Re: Multiple disks: RAID 5 or PG Cluster

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.