From: Gregory Leblanc Subject: Re: Tiobench results LOWER with more threads Date: Thu, 17 Oct 2002 19:31:37 -0700 Sender: linux-raid-owner@vger.kernel.org Message-ID: <20021018023137.GC18294@peeps.cable.rcn.com> References: <3DABCFE7.9070701@webmail.co.za> <20021017215014.GA7875@unthought.net> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20021017215014.GA7875@unthought.net> To: Vladimir Milovanovic , linux-raid List-Id: linux-raid.ids On Thu, Oct 17, 2002 at 11:50:14PM +0200, Jakob Oestergaard wrote: > On Tue, Oct 15, 2002 at 10:20:55AM +0200, Vladimir Milovanovic wrote: > > OK, just joined the list and rad the faq, and something caught my eye. > > Tiobench results are apparently supposed to INCREASE when there are more > > threads. > > No, what gave you that idea? > > It is so much easier for the kernel to handle one sequential stream of > I/O, instead of many streams. > > If you have more than one stream, you need to seek. Seeking is bad. One > sequential I/O is almost always (with the notable exception of RAID-1 > reads) faster in total sustained throughput, and always (as in really > always) faster in per-thread sustained throughput. Err, so are you saying that a single sequential I/O is slower on RAID 1 when compared with a single disk? That doesn't make a lot of sense. There -are- instances where parallel I/O is required. In these cases, any RAID-1 should be much faster than a single disk, as should RAID 10. I'm not sure that RAID 5 should give a similar benefit, but given the cost of disks, I don't care about RAID 5. > > Celeron 633 > > 196 MB PC 133 > > Adaptec 29160 SCSI controller (PCI) > > 5 IBM Ultrastar 18XP (18gig, SCSI-3) disks hanging off the Adaptec > > controller > > Red Hat 7.3 Linux (2.4.18-3) > > > > Experimenting with different RAID configurations, I have found that I > > can not get more than 32 MB/s from this array with 4 disks, one spare. I > > have actually found out that the disks set the SCSI bus at 40 MB/s > > (since the disks are old) and that in RAID 0 it scales well, the speed > > doubles for two disks, and then the third disk brings in a little more > > performance, and then things topp off at 32 MB/s with four disks. Adding > > the fifth disk gains no extra performance. Interesting. How fast are you getting single-threaded reads from a single disk? And what are the real specs on the drives (RPM, cache, SCSI version support (SCSI 3 is a family, not a version)). > > Apparently VIA chipsets have problems with PCI bursting, so that is why > > I can't see the full 40 MB/s. That's fine. > > There's some SCSI overhead as well. And probably you have some RAM > bandwidth limitation also - although that is probably not very important > at the speed you're seeing. But it all adds up. Yeah, if this is a 40MB/sec bus, you shouldn't expect to get more than 35MB/sec out of the bus. It'd be slick to get more, but overhead eats the rest. Greg