On Thu, Jan 24, 2008 at 05:59:39PM -0800, malahal@xxxxxxxxxx wrote: > 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. Is there any way (yet) to increase the bio size? It would be nice if individual paths could perform large I/O's when needed. Andy -- dm-devel mailing list dm-devel@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/dm-devel
