a very use full tool
make sure there is no error when it is done
make sure you have done a
bosboot -ad hdisk0 and also the mirror disk
mksysb only backup anything in rootvg
you can check this by doing lsvg -l rootvg
you can reduce the size by excluding any filesystems that you do not need to backup by adding a file in /etc/exclude.rootvg
by default there are some basic stuff in there
you can then just use the exclude=yes in smitty mksysb
if you have a dvd ram disk you can use smitty mkdvd as well
Hope this helps