System locks up while moving file

[John Elliot V <jj5@jj5.net>]

Hey there. Maybe someone can help me with this.

I'm running Debian 9.0 (stretch). I have a few scripts which
periodically create and then move a file from my 1TB M.2 drive to a MD
RAID1 device comprised of 2x 6TB Seagate IronWolf drives. Full details
are here [1].

My disks look something like this:

jj5@tact:~$ df -h
Filesystem      Size  Used Avail Use% Mounted on
/dev/md0        469G   86G  378G  19% /
/dev/md1        5.5T  4.0T  1.6T  73% /data
/dev/nvme0n1p1  931G   78G  853G   9% /data/fast

An example of the move command which locks everything up is:

 mv /data/fast/temp/blob-tact.tgz \
    /data/temp/blob-tact.tgz.8157-12003.tmp

So while this mv command is being processed my mouse doesn't respond, my
KDE Plasma session locks up, everything grinds to a halt.

There seems to be plenty of RAM available, and plenty of CPU. So must be
some I/O thing..? Anyway, I'm a bit stumped. I haven't tried running the
mv command with nice, because it doesn't seem to be a CPU thing... but
maybe I should..? Anyone got any ideas about how to keep my system
responsive while the mv command is processed?

Regards,
John Elliot V

[1] https://www.jj5.net/sixsigma/Workstation#Disk

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