Just to add to the many descriptions:
On SGI, if you want to be able to boot from a disk, you are not allowed
to put a partition for an ordinary file system (EFS or XFS) at 0.
Instead you make a special partition starting at 0 which they call
Volume Header (as far as I can tell this is still just a partition as
It's just a small partition where they store the 'sash' second stage boot
loader, and whatever other stand-alone tools you would want there, like
system testers, fx (disk partitioner) etc.
You can make the partition as big as you want, usually it's one or two
megabytes. They use a special program to copy files to/from there.