mkfs /dev/md0 can take a very long time, if /dev/md0 is a very large
disk that supports TRIM/DISCARD (erase whichever is inappropriate).
That is because mkfs issues a TRIM/DISCARD (erase whichever is
inappropriate) for the entire partition. As far as I can tell, md
converts the large TRIM/DISCARD (erase whichever is inappropriate) into
a large number of TRIM/DISCARD (erase whichever is inappropriate)
requests, one per chunk-size worth of disk, and issues them to the RAID
components individually.
It seems to me that md can convert the large TRIM/DISCARD (erase
whichever is inappropriate) request it gets into one TRIM/DISCARD (erase
whichever is inappropriate) per RAID component, converting an O(disk
size / chunk size) operation into an O(number of RAID components)
operation, which is much faster.
I observed this with mkfs.xfs on a RAID0 of four 3TB NVMe devices, with
the operation taking about a quarter of an hour, continuously pushing
half-megabyte TRIM/DISCARD (erase whichever is inappropriate) requests
to the disk. Linux 4.1.12.
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