hi,
also it is best to backup just the root filesystem when doing a mksysb rather than the whole lot i.e. other filesystems etc.. to avoid this you can create a file in
/etc/ called exclude.rootvg ( more info do man mksysb)
so you back up / , tmp , home , usr, var
But as victorv has sais its is best if no users are on the system , so they don't have opened files .