I understand that ZFS prefers to have as much data about the drives as possible, and that the best thing to do is turn off RAID.

The hardware environment is a Dell PowerEdge 2850 with PERC 4/DC and four drives (73Gb each) installed out of six possible. The software is FreeNAS 8.0.2 with ZFS booting from USB key.

I've configured the RAID this way: each physical drive is a logical drive in a RAID 0. No special configurations were made beyond this.

Is this optimal for ZFS? How do I properly set this up under FreeNAS as a ZRAID? Do I want to?

In my reading it was said that one can't add a new disk to a ZRAID pool; is this still true? How would you go about adding two new disks in a redundant fashion to a zpool in FreeNAS?

2 Answers 2


No, this is not optimal for ZFS. This is outlined here on Server Fault at: ZFS SAS/SATA controller recommendations

The PERC 4/DC controller is a basic PCI-X parallel SCSI RAID controller. ZFS prefers to handle whole-disk management, so the better option in your case is to use a basic SCSI HBA. In lieu of that, the solution you've come up with by creating multiple RAID 0 logical drives will work. The only issue comes up when a drive fails. The replacement disk will need to be reinitialized as a RAID 0 logical drive in order to be recognized by ZFS. You will probably need a reboot in order to do this. It's something to consider.

Disk expansion is possible in certain cases. You can add vdevs (groupings of RAID disks) to a pool. For instance, if you have a zpool comprised of striped-mirrors (RAID 1+0), you can add additional mirrored pairs to that pool. You cannot expand a RAIDZ1/2/3 vdev.

Also see: ZFS: Mirror vs. RAID-Z and How can I add one disk to an existing raidz zpool?

  • So, if I read this right, given that I don't want to change the hardware configuration completely, I should probably set up a PERC 4/DC mirror and put ZFS on top? Or is that no different than using RAID5 on the PERC 4/DC? The questions you pointed to are good, but none seem to clarify if they mean a ZFS mirror or a hardware mirror....
    – Mei
    Dec 22, 2011 at 14:36
  • Everyone is referring to ZFS mirrors. You could bypass the ZFS RAID entirely and handle everything at the hardware level. Just make a RAID 1+0 (or RAID 5) array on your Perc and present that to ZFS. You lose some ZFS features (self healing, intelligent RAID), but it depends on why you're using ZFS and what features are important to you.
    – ewwhite
    Dec 22, 2011 at 14:43
  • I want the self-healing features of ZFS but still retain the ability to hot-swap drives when needed. If I've two PERC 4/DC mirrors and present both to ZFS as two drives, will this work?
    – Mei
    Dec 22, 2011 at 14:56
  • No. That would give you a stripe over hardware mirrors. I don't know if you'd get the ZFS protection in that case. I'd go all or nothing... Obtain the right type of controller or use hardware RAID... Or live without hotswap...
    – ewwhite
    Dec 22, 2011 at 15:07

I was able to allow the SCSI disks to pass thru the RAID by going into the config tool (Ctrl+M at boot) and disabling the controller bios. ZFS was not able to run smartd on disks behind the raid controller.

This is a 2850 with the 4/DC card controlling the scsi target for a powervault 220s. The disks in the array need to be JBOD and no RAID for ZFS to be able to get a full hold of it.

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .