ohb - Question About ZFS Root Dataset

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I have recently started trying out FreeBSD and I have a ZFS question.

In the third edition of Absolute FreeBSD, as well as the FreeBSD Handbook, a "zfs list" command shows something like this (I have copied this from the FreeBSD Handbook, which mirrors Michael's book):

========================================================
# zfs list
NAME                  USED  AVAIL  REFER  MOUNTPOINT
mypool                781M  93.2G   144K  none
mypool/ROOT           777M  93.2G   144K  none
mypool/ROOT/default   777M  93.2G   777M  /
mypool/tmp            176K  93.2G   176K  /tmp
mypool/usr            616K  93.2G   144K  /usr
mypool/usr/home       184K  93.2G   184K  /usr/home
mypool/usr/ports      144K  93.2G   144K  /usr/ports
mypool/usr/src        144K  93.2G   144K  /usr/src
mypool/var           1.20M  93.2G   608K  /var
mypool/var/crash      148K  93.2G   148K  /var/crash
mypool/var/log        178K  93.2G   178K  /var/log
mypool/var/mail       144K  93.2G   144K  /var/mail
mypool/var/tmp        152K  93.2G   152K  /var/tmp
========================================================

As you can see the root dataset is unmounted and set to having a mountpoint of "none". But when I installed FreeBSD 12.0, the root dataset is now mounted by default:

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% zfs list
NAME                 USED  AVAIL  REFER  MOUNTPOINT
zroot                459M  17.5G    88K  /zroot
zroot/ROOT           458M  17.5G    88K  none
zroot/ROOT/default   457M  17.5G   457M  /
zroot/tmp             88K  17.5G    88K  /tmp
zroot/usr            388K  17.5G    88K  /usr
zroot/usr/home       124K  17.5G   124K  /usr/home
zroot/usr/ports       88K  17.5G    88K  /usr/ports
zroot/usr/src         88K  17.5G    88K  /usr/src
zroot/var            572K  17.5G    88K  /var
zroot/var/audit       88K  17.5G    88K  /var/audit
zroot/var/crash       88K  17.5G    88K  /var/crash
zroot/var/log        132K  17.5G   132K  /var/log
zroot/var/mail        88K  17.5G    88K  /var/mail
zroot/var/tmp         88K  17.5G    88K  /var/tmp
========================================================

Based on what I have read about ZFS so far, I can't see a benefit to this change, but I am a newbie so I am probably missing something. Can you can tell me why the root dataset is now mounted by default on new installs?


Thanks

Owen
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