2019-12-23: Faulty hard drive replacement
I got alerts from mdadm this morning indicating, via a pretty terse DegradedArray message, that /dev/sda had died.
I want to praise OVH for how fast they fixed the issue. Within a few hours, the fault was confirmed, the hard drive physically replaced and as of this post, the RAID array is happily rebuilding.
Apart from the fact they had to reboot the server around 15:47, there were no other issues. But as usual, do yell at me if something seems missing or corrupted.