> I say go with RAID 5, on a controller card.
> Mirroring just gives you backup, and you lose half your diskspace. It
offers
> no performance benefit, and actually the computer might have to work
harder
> to make sure the drives are in sync.
I do not think it is true that mirroring gives no performance benefit (on a
well implemented controller). For reads, the raid controller can read
either copy of the data, so that effectively two reads can be in progress
at the same time, doubling read performance. On the write side, for small
writes a raid 5 has to read the overwritten data (in order to remove it
from the parity) then do a read/modify/write on the parity. Performance
again should be doubled (two writes on the mirrored system, read/overwrite
and read/modify/write on the Raid5 system, with the two halves of each
operation requiring a full rotation between them). For large writes, the
Raid5 system catches up, because the parity can be entirely calculated from
the data sent, so it does not need to do the reads.
I would therefore expect a mirrored system to approach, but not reach,
twice the performance of a Raid5 system - at the cost of nearly doubling
the number of disks. This does depend on appropriate intelligence in the
Raid controller. A badly designed controller can fail to take advantage of
these gains. If you are concerned abut ultimate performance, it would be
well worth benchmarking the actual raid controllers you are considering.
Alec
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