When I do "cat /proc/mdstat", there seems no such information as to tell which disks are in mirror.
% cat /proc/mdstat
md126 : active raid10 sda1[0] sdd1[3] sdc1[2] sdb1[1]
... near-copies
Thanks for the hint.
Does "near-copies" implies sda1 and sdd1 are mirror, and so are sdc and sdb?
Add:
It looks like the same question has been raised before (https://serverfault.com/questions/200725/mirrored-and-stripped-hd-drives-in-raid10) and seems no clear answer.
First, the number [#] beside each device seems to be related to the order in which they appear in "mdadm --create", don't know why '/proc/mdstat' has to order them strangely. If so, then by "near-copies", it might be concluded that 'sda1' and 'sdb1' are in mirror, and so are 'sdc1' and 'sdd1', correct?
Add:
And it was (partially) confirmed by the test here (http://www.linuxquestions.org/questions/linux-server-73/software-raid10-does-the-disk-order-in-mdadm-matter-671016/).