Rogier Wolff <R.E.Wolff@xxxxxxxxxxxx> writes: > On Thu, Dec 23, 2010 at 09:40:54AM -0500, Jeff Moyer wrote: >> > In my performance calculations, 10ms average seek (should be around >> > 7), 4ms average rotational latency for a total of 14ms. This would >> > degrade for read-modify-write to 10+4+8 = 22ms. Still 10 times better >> > than what we observe: service times on the order of 200-300ms. >> >> I didn't say it would account for all of your degradation, just that it >> could affect performance. I'm sorry if I wasn't clear on that. > > We can live with a "2x performance degradation" due to stupid > configuration. But not with the 10x -30x that we're seeing now. Wow. I'm not willing to give up any performance due to misconfiguration! >> > > md1 : active raid5 sda2[0] sdd2[3](S) sdb2[1] sdc2[4] >> >> > 39067648 blocks super 1.2 level 5, 512k chunk, algorithm 2 [3/3] >> >> > [UUU] >> >> >> >> A 512KB raid5 chunk with 4KB I/Os? That is a recipe for inefficiency. >> >> Again, blktrace data would be helpful. >> > >> > Where did you get the 4kb IOs from? You mean from the iostat -x >> > output? >> >> Yes, since that's all I have to go on at the moment. >> >> > The system/filesystem decided to do those small IOs. With the >> > throughput we're getting on the filesystem, it better not try to write >> > larger chuncks... >> >> Your logic is a bit flawed, for so many reasons I'm not even going to >> try to enumerate them here. Anyway, I'll continue to sound like a >> broken record and ask for blktrace data. > > Here it is. > > http://prive.bitwizard.nl/blktrace.log > > I can't read those yet... Manual is unclear. OK, I should have made it clear that I wanted the binary logs. No matter, we'll work with what you've sent. > My friend confessed to me today that he determined the "optimal" RAID > block size with the exact same test as I had done, and reached the > same conclusion. So that explains his raid blocksize of 512k. > > The system is a mailserver running on a raid on three of the disks. > most of the IOs are generated by the mail server software through the > FS driver, and the raid system. It's not that we're running a database > that inherently requires 4k IOs. Apparently what the > system needs are those small IOs. The log shows a lot of write barriers: 8,32 0 1183 169.033279975 778 A WBS 481958 + 2 <- (8,34) 8 ^^^ On pre-2.6.37 kernels, that will fully flush the device queue, which is why you're seeing such a small queue depth. There was also a CFQ patch that sped up fsync performance for small files that landed in .37. I can't remember if you ran with a 2.6.37-rc or not. Have you? It may be in your best interest to give the latest -rc a try and report back. Cheers, Jeff -- To unsubscribe from this list: send the line "unsubscribe linux-ide" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
