Date: Wed, 17 Nov 1999 17:28:51 -0500 From: Greg Lehey <grog@mojave.sitaranetworks.com> To: Scott Hess <scott@avantgo.com>, freebsd-fs@FreeBSD.ORG Subject: Re: vinum, MYSQL, and small transaction sizes. Message-ID: <19991117172851.06023@mojave.sitaranetworks.com> In-Reply-To: <166101bf3121$76518900$1e80000a@avantgo.com>; from Scott Hess on Wed, Nov 17, 1999 at 09:30:37AM -0800 References: <166101bf3121$76518900$1e80000a@avantgo.com>
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On Wednesday, 17 November 1999 at 9:30:37 -0800, Scott Hess wrote: > I've been experimenting with vinum striping as a means of improving MYSQL > performance, and am having some odd results. > > Running a particular workload and a particular set of disks, at overload > iostat shows the disk doing about 185 tps, and about 8KB/t. When I run the > workload on a 256k striped volume made up of two drives, I'm finding that > each drive does about 95 tps. I've also run the tests with slower drives, > which do 155 tps for the single-drive test, and 80 tps for the striped > test. > > I didn't expect to double the tps of the entire system - but getting no > increase at all seems very suspect. It's frequently the system's way of saying "the disk is not the bottleneck". > Based on the transaction sizes iostat is reporting, I have tried > restriping with 8k stripes, which gives me about 105 tps per disk, > which is marginally better. Going the other direction, with 1m > stripes, gave the same results as for 256k stripes. I think this is probably a red herring. It's very unlikely that you'll get better performance from an 8k stripe than a 256k stripe. The fact that there's not a significant degradation with such small stripes again points to the likelihood that the disks aren't the bottleneck, though it could also indicate that the transfers are very small (as you indicate in the Subject: line). How big are the transfers? > In an attempt to isolate the problem, I tried cat'ing very large > files in parallel. The files were large enough to not fit in > memory, and I ran four cat commands at the same time on different > files. I found that running them all from a single disk gave 380tps > (24M/s), running 4 on one drive and 4 on the other gave 200tps > (12M/s) for each drive, 400tps (24M/s) aggregate, and running them > on a 256k volume striped across the disks gave 100tps (6M/s) for > each drive, 200tps (12M/s) aggregate. Hmm. The arithmetic at the end suggests that you only striped across 2 disks. What kind of disks are they? You'll run into significant contention problems with IDE, for example. Also, what version of FreeBSD? Greg -- Finger grog@lemis.com for PGP public key See complete headers for address and phone numbers To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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