Il 08/09/2014 18:18, Peter Lieven ha scritto: >> > When copying data, gparted will try using very large I/O sizes. Of >> > course if something breaks it will just use a smaller size, but it would >> > make performance worse. >> > >> > I tried now (with local storage, not virtual---but with such large block >> > sizes it's disk bound anyway, one request can take 0.1 seconds to >> > execute) and a 2 MB block size is 20% slower than 16 MB block size on >> > your usual 3.5" rotational SATA disk. >> > > can you share with what command exactly you ran these tests? > > i tried myself and found that without multiwrite_merge i was not able to > create a request bigger than 0xffff sectors from inside linux.
On a different machine: $ time dd if=/dev/zero of=test bs=16777216 count=30 oflag=direct real 0m13.497s user 0m0.001s sys 0m0.541s $ time dd if=/dev/zero of=test2 bs=1048576 count=480 oflag=direct real 0m15.835s user 0m0.005s sys 0m0.770s The bigger block size is 17% faster; for disk-to-disk copy: $ time dd if=test of=test3 bs=16777216 count=30 iflag=direct oflag=direct real 0m26.075s user 0m0.001s sys 0m0.678s $ time dd if=test2 of=test4 bs=1048576 count=480 iflag=direct oflag=direct real 0m45.210s user 0m0.005s sys 0m1.145s The bigger block size is 73% faster. Paolo
