...
2- not putting everything of "/var" under "zroot/var" dataset (with "canmount=on" of course)?

One of the reasons offered, on IRC, was that other datasets would cause churn in boot environment.
-- so does installation|upgrade of packages in boot environment.

#FreeBSD #var_strewn_about #bootEnvironment #boot_env

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- 5 datasets & related mountpoints as children of "zroot/var" dataset (mounted under "/var" as expected; https://wiki.freebsd.org/BootEnvironments#Pool_Layout.2FDesign ).

(Observe different source datasets yourself if installed with ZFS options as mentioned: df -h /var/* | sort -u )

❔What is the justification for ...

1- putting some parts of "/var" in a root directory (boot environment);

#FreeBSD #var_strewn_about #bootEnvironment #boot_env
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BootEnvironments - FreeBSD Wiki

High-level overview of ZFS Boot Environment setup