[K12OSN] Linux "Software RAID"
Les Mikesell
lesmikesell at gmail.com
Fri Aug 8 14:56:58 UTC 2008
Rob Owens wrote:
>
>>
>> No, I don't believe so. For one thing, as Dan Young put it, it's much
>> easier to deal with swapping a failed disk out with a dedicated card.
>> That by itself is a *BIG DEAL*. Additionally, if you do have a disk
>> fail, your CPU will take an especially big hit, because then it's got
>> to reconstruct data from the parity info for *all* disk accesses, not
>> just writes. Oops....
>>
> I realize the ease-of-use advantage of hardware RAID, but I really was
> talking about the performance hit only.
The performance hit mostly goes away if you use RAID1 instead of 5 (or
the other direction - raid5 usually hurts performance unless you have 12
or more drives in the set), but then you lose disk space.
> Realistically, a hardware RAID costs about $300, and that could buy me a
> 2nd 4-core processor. I have to believe that a modern 4-core processor
> is way more capable of handling a RAID rebuild than whatever chip is
> onboard a hardware RAID controller. Of course, I haven't done any
> testing...
I'd think in terms of the difference in cost of software raid1 vs.
hardware raid5 instead - that is, the extra disk space for full
redundancy vs parity - and perhaps more drive bays in the server. If you
have extra money in the mix, throwing in more RAM will almost always be
a win in performance since it acts as a disk buffer and will eliminate
the need for a lot of seeks and reads.
> Even though we've debated software vs. hardware RAID on this list many
> times, these discussions always hold my interest.
The technology and pricing makes it a moving target. A lot of
server-class machines will have hardware RAID on the motherboard so it
may not even be an extra-cost item, and some of the newer ones will mix
and match SAS and SATA so you can balance performance and cost on the
same hot-swap backplane.
--
Les Mikesell
lesmikesell at gmail.com
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