On Saturday 07 October 2006 02:38, Richard Broersma Jr wrote:
> I thought it might be nice to recap the characteristics of the possible
> RAID configurations. Please feel free to correct any points that are
> incorrect or inaccurate.
> (4) Disk RAID5:
>
> Total Array Capacity: 750G
> Reading Throughput: <= 2 x a single drive read cap. (similar to RAID0)
> 1 file can be read at-most 1 time
> or 4 separate file can be read simultaneously
> Write Throughput: <= 1 x a single drive write cap.
> Fault tolerance: can survive 1 disk failures
I'm not sure of the raid 5 figures. For very big writes, i.e., lot's of
full
stripes, you'll get up to 3x write performance ( if your controller can write
to all disk in paralell and is smart enough to calculate parity in memory ),
but for small writes ( i.e, directory entries or DB updates ), you'll get
about 0.5x ( exact figure is difficult, but for a single sector update a raid
5 has to make 2 reads ( old sector, old parity ), then recalculate new parity
as and then two writes AFTER them ( new sector, new parity ). Also read
performance should be <= 3x if you give <= 4x for raid 0.
F.O.S.
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