Component size difference with mdadm

Component size difference with mdadm

[Justin Maggard]

I was working on recovering a damaged array today, and I used mdadm
3.1.2 with --assume-clean to get it up and running.  I was surprised
when I wasn't able to mount the filesystem on the array after starting
it, so I did a closer comparison of a dump of the original array
configuration vs. the new one.  I found that the main difference was
that the component size was 1KB smaller now than it used to be.  The
array was originally created with mdadm 2.6.8.  So I compiled the old
2.6.8 version of mdadm and used the same mdadm --assume-clean command,
and bingo, the component size matched the original array config and it
mounted just fine.

So, I suspect there is some new code in mdadm 3.1 that wasn't in 2.6.x
to try to make I/O 4KB-aligned on non-aligned partitions.  Is that
accurate?  If so, is there a way to tell mdadm to behave like older
mdadm versions in these cases?

-Justin