What is METALOG?
When you 'make installworld -DNO_ROOT DESTDIR=blah', the system will create a $DESTDIR/METALOG file. This file contains all the permission and modes for the files. Normally, installworld requires root permission. -DNO_ROOT instructs the build system to install them as the user and to note what permissions, etc in a METALOG.
How to use METALOG
Creating a UFS partition with no privs
If you have your own tooling around image creation, you can use the METALOG to supply the permissions and other filesystem metadata to that process. makefs can be used by a non-privileged user to a UFS partition image. Coupled with mkimg, you can create an entire bootable system image without needing root. Look at the -F flag to makefs(8) for how to use this functionality.
Package Base Use
METALOG is also used by the pkgbase initiative to slice up the system. Part of the metadata that's included is what package each of the installed files belongs to. This is all transparent when you do a 'make packages' to generate these packags.
Taring up an installworld
If you are looking for a quick and dirty way to udpate a VM, you can often just create a tarball from the METALOG. Tar was enhanced a number of years ago to understand mtree files. The METALOG is one giant MTREE file. To create a tarball that's a copy of the image with all the right permissions:
cd $DESTDIRThis will create a xz compressed base.txz similar to what the release images create. This one tarball has everything (unlike the base.txz from the release build process), and is about 800MB.
tar cfJ base.txz @METALOG