> -----Original Message----- > From: Ilya Dryomov [mailto:idryomov@xxxxxxxxx] > Sent: 10 June 2015 14:06 > To: Nick Fisk > Cc: ceph-users > Subject: Re: krbd splitting large IO's into smaller IO's > > On Wed, Jun 10, 2015 at 2:47 PM, Nick Fisk <nick@xxxxxxxxxx> wrote: > > Hi, > > > > Using Kernel RBD client with Kernel 4.03 (I have also tried some older > > kernels with the same effect) and IO is being split into smaller IO's > > which is having a negative impact on performance. > > > > cat /sys/block/sdc/queue/max_hw_sectors_kb > > 4096 > > > > cat /sys/block/rbd0/queue/max_sectors_kb > > 4096 > > > > Using DD > > dd if=/dev/rbd0 of=/dev/null bs=4M > > > > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz > > avgqu-sz await r_await w_await svctm %util > > rbd0 0.00 0.00 201.50 0.00 25792.00 0.00 256.00 > > 1.99 10.15 10.15 0.00 4.96 100.00 > > > > > > Using FIO with 4M blocks > > Device: rrqm/s wrqm/s r/s w/s rkB/s wkB/s avgrq-sz > > avgqu-sz await r_await w_await svctm %util > > rbd0 0.00 0.00 232.00 0.00 118784.00 0.00 1024.00 > > 11.29 48.58 48.58 0.00 4.31 100.00 > > > > Any ideas why IO sizes are limited to 128k (256 blocks) in DD's case > > and 512k in Fio's case? > > 128k vs 512k is probably buffered vs direct IO - add iflag=direct to your dd > invocation. Yes, thanks for this, that was the case > > As for the 512k - I'm pretty sure it's a regression in our switch to blk-mq. I > tested it around 3.18-3.19 and saw steady 4M IOs. I hope we are just missing > a knob - I'll take a look. I've tested both 4.03 and 3.16 and both seem to be split into 512k. Let me know if you need me to test any other particular version. > > Thanks, > > Ilya _______________________________________________ ceph-users mailing list ceph-users@xxxxxxxxxxxxxx http://lists.ceph.com/listinfo.cgi/ceph-users-ceph.com
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