of data, I guess you would only need to read
from one disk.
So my assertion that adding more drives doesn't help is pretty
wrong... particularly with OLTP because it's always dealing with
blocks that are smaller that the stripe size.
Alex Turner
netEconomist
On 4/18/05, Jacques Caron <jc@directinfos.com> wrote:
> Hi,
>
> At 18:56 18/04/2005, Alex Turner wrote:
> >All drives are required to fill every request in all RAID levels
>
> No, this is definitely wrong. In many cases, most drives don't actually
> have the data requested, how could they handle the request?
>
> When reading one random sector, only *one* drive out of N is ever used to
> service any given request, be it RAID 0, 1, 0+1, 1+0 or 5.
>
> When writing:
> - in RAID 0, 1 drive
> - in RAID 1, RAID 0+1 or 1+0, 2 drives
> - in RAID 5, you need to read on all drives and write on 2.
>
> Otherwise, what would be the point of RAID 0, 0+1 or 1+0?
>
> Jacques.
>
>