> > On 3/2/2011 12:13 PM, Jonathan Tripathy wrote: > > I know the IO is only being caused by a "cp -a" command, but the > > issue is why all the reads? It should be 99% writes. cp has to read something before it can write it elsewhere. -- Ray Morris support@bettercgi.com Strongbox - The next generation in site security: http://www.bettercgi.com/strongbox/ Throttlebox - Intelligent Bandwidth Control http://www.bettercgi.com/throttlebox/ Strongbox / Throttlebox affiliate program: http://www.bettercgi.com/affiliates/user/register.php On Wed, 02 Mar 2011 18:00:53 -0500 Dave Sullivan <dsulliva@redhat.com> wrote: > http://sourceware.org/systemtap/examples/ > > look at traceio.stp and disktop.stp > > http://docs.redhat.com/docs/en-US/Red_Hat_Enterprise_Linux/5/html/SystemTap_Beginners_Guide/index.html > > On 03/02/2011 05:19 PM, Bart Kus wrote: > > On 3/2/2011 12:13 PM, Jonathan Tripathy wrote: > >> I once used a tool called dstat. dstat has modules which can tell > >> you which processes are using disk IO. I havenât used dstat in a > >> while so maybe someone else can chime in > > > > I know the IO is only being caused by a "cp -a" command, but the > > issue is why all the reads? It should be 99% writes. Another > > thing I noticed is the average request size is pretty small: > > > > 14:06:20 DEV tps rd_sec/s wr_sec/s avgrq-sz > > avgqu-sz await svctm %util > > [...snip!...] > > 14:06:21 sde 219.00 11304.00 30640.00 191.53 > > 1.15 5.16 2.10 46.00 > > 14:06:21 sdf 209.00 11016.00 29904.00 195.79 > > 1.06 5.02 2.01 42.00 > > 14:06:21 sdg 178.00 11512.00 28568.00 225.17 > > 0.74 3.99 2.08 37.00 > > 14:06:21 sdh 175.00 10736.00 26832.00 214.67 > > 0.89 4.91 2.00 35.00 > > 14:06:21 sdi 206.00 11512.00 29112.00 197.20 > > 0.83 3.98 1.80 37.00 > > 14:06:21 sdj 209.00 11264.00 30264.00 198.70 > > 0.79 3.78 1.96 41.00 > > 14:06:21 sds 214.00 10984.00 28552.00 184.75 > > 0.78 3.60 1.78 38.00 > > 14:06:21 sdt 194.00 13352.00 27808.00 212.16 > > 0.83 4.23 1.91 37.00 > > 14:06:21 sdu 183.00 12856.00 28872.00 228.02 > > 0.60 3.22 2.13 39.00 > > 14:06:21 sdv 189.00 11984.00 31696.00 231.11 > > 0.57 2.96 1.69 32.00 > > 14:06:21 md5 754.00 0.00 153848.00 204.04 > > 0.00 0.00 0.00 0.00 > > 14:06:21 DayTar-DayTar 753.00 0.00 153600.00 > > 203.98 15.73 20.58 1.33 100.00 > > 14:06:21 data 760.00 0.00 155800.00 205.00 > > 4670.84 6070.91 1.32 100.00 > > > > Looks to be about 205 sectors/request, which is 104,960 bytes. > > This might be causing read-modify-write cycles if for whatever > > reason md is not taking advantage of the stripe cache. > > stripe_cache_active shows about 128 blocks (512kB) of RAM in use, > > per hard drive. Given the chunk size is 512kB, and the writes > > being requested are linear, it should not be doing > > read-modify-write. And yet, there are tons of reads being logged, > > as shown above. > > > > A couple more confusing things: > > > > jo ~ # blockdev --getss /dev/mapper/data > > 512 > > jo ~ # blockdev --getpbsz /dev/mapper/data > > 512 > > jo ~ # blockdev --getioopt /dev/mapper/data > > 4194304 > > jo ~ # blockdev --getiomin /dev/mapper/data > > 524288 > > jo ~ # blockdev --getmaxsect /dev/mapper/data > > 255 > > jo ~ # blockdev --getbsz /dev/mapper/data > > 512 > > jo ~ # > > > > If optimum IO size is 4MBs (as it SHOULD be: 512k chunk * 8 data > > drives = 4MB stripe), but maxsect count is 255 (255*512=128k) how > > can optimal IO ever be done??? I re-mounted XFS with > > sunit=1024,swidth=8192 but that hasn't increased the average > > transaction size as expected. Perhaps it's respecting this maxsect > > limit? > > > > --Bart > > > > PS: The RAID6 full stripe has +2 parity drives for a total of 10, > > but they're not included in the "data zone" definitions of stripe > > size, which are the only important ones for figuring out how large > > your writes should be. > > > > _______________________________________________ > > linux-lvm mailing list > > linux-lvm@redhat.com > > https://www.redhat.com/mailman/listinfo/linux-lvm > > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ > > _______________________________________________ > linux-lvm mailing list > linux-lvm@redhat.com > https://www.redhat.com/mailman/listinfo/linux-lvm > read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/ > _______________________________________________ linux-lvm mailing list linux-lvm@redhat.com https://www.redhat.com/mailman/listinfo/linux-lvm read the LVM HOW-TO at http://tldp.org/HOWTO/LVM-HOWTO/
