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. > 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? 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... 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.) Roger. -- ** R.E.Wolff@xxxxxxxxxxxx ** http://www.BitWizard.nl/ ** +31-15-2600998 ** ** Delftechpark 26 2628 XH Delft, The Netherlands. KVK: 27239233 ** *-- BitWizard writes Linux device drivers for any device you may have! --* Q: It doesn't work. A: Look buddy, doesn't work is an ambiguous statement. Does it sit on the couch all day? Is it unemployed? Please be specific! Define 'it' and what it isn't doing. --------- Adapted from lxrbot FAQ -- 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
