Rebuild mdadm RAID5 array with fewer disks

According to the power of Google, which brought me to this article, it's possible to resize RAID5 arrays under Linux since mdadm version 3.1.something.

1. First shrink the filesystem (using resize2fs -M).
2. Use mdadm ... --grow --array-size= to resize the array down so that it's the correct size for three physical volumes.
3. Use mdadm ... --grow --raid-devices=3 --backup-file=/tmp/backup to change the number of devices in the array.

I have never tested this myself, and until now I didn't even realize this operation was supported. So, I've learned something. Good luck!