More Controllers != Higher Through Put

[Chuck Swiger]

Hi--

On Jul 6, 2010, at 4:35 PM, Stephen Sanders wrote:
> I'm wondering if anyone has heard of this.
> 
> I've a system with a 3ware 9650 servicing 4 7200RPM Segate 1TB drives and the motherboard servicing 2 7200 RPM Segate 1TB drives.
> 
> The 4 disk array is RAID 6 while the 2 disk array is RAID 1.  The drives should deliver about 100MB/s.
> 
> 1.  The most the 4 disk array is developing is 250MB/s write performance while the 2 disk array is coming in at 90MB/s write performance.  The 4 disk array seems slow.
> 
> 2. Attempting to write to both arrays simultaneously causes the rate on the 4 disk array to drop to 150MB/s and the 2 disk array drops to 60MB/s
> 
> I'd expect the 4 disk array should look more like 300+MB/s while the 2 disk array is about right.
> 
> I don't get why there should be a 'coupling' between the rates on separate controllers. 


I'd imagine you are contending for memory bandwidth or similar bottleneck when you try to run both drive arrays at full tilt.

As for the other question, if you want good write performance out of 4 drives, use RAID-10, not RAID-5 or -6.  I regard it as normal for RAID-5 to perform slower than even a single drive when doing writes, and RAID-6 should be similar, although I haven't done anywhere near as much benchmarking with RAID-6 as with the other levels...

Regards,
-- 
-Chuck