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04-19-2008, 07:14 AM
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 Re: Can anyone explain this pgbench results? Jim C. Nasby wrote: > Well, the problem is that you're using RAID5, which has a huge write > overhead. You're unlikely to get good performance with it. Apparently. But I had no idea that the performance hit would be that big. Running bonnie or copying a large file with dd show that the card can do 30-50 MB/sec. Running a large update on my postgresql database however, show a throughtput of ~ 2MB/sec, doing between ~ 2500 - 2300 writes/second (avarage). with an utilisation of almost always 100%, and large await times ( almost always > 700), large io-wait percentages (>50%), all measured with iostat. > Also, it sounds like sda and sdb are not mirrored. If that's the case, > you have no protection from a drive failure taking out your entire > database, because you'd lose pg_xlog. > > If you want better performance your best bets are to either > setup RAID10 or if you don't care about the data, just go to RAID0. Because it is just my development machine I think I will opt for the last option. More diskspace left. Groeten, Joost Kraaijeveld Askesis B.V. Molukkenstraat 14 6524NB Nijmegen tel: 024-3888063 / 06-51855277 fax: 024-3608416 e-mail: J.Kraaijeveld@Askesis.nl web: www.askesis.nl ---------------------------(end of broadcast)--------------------------- TIP 9: In versions below 8.0, the planner will ignore your desire to choose an index scan if your joining column's datatypes do not match      |
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04-19-2008, 07:14 AM
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| Re: Can anyone explain this pgbench results? On Tue, Mar 07, 2006 at 09:15:37PM +0100, Joost Kraaijeveld wrote: > Jim C. Nasby wrote: > > Well, the problem is that you're using RAID5, which has a huge write > > overhead. You're unlikely to get good performance with it. > Apparently. But I had no idea that the performance hit would be that big. > > Running bonnie or copying a large file with dd show that the card can do 30-50 MB/sec. Running a large update on my postgresql database however, show a throughtput of ~ 2MB/sec, doing between ~ 2500 - 2300 writes/second (avarage). with an utilisation of almost always 100%, and large await times ( almost always > 700), large io-wait percentages (>50%), all measured with iostat. While there are some issues with PostgreSQL not getting as close to the theoretical maximum of a dd bs=8k (you did match the block size to PostgreSQL's page size, right?  ), a bigger issue in this case is thatbetter cards are able to remove much/all of the RAID5 write penalty in the case where you're doing a large sequential write, because it will just blow entire stripes down to disk. This is very different from doing a more random IO. And it's also very possible that if you use a block size that's smaller than the stripe size that the controller won't be able to pick up on that. In any case, RAID0 will absolutely be the fastest performance you can get. -- Jim C. Nasby, Sr. Engineering Consultant jnasby@pervasive.com Pervasive Software http://pervasive.com work: 512-231-6117 vcard: http://jim.nasby.net/pervasive.vcf cell: 512-569-9461 ---------------------------(end of broadcast)--------------------------- TIP 1: if posting/reading through Usenet, please send an appropriate subscribe-nomail command to majordomo@postgresql.org so that your message can get through to the mailing list cleanly |
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