Re: Software vs Hardware RAID 10?
On Sun, Aug 26, 2007 at 02:01:24AM -0400, Lennart Sorensen wrote:
> Linux software raid (at least for raid 0 and 1 and combinations) is
> often faster than what a hardware raid card can do, and almost certainly
> better than what any fakeraid pulls off (since their drivers are often
> crap at doing the raid in software). raid 5 on the other hand can often
> be done faster by a dedicated xor engine, and since you don't have to do
> as much reading to write you reduce bus bandwidth. Probably the main
> advantage of hardware raid is reduced [b]us usage since only the actual
> desired data goes over the bus, while with software raid, raid 1
> involves twice as much bus traffic to write to both devices.
If the raid card was PCI-e, would it matter?
Worst case for the bus would be data to and from the drive's cache. For
SATA-II, that's 300 MB/s. So to write to two drives at once it takes
600 MB/s. Since each lane of a PCI-e has a raw max rate of 250 MB/s,
each drive needs two lanes to achieve this. So for a 5-port raid card,
it would need to be x8 (300 MB/s x 5 / 250 MB/s = 6). Are they
available? Or would there be contention within the north/south bridge
where all the buss lanes meet?
How do file servers address this?
Doug.
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