Re: raid io requests not parallel?

[Avi Kivity <avi@argo.co.il>]

Jonathan Baccash wrote:
>
> I'm using kernel linux-2.6.15-gentoo-r1, and I noticed performance of
> the software RAID-1 is not as good as I would have expected on my two
> SATA drives, and I was wondering if anyone has an idea what may be
> happening. The test I run is 1024 16k direct-IO reads/writes from
> random locations within a 1GB file (on a RAID-1 partition), with my
> disk caches set to
> write-through mode. In the MT (multi-threaded) case, I issue them from
> 8 threads (so it's 128 requests per thread):
>
> Random read: 10.295 sec
> Random write: 19.142 sec
> MT Random read: 5.276 sec
> MT Random write: 19.839 sec
>
> As expected, the multi-threaded reads are 2x as fast as single-threaded
> reads. But I would have expected (assuming the write to both disks can
> occur in parallel) that the random writes are about the same speed (10
> seconds) as the single-threaded random reads, for both the
> single-threaded and multi-threaded write cases. The fact that the
> multi-threaded reads were
> twice as fast indicates to me that read requests can occur in parallel.
>
> So.... why doesn't the raid issue the writes in parallel? Thanks in
> advance for any help.
>
The writes are issued in parallel, but both disks have to be written.  
Each head has to service 1024 write requests (compared to just 512 read 
requests).


-- 
Do not meddle in the internals of kernels, for they are subtle and quick to panic.