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fyi, btrfs as /var/lib/docker/btrfs has a disadvantage under certain circumstances: running commands (du,rsync,..) on the subvolume folders altering the access time will incur a storage cost if the filesystem is mounted without noatime option: https://github.com/moby/moby/issues/39815 - not sure if it applies to ZFS as well (see https://lwn.net/Articles/499648/)

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Another difference that may matter for some:

on block based CoW FS, shared executables and libs will not be shared among containers, because file cache is file based not block based.

Using Overlayfs - or other file level CoW storage, when one starts several containers from the same image, shared libraries and executables will be loaded only once(so will be actually shared) across containers.

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