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 (link below)
http://postimg.org/image/ilxyf2r8x/
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 ~175MB/s max 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.
I'm thinking this has to do with the default Storage Spaces interleave of 256K and I've done some more tests with IOMeter to confirm and in IOMeter I can get ~350MB/s performance anywhere above a 512K transfer size (where you'd expect to start seeing parallel reads of slices from 2 disks at once)
So is there a way to tell the Windows file copy to increase its transfer size somehow to achieve full parallelism on large files? The files I'm testing with are between 5GB and 10GB.