On Thu, 2006-03-02 at 01:45 +0100, Jeroen Geilman wrote:
> > With 4 disks you could also build a raid5 with little overhead, takes a
> > tad more cpu and gives you 750G capacity (or 500 with one hotspare)
> >
> Take note that both using RAID5 and RAID10 in software will use a
> significant amount of CPU*;
RAID5:
On a 500Mhz P3 ~35% CPU at 40MB/s (array to array copy)
(4 CPUs in that box --> <10% real load :-) )
1Ghz Athlon ~8% at 12,5MB/s (saturating a 100Mbit line)
2Ghz AMD64 ~1% - never managed to push it beyond that.RAID1 takes about 0,3% CPU on a 1,5Ghz AthlonXP at disk-speed (25-40MB/s) RAID0 takes, as far as I can tell, about the same > normally speaking (in a hardware > configuration) RAID10 would outperform RAID5 by 30% or more, Depends on read patterns etc. - I get about 95% native disk speeds on an AMD64 (3-disk+hotspare) raid5 (~70M/s) where a RAID0 would be limited to about the same (2 disks+ raid1) > but since it's in software the RAID0 has to be layered on top of the RAID1, > increasing its overhead by no small amount. negligible > I'd go with the RAID1 with LVM solution mentioned earlier if you intend > to retain any performance worth mentioning. > > If there are decent Linux drivers for it, I'd highly recommend a RAID > card that can do RAID5 or RAID10 in hardware. That depends a lot. Older 3ware hardware-raid cards are severely CPU-limited compared to an amd64. And if a hardware-based raid fails you have much more trouble (I've seen an 8-disk raid1+0 die horribly as all disks decided to go bad within a day) > *Actually, the RAID10 solution won't use nearly as much CPU as the > RAID5, but the RAID10 will spend a lot more time waiting on disk I/O, so > the net result will likely be similar, if not actually worse. See my performance numbers above It used to be that RAID5 ate CPU for breakfast, but calculating XOR (at 3,5GB/s according to dmesg) for a small 40MB/s stream should be less taxing than watching a DVD :-) Patrick -- Stand still, and let the rest of the universe move
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