Re: Multiple disks: RAID 5 or PG Cluster

Of course these numbers are not true as soon as you exceed the stripe size
for a read operation, which is often only 128k. Typically a stripe of
mirrors will not read from seperate halves of the mirrors either, so RAID 10
is only N/2 best case in my experience, Raid 0+1 is a mirror of stripes and
will read from independant halves, but gives worse redundancy.

Alex Turner
NetEconomist

On 6/18/05, Jacques Caron <jc(at)directinfos(dot)com> wrote:
>
> Hi,
>
> At 18:00 18/06/2005, PFC wrote:
> > I don't know what I'm talking about, but wouldn't mirorring be
> > faster
> >than striping for random reads like you often get on a database ? (ie.
> the
> >reads can be dispatched to any disk) ? (or course, not for writes, but if
> >you won't use fsync, random writes should be reduced no ?)
>
> Roughly, for random reads, the performance (in terms of operations/s)
> compared to a single disk setup, with N being the number of drives, is:
>
> RAID 0 (striping):
> - read = N
> - write = N
> - capacity = N
> - redundancy = 0
>
> RAID 1 (mirroring, N=2):
> - read = N
> - write = 1
> - capacity = 1
> - redundancy = 1
>
> RAID 5 (striping + parity, N>=3)
> - read = N-1
> - write = 1/2
> - capacity = N-1
> - redundancy = 1
>
> RAID 10 (mirroring + striping, N=2n, N>=4)
> - read = N
> - write = N/2
> - capacity = N/2
> - redundancy < N/2
>
> So depending on your app, i.e. your read/write ratio, how much data can be
> cached, whether the data is important or not, how much data you have, etc,
> one or the other option might be better.
>
> Jacques.
>
>
>
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