Dmesg shows "md: serializing resync, md4 has overlapping physical units with md5". What does this mean?

In that message "physical units" refers to disks, and not to blocks on the disks. Since there is more than 1 RAID array that needs resyncing on a disk, the RAID code is going to sync md4 first, and md5 second, to avoid excessive seeks (also called thrashing), which would drastically slow the resync process. More.

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