>but i have many times read that it's not a good way to backup
>aliving [sic] system

I've always made backups of live systems.  If they have a lot of activity
you might get an inconsistent backup, but it has *never* happened to me.

>its possible that i boot from cdrom and make a=20
>dump of /dev/sd0a /dev/rmt/0 (i thing so i can take the hole slice)
>or its a other way todo this ???

You can boot single-user from the hard drive and do a backup.
You could also boot from other media; your choice as to whether this
is floppy, CD-ROM, or another root partition.  But you probably want
to have the same /etc/fstab so you back up the right things.

If you want it really automated you could make a boot floppy that
automatically starts backing up when it boots.  I'm working on some
custom floppies like this, but it's tricky to get all the programs
you want on them.