On Tue, Sep 08, 2009 at 12:47:33PM -0400, Rik van Riel wrote: > Nauman Rafique wrote: > >> I think this is probably the key deal breaker. dm-ioband has no >> mechanism to anticipate or idle for a reader task. Without such a >> mechanism, a proportional division scheme cannot work for tasks doing >> reads. > > That is a really big issue, since most reads tend to be synchronous > (the application is waiting for the read), while many writes are not > (the application is doing something else while the data is written). > > Having writes take precedence over reads will really screw over the > readers, while not benefitting the writers all that much. > I ran a test to show how readers can be starved in certain cases. I launched one reader and three writers. I ran this test twice. First without dm-ioband and then with dm-ioband. Following are few lines from the script to launch readers and writers. ************************************************************** sync echo 3 > /proc/sys/vm/drop_caches # Launch writers on sdd2 dd if=/dev/zero of=/mnt/sdd2/writezerofile1 bs=4K count=262144 & # Launch writers on sdd1 dd if=/dev/zero of=/mnt/sdd1/writezerofile1 bs=4K count=262144 & dd if=/dev/zero of=/mnt/sdd1/writezerofile2 bs=4K count=262144 & echo "sleeping for 5 seconds" sleep 5 # launch reader on sdd1 time dd if=/mnt/sdd1/testzerofile1 of=/dev/zero & echo "launched reader $!" ********************************************************************* Without dm-ioband, reader finished in roughly 5 seconds. 289533952 bytes (290 MB) copied, 5.16765 s, 56.0 MB/s real 0m5.300s user 0m0.098s sys 0m0.492s With dm-ioband, reader took, more than 2 minutes to finish. 289533952 bytes (290 MB) copied, 122.386 s, 2.4 MB/s real 2m2.569s user 0m0.107s sys 0m0.548s I had created ioband1 on /dev/sdd1 and ioband2 on /dev/sdd2 with weights 200 and 100 respectively. Thanks Vivek -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
