Speaking as someone that has built experiments on top of zfs zvols (https://xeiaso.net/blog/waifud-progress-report-2/), I'm not entirely sure what the main usecase of them is. They seem to really hate journaled filesystems (nearly all of the filesystems currently in use). It's also really easy to run into cases where LVM inside a zvol for a VM makes the host kernel eagerly pick it up and pin it as active on reboot, preventing you from deleting the zvols in question (this took me a week to properly diagnose).
They probably made sense back when filesystems weren't journaled, but we don't live in that world anymore.
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(I work for fly.io, but not on Machines)
Speaking as someone that has built experiments on top of zfs zvols (https://xeiaso.net/blog/waifud-progress-report-2/), I'm not entirely sure what the main usecase of them is. They seem to really hate journaled filesystems (nearly all of the filesystems currently in use). It's also really easy to run into cases where LVM inside a zvol for a VM makes the host kernel eagerly pick it up and pin it as active on reboot, preventing you from deleting the zvols in question (this took me a week to properly diagnose).
They probably made sense back when filesystems weren't journaled, but we don't live in that world anymore.