When piping to awk, it's almost always an anti-pattern to use 'grep' first.

When not in a pipeline, sometimes it is faster to use tail, as awk must process all the lines, and won't 'seek'. In a pipeline, both must process all lines, so we might as well skip the extra process creation and just use awk for all the processing.

Verify that the output between:

zpool list -v $rootdev | tail -n 1 | awk '{print $1}'

and

zpool list -v $rootdev | awk 'END {print $1}'

Are the same on all my systems. The output is the same.