pfSense

Due to various changes in geom and how things are checked, a whole-disk gmirror may break when the base OS is updated to FreeBSD 9.x. There are some installs out there that will probably be affected.

The most common workaround to let such a mirror keep functioning is to set this in loader.conf:
kern.geom.part.check_integrity=0

People who setup mirrors following the FreeBSD handbook (and our old instructions), may encounter such errors. I believe the way the installer creates a gmirror is OK, but it needs testing.

In 9.x a mirror created on disks after they were already active would be considered corrupt since the gmirror metadata resides over actual FS data at the end of the disk. However if the mirror is created before the filesystems, this may work out.

The better way in 9.x is to use gpt to setup the partitions, and then use gmirror to mirror the partitions rather than the whole disk, but that may not yet be needed if the current installer method still works.