I followed Building OmniOS to build OmniOS, but met following errors:
$ pfexec /opt/ooce/bin/omni build_world
......
./buildctl: fork: Not enough space
../lib/functions.sh: line 627: wait_for: No record of process 22984
../lib/functions.sh: fork: Not enough space
Waiting for illumos build...
......
The reason is the virtual machine which I used has only ~1.5G memory, and it is not enough. Increase the memory to 4G, then it is OK to build.