What software should I use to backup a Linux system ?

I have a live server running RH9
on a Dell machine.The HDD are in a RAID5 setup using a hardware RAID controller.

For my peace of mind I would like to create an image of the whole system and store it on a drive in case something happens to the server. This would allow me a faster restore using a bootable media.

I should mention that I cannot afford to take the server down because I am running DNS and Email servers on it. I can maybe have a short maintenence window to get this done.

Could you please recommend me a software that you’ve used to image a live Linux machine???
or maybe some other solutions.

Thank you.