I'm building out a FreeNAS based server that will primarily be used to serve up iSCSI disks to Xen virtual machines and dedicated servers. My current data need is for 8TB of space, growing at about 2TB per year. I have a Supermicro X6DHE-XB 3U enclosure with 4G of RAM, 16 SATA hot-swap bays that I will be using. It comes with 2x8 port 3Ware RAID cards, but I'm planning on just using the ZFS capabilities instead of the hardware RAID. My initial drive set will be 8x2TB HITACHI Deskstar 7K3000 HDS723020BLA642 drives.

I have 2 questions

  1. I will want to add additional drives to this server in the future and have them added to the storage pool. Ideally, I'd like to do this live without having to reboot or take the system off line. Are there any limitations or advantages to a ZFS mirror setup vs a RAID-Z setup for expanding the storage pool? With the hot-swap SATA ports, can I add a disk and have it show up in the FreeNAS GUI as available?
  2. If the performance penalty is less than 5% or so, I'd prefer to go with a RAID-Z2 setup so my initial volume size would be 12TB instead of 8TB with the mirrored config. This server will be pretty busy (serving up around 60 iSCSI targets), and unfortunately I have no way of testing it with a production load, besides putting it into production. I'd appreciate any real-world performance experience with RAID-Z serving large numbers of iSCSI targets.

2 Answers 2


Sorry I dont know anything about your hardware but If your OS and device can add the drives on the fly then ZFS will handle the disk additions and filesystem increase no problems. Infact it should be a very pleasing experience :-)

I do not know of any limitations with Mirror compaired to RAIDZ other than the usual mirror vs RAIDz conversations regarding space/redunancy and speed. Which ever you go for just give ZFS the indivdual disks and dont use the RAID , or if your not happy with that then maybe the second best options would be stripe to LUNS in your RAID and let ZFS mirror them ?

Your choice to let ZFS do the redundancy not your Hardware RAID seems to be the accepted method with ZFS if you dont have a mirror or RAIDZ in ZFS then you lose the filesystem self healing abilities that ZFS boasts about.

A old conversation can be found here. Its not the same type of hardware but its the same situation as yours http://web.archiveorange.com/archive/v/LmwutnAQ0OKwhALUNimw

Regards Andy


If you are running raidz you can only add another set of 3 drives. You cannot like in linux (mdadm) grow a raid5 array to house another disk.

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