Rogier Wolff <R.E.Wolff@xxxxxxxxxxxx> writes: > On Wed, Dec 22, 2010 at 11:27:20AM -0500, Jeff Moyer wrote: >> Rogier Wolff <R.E.Wolff@xxxxxxxxxxxx> writes: >> >> > Unquoted text below is from either me or from my friend. >> > >> > >> > Someone suggested we try an older kernel as if kernel 2.6.32 would not >> > have this problem. We do NOT think it suddenly started with a certain >> > kernel version. I was just hoping to have you kernel-guys help with >> > prodding the kernel into revealing which component was screwing things >> > up.... >> [...] >> > ata3.00: ATA-8: WDC WD10EARS-00Y5B1, 80.00A80, max UDMA/133 >> >> This is an "Advanced format" drive, which, in this case, means it >> internally has a 4KB sector size and exports a 512byte logical sector >> size. If your partitions are misaligned, this can cause performance >> problems. > > This would mean that for a misalgned write, the drive would have to > read-modify-write every super-sector. > > 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. > > 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. > I have benchmarked my own "high bandwidth" raid arrays. I benchmarked > them with 128k, 256, 512 and 1024k blocksize. I got the best > throughput (for my benchmark: dd if=/dev/md0 of=/dev/null bs=1024k) > with 512k blocksize. (and yes that IS a valid benchmark for my > usage of the array.) Sorry, I'm not sure I understand how this is relevant. I thought we were troubleshooting a problem on someone else's system. Further, the window into the workload we saw via iostat definitely shows that smaller I/Os are issued. Anyway, it will be much easier to debate the issue once the blktrace data is gathered. Happy holidays. -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
