Hi, I'm configuring a FreeBSD server hosting virtualbox serving half dozen mission critical busy mail servers. I just learned ZFS, I'm quite attracted, but have a few questions:

1. what is the CPU overhead of ZFS? I googled and found little (or no) benchmark for that.

2. from what I learned, when ZFS updates files, it keeps the old file as snapshot, and write the updated part for the new version. However that would mean for each snapshot it keeps that require significant storage overhead. How much is this storage overhead? For example, suppose I have 2TB usable space, how much space can actually be used for the latest version of files one year later?

3. is FreeBSD with ZFS hosting virtualbox serving half dozen busy guest mission critical mail servers a reasonable combination? Anything particular to be careful with? And can I still choose ZFS for the guest OSs? This is because I may build another identical such box for redundancy, and will need to do some mirroring between each pair of the guest systems across the boxes.

4. I'm trying to configure a Dell R710 for this. From what I learned, I shouldn't choose any RAID at all, is that true? In that case, are the drives still arrive hot swappable?

5. this may sounds a bit pathetic, but since I have no experience with ZFS at all, and this is a mission critical server, so just ask just in case: I'm choosing twin Intel L5630 processors, and 6 x 600GB 15K RPM Serial-Attach SCSI drives. If I need more space in the future, I would just hot swap some drivers with larger capacity to expand the storage. There is no problem with these, right?