By default, it is not easily possible to recursively determine the total space used by a ZFS snapshot. After taking a recursive snapshot, the used space can be displayed on each individual filesystem, but not all at once.
The reason for this odd behaviour is simple: While snapshots can be recursively created and deleted, they, unlike the filesystems themselves, do not acutally depend on one another. Technically, what looks like one big snapshot across a ZFS hierachy, is instead a collection of independant snapshots with the same name. For instance, a snapshot can be deleted on one dataset but kept on a descendant - that is not possible with datasets themselves.
Consequently, the "used" space of a snapshot does not include the space used by its incarnations on descendant filesystems. A snapshot, unlike a filesystem, does not own or contain its seeming descendants.
This script aims to solve the described issue by simply adding up the
space used by all incarnations of a snapshot, giving us the total size.
This will be the unique, deduplicated size of a snapshot, i.e. the space
that is freed up when deleting it. Adding up the sizes this script
returns for individual snapshots will not result in the space used by
all snapshots, that information is stored in the usedbysnapshots
property.
znapsize.sh [flags] filesystem
-h: Human-readable output - SI unit prefixes
-u: Display the help screen
zroot@2020-07-19_00-00-00 394M
zroot@2020-07-26_00-00-00 396M
zroot@2020-08-02_00-00-00 396M
zroot@2020-08-09_00-00-00 173M
zroot@2020-08-16_00-00-00 481M
zroot@2020-08-22_10-23-44 21.5M
zroot@2020-08-23_00-00-00 13.7M
zroot@2020-08-27_08-19-10 199M
install.sh installs znapsize.sh to /usr/local/bin/znapsize and sets
the appropriate permissions (+x to make it executable). You will be asked
to overwrite if it already exists.