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Date: Sun, 16 Jul 2006 08:46:50 +0300 From: Avi Kivity <avi@...o.co.il> To: Jonathan Baccash <jbaccash@...il.com> CC: linux-kernel@...r.kernel.org Subject: Re: raid io requests not parallel? 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. - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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