ATTO benchmark read speed slow below 4MB transfer size on 2-way mirror

Question

I've set up two Toshiba DT01ACA300 HDDs in Windows 8.1 Storage Spaces as a 2-way mirror formatted as a NTFS volume.

Benchmarking with ATTO shows this (volume is only a quarter full of data)

^{Click to enlarge}

For the 4MB and 8MB block size I get the read speed I would expect from a 2 drive setup. A drive by itself can push about 170MB/s to 180MB/s when reading near the beginning of the disk.

So why is anything below 4MB reading only as fast as a single disk? This speed is correct as verified by Windows file copy, Robocopy and other file copying tools when copying to a fast SSD so as to max out the HDD array.

Answer 1

OP here. I think I found the answer to this one.

The reason for the ATTO benchmark behavior is due to the fact that 2-way mirrored spaces are not optimized with any load-balancing/round-robin algorithm. This is similar with any other RAID1 setup as there's few implementing this advanced feature.

So what happens is that it all depends on the number of outstanding IOs. Up until the simultaneous IO requests are exceeding a certain threshold, Windows will utilize only one drive to read from. Beyond the threshold, Windows starts reading from the second drive at the same time doubling the read speed.

I confirmed this with IOMeter.