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Re: [K12OSN] Linux "Software RAID"



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 gmail com


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