I encountered this error in my Triton instance
And the community at irc.freenode.org #smartos helped me solve it.
The root cause is running out of space on dump device.
so here are the steps to solve it:
# zpool import zones # zfs destroy zones/dump # zfs create -V ( how much space you want the dump to be )G zones/dump # dumpadm -y -d /dev/zvol/dsk/zones/dump
# reboot