Cloning an active partition?
 

I would like to clone a partition that is in use and active.

I do this on Solaris by upgrading the partition to a one way mirror (which takes a reboot), but then I can add a partition live to make it two way and let the mirror sync while the partition is in use.

Can I do something like this on Linux?

Yes, it involves setting up a free partition as a degraded RAID 1, copying the files of the active partition to it (in single-user mode), switching from the live partition to the newly-created RAID 1, then adding the live partition to the RAID 1 set. You can start by searching for software raid on linux.

If I have to copy the partition in single user mode that defeats my purpose. The partition is 100's of GB and will take many hours to copy, and if I did that I really would be done without doing the raid. I just want a consistent snapshot of the partition.

On solaris it would take a few minutes for the reboot, it would sync the mirror for hours multiuser and then I could detach the spare at the moment I wanted to freeze the clone.