Re: Hardware purchase question

Madison Kelly <linux@alteeve.com> writes:

> Without it, specially in a failure state, the performance can collapse as
> the CPU performs all that extra math.

It's really not the math that makes raid 5 hurt. It's that in order to
calculate the checksum block the raid controller needs to read in the existing
checksum block and write out the new version. So every write causes not just
one drive seeking and writing, but a second drive seeking and performing a
read and a write.

The usual strategy for dealing with that is stuffing a huge nonvolatile cache
in the controller so those reads are mostly cached and the extra writes don't
saturate the i/o throughput. But those kinds of controllers are expensive and
not an option for software raid.

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greg