Bill Davidsen wrote:

One of the advantages of mirroring is that if there is heavy read load 
when one drive is busy there is another copy of the data on the other 
drive(s). But doing 1MB reads on the mirrored device did not show that 
the kernel took advantage of this in any way. In fact, it looks as if 
all the reads are going to the first device, even with multiple 
processes running. Does the md code now set "write-mostly" by default 
and only go to the redundant drives if the first fails?

No, it doesn't use write-mostly by default. The way raid1 read balancing 
works (in recent kernels) is this:

- sequential reads continue to go to the first disk

- for non-sequential reads, the code tries to pick the disk whose head 
is "closest" to the sector that needs to be read

So even if the reads aren't exactly sequential, you probably still end 
up reading from the first disk most of the time. I imagine with a more 
random read pattern you'd see the second disk getting used.

--
Paul