The 32-bit host ID of the machine, formatted as 8 hexadecimal characters. You should try to make this ID unique among your machines. You can generate a random 32-bit ID using the following commands: head -c 8 /etc/machine-id (this derives it from the machine-id that systemd generates) or head -c4 /dev/urandom | od -A none -t x4 The primary use case is to ensure when using ZFS that a pool isn’t imported accidentally on a wrong machine.

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"4e98920d"