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@directinfos.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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