> Average write operation duration decreased by 50% on our fullest machines

This is probably not a fair comparison.

On the existing lz4 machines, the zfs pools are already badly fragmented, making it difficult to find regular blocks: https://utcc.utoronto.ca/~cks/space/blog/solaris/ZFSGangBloc...

On the new zstd machines, the zfs pools are still pristine, after being restored from backup.

If we want to isolate the effects of migrating from lz4 to zstd, we also need some new lz4 machines for comparison.