Date: Wed, 17 Nov 1999 15:15:12 -0800 From: "Scott Hess" <scott@avantgo.com> To: "Greg Lehey" <grog@lemis.com>, <freebsd-fs@FreeBSD.ORG> Subject: Re: vinum, MYSQL, and small transaction sizes. Message-ID: <17e101bf3151$99554ec0$1e80000a@avantgo.com> References: <166101bf3121$76518900$1e80000a@avantgo.com> <19991117172851.06023@mojave.sitaranetworks.com>
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Greg Lehey <grog@mojave.sitaranetworks.com> wrote: > On Wednesday, 17 November 1999 at 9:30:37 -0800, Scott Hess wrote: > > 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". Memory is not an issue, CPU time is not an issue. AFAICT, the disk _is_ the bottleneck, because when I upgrade to faster disks, the tps goes up - both for the single-disk test (155->185), and for the vinum'ed test (80->95). I can't think of another way I'd see those results. > > 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? iostat reports that the average transfer size is 8k. I can't tell for certain what the distribution is, but I am pretty certain it is basically everything at 8k, with a couple 16k transfers (lots of short bits of data). > > 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? 10k 18Gig Seagate disks, on an NCR 875 controller. The disks by themselves kick ass. The disks both being used at the same time kick ass. The disks when used with vinum do not kick ass. Again, I don't expect to double performance, but my experience did lead me to believe we should have added 50% or so with the second disk, perhaps more given the nature of our use. Later, scott To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-fs" in the body of the message
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