Reason for md raid 01 blksize limited to 4 KiB?

Hi list,

I'm wondering why stacking raid1 above raid0 limits the block sizes in
the blkio queue to 4 KiB both read and write.

The max_sectors_kb is at 512. So it's not a matter of limits.

Could someone explain, please? Or could someone pinpoint me to the
related location in the source code?

We've thought of using this for replication via InfiniBand/SRP. 4 KiB
chunks are completely inefficient with SRP. We wanted to do this with
DRBD first, but this is also extremely inefficient, because of chunk
sizes in the blkio queue.

I can reproduce the small 4 KiB chunks also in a file copy benchmark
with raid 01 on ram disks.

Cheers,
Sebastian