Andy [genanr@xxxxxxxxxxxx] wrote: > I did some basic dd tests to get an idea of the speed of multipath vs > individual devices, some of the numbers seem not to make sense. > > I ran several test with dd, 4 at a time reading different parts of the > device. > > multibus - round robin among paths rr_min_io=1, 4 paths > individual - each dd going to a separate path > > dd with iflag=direct > multibus : 120 MB/s > individual : 115 MB/s > > dd without direct > multibus : 44MB/s (why is this so bad) > individual : 160MB/s (why is this so good) > > Why is multibus without direct the worst, yet individual devices without > direct is the fastest. And why doesn't multibus perform about the same as > going to the individual devices (25% performance hit using multibus)? I am assuming that you are only reading from the device based on your iflag. When you do O_DIRECT, you inherently disable read-ahead. Also note that the read-ahead benefit would be insignificant as you increase the block size. without direct I/O, you end up with very big I/O's in individual mode. Same can't be said in multibus mode as it tries to send small I/O's (actually bio's) on different paths and it makes the system not to merge any adjacent requests. Use iostat to see the average I/O size in each case. You should be able to get the same behavior even in direct I/O mode provided you use very block size with dd. -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
