Hmm, it shows "4-EBS RAID" getting around 2.6x speedup for
4k writes. They don't say what RAID configuration they are using, but it sounds odd.
A 4k write has to be synced to all the disks unless they have a <=4k stripe size AND are using RAID-0
AND are using stripe-aligned IO ops. It's also
possible they use 4k writes to cache that end up forming large dirty blocks which the OS then syncs as larger I/Os.
But that would be measuring something else than the benchmark claims.
Correcting myself: they do say in the methodology section
that they are using RAID 0 and say that the write benchmark
is multithreaded. So that explains it, it's measuring
write throughput for 4k writes.
A 4k write has to be synced to all the disks unless they have a <=4k stripe size AND are using RAID-0 AND are using stripe-aligned IO ops. It's also possible they use 4k writes to cache that end up forming large dirty blocks which the OS then syncs as larger I/Os. But that would be measuring something else than the benchmark claims.