> is there a way to batch explicitely write requests raid5 issues?

sort of like TCP_CORK?

> for example, there is a raid5 built from 3 disks with chunk=64K.
> one types dd if=/dev/zero of=/dev/md0 bs=128k count=1

OK, so this is an aligned, whole-stripe write.

>and 128K
> bio gets into the raid5. raid5 processes the request, does xor
> for parity stripe, then issues 2 64KB requests down to lower level.

don't you mean _3_ chunk-sized writes?  if so, are you actually
asking about the case when you issue an aligned two-stripe write?
(which might get broken into 6 64K writes, not sure, rather than 
three 2-chunk writes...)

_not_ that I know this code at all!

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