On 11/18/13, 10:02 AM, Martin Boutin wrote: > Dear list, > > I am writing about an apparent issue (or maybe it is normal, that's my > question) regarding filesystem write speed in in a linux raid device. > More specifically, I have linux-3.10.10 running in an Intel Haswell > embedded system with 3 HDDs in a RAID-5 configuration. > The hard disks have 4k physical sectors which are reported as 512 > logical size. I made sure the partitions underlying the raid device > start at sector 2048. (fixed cc: to xfs list) > The RAID device has version 1.2 metadata and 4k (bytes) of data > offset, therefore the data should also be 4k aligned. The raid chunk > size is 512K. > > I have the md0 raid device formatted as ext3 with a 4k block size, and > stride and stripes correctly chosen to match the raid chunk size, that > is, stride=128,stripe-width=256. > > While I was working in a small university project, I just noticed that > the write speeds when using a filesystem over raid are *much* slower > than when writing directly to the raid device (or even compared to > filesystem read speeds). > > The command line for measuring filesystem read and write speeds was: > > $ dd if=/tmp/diskmnt/filerd.zero of=/dev/null bs=1M count=1000 iflag=direct > $ dd if=/dev/zero of=/tmp/diskmnt/filewr.zero bs=1M count=1000 oflag=direct > > The command line for measuring raw read and write speeds was: > > $ dd if=/dev/md0 of=/dev/null bs=1M count=1000 iflag=direct > $ dd if=/dev/zero of=/dev/md0 bs=1M count=1000 oflag=direct > > Here are some speed measures using dd (an average of 20 runs).: > > device raw/fs mode speed (MB/s) slowdown (%) > /dev/md0 raw read 207 > /dev/md0 raw write 209 > /dev/md1 raw read 214 > /dev/md1 raw write 212 > > /dev/md0 xfs read 188 9 > /dev/md0 xfs write 35 83 > > /dev/md1 ext3 read 199 7 > /dev/md1 ext3 write 36 83 > > /dev/md0 ufs read 212 0 > /dev/md0 ufs write 53 75 > > /dev/md0 ext2 read 202 2 > /dev/md0 ext2 write 34 84 > > Is it possible that the filesystem has such enormous impact in the > write speed? We are talking about a slowdown of 80%!!! Even a > filesystem as simple as ufs has a slowdown of 75%! What am I missing? One thing you're missing is enough info to debug this. /proc/mdstat, kernel version, xfs_info output, mkfs commandlines used, partition table details, etc. If something is misaligned and you are doing RMW for these IOs it could hurt a lot. -Eric > Thank you, > -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
