What does `mdadm --assemble --scan` do?

Unix
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2

-A, --assemble
Assemble a pre-existing array.

The --assemble parameter tells mdadm to «assemble» (start) the found RAID arrays.
If there are RAID arrays in the system that were not started automatically (for example, during system boot), this command will attempt to restore their operation.

-s, --scan
Scan config file or /proc/mdstat for missing information.
In general, this option gives mdadm permission to get any missing information (like component devices, array devices, array identities, and alert destination) from the configuration file (see previous option); one exception is MISC mode when using --detail or --stop, in which case --scan says to get a list of array devices from /proc/mdstat.

The --scan parameter instructs mdadm to examine all available devices (usually these are disk partitions), as well as the configuration file (usually /etc/mdadm.conf), in search of RAID metadata.