I recently created a ZFS mirror on a Proxmox host. I then noticed my new 500 GB VM disk reserved 508 GB, instead of just holding only a few gigabytes of data. This led to writing up a list of checks for ZFS, including thin provisioning, block sizes, LZ4 compression, ARC memory limits, and the existing scrub schedule. This post covers what I found, and how you can check these settings. We see which changes affect existing disks you have created vs. new ones.