February 26th, 2009 Leave a comment Go to comments

What is ZFS?

ZFS is a file system designed by Sun Microsystems for the Solaris Operating System. ZFS is open-source software and has therefore also been ported to FreeBSD the OS behind FreeNAS.

A traditional file systems resides on single hard drive and if you want to use more than one hard drive they need to be combined either with RAID or with a volume manager.

ZFS is different, all ZFS filesystems are built on top of virtual storage pools called zpools. A zpool is constructed of virtual devices, which are themselves constructed of physical hard drives (or indeed files or hard drive partitions).

Hard drives within a virtual device may be configured in different ways, depending on needs and space available: non-redundantly (similar to RAID 0), as a mirror (RAID 1) of two or more devices, as a RAID-Z group of three or more devices (which is similar to RAID 5), or as a RAID-Z2 group of four or more devices (which is similar to RAID 6).

You can find more technical details on ZFS at Wikipedia.

What is iSCSI?

iSCSI (Internet SCSI) is an evolution of the SCSI protocol, which allows SCSI commands to be sent over a network. It allows two hosts to negotiate and then exchange SCSI commands using IP networks. The result is that a remote device with iSCSI capabilities can be seen to be a local disk drive but the commands and data for that device are being sent over the network rather than down a cable in the machine.

In iSCSI clients (called initiators) are able to send SCSI commands to SCSI storage devices (targets) on remote servers.

FreeNAS can act as an iSCSI initiator or and iSCSI target.

What is RAID?
Redundant Array of Inexpensive Disks or Redundant Array of Independent Disks (as it is more commonly called now) is a technology that uses two or more hard disk drives to achieve greater levels of performance and reliability.

A RAID distributes data across several physical disks which look to the operating system and the user like a single disk. Several different arrangements are possible (called RAID levels). Usually all the disks are of the same capacity.

RAID Levels
RAID levels 0, 1, and 5 are the most commonly found, and cover most requirements.

RAID 0 (striped disks) distributes data across several disks in a way which gives improved speed and full capacity, but all data on all disks will be lost if any one disk fails.

RAID 1 (mirrored disks) uses two (possibly more) disks which each store the same data, so that data is not lost so long as one disk survives. Total capacity of the array is just the capacity of a single disk. The failure of one drive, in the event of a hardware or software malfunction, does not increase the chance of a failure or decrease the reliability of the remaining drives (second, third, etc).

RAID 5 (striped disks with parity) combines three or more disks in a way that protects data against loss of any one disk; the storage capacity of the array is reduced by one disk. The less common RAID 6 can recover from the loss of two disks.

  1. August 19th, 2009 at 13:29 | #1

    Thanks for all of the great information. I have a couple of questions. If I set up 3 disks using RAID 5, can I add one after it has already been setup? Also, say I have 3 drives, 2x1TB and one 640GB. If I want to replace the 640GB drive with a terrabyte drive, is that possible?

    Thanks again!

  2. December 12th, 2009 at 03:49 | #2

    @Eric Lightbody
    to my knowledge, you can add up to 8 total disks in RAID5… also believe that all disks in the RAID group will autoresize to the lowest common size… in this case, 640gb… again, this is based on my knowledge of working with RAID and storage appliances (NetApp)…

  3. E Hepler
    December 15th, 2009 at 06:45 | #3

    @Eric Lightbody

    If you setup 3 drives of 2x1TB and 1x640GB I think you’ll get a raid system built on 3 drives using 3x640GB with no failover. I’d wait till you get 2 additional 1TB drives, and make the array 4x1TB. Otherwise the Raid is pretty ineffective.

  4. Mathew
    March 28th, 2010 at 16:21 | #4

    Sorry for posting the question in the wrong area. I was wondering if you could use lvm with freenas?

  5. June 24th, 2010 at 06:04 | #5

    Does FreeNAS support removable hard disks?

  6. William
    July 5th, 2010 at 21:50 | #6

    I had created a freeNAS disk of 379GB. When it came time withing the VMware ESXi to create a VM Windows 2003 server it said that the maximum allowed by the storage type was 250GB. Is there a limit into the size that freeNAS supports? Can I create a 2Tb disk with freeNAS and use it with a VM?

  7. September 20th, 2010 at 09:56 | #7

    Sounds like the ESXi storage container you are working with has a 1M block size and is limited to 256G per drive image. My website has some info on the topic.

  8. Joe
    December 1st, 2010 at 00:36 | #8

    Will there be an update to the Learning FreeNAS book written in 2008?

  9. admin
    December 1st, 2010 at 00:48 | #9


    I have been talking to the publishers PacktPub about doing a new edition to cover FreeNAS 0.8… If they don’t go for it I am considering going ahead and publishing privately on Amazon.

    If you want to see a new edition, please contact PacktPub at http://www.packtpub.com/contact

    Thanks, Gary

  10. Mputu
    December 2nd, 2010 at 20:50 | #10

    I was wondering if anyone can help, can I setup Freenas on my laptop? if so, how would setup using either my thumb drive or any other USB external? The issue I am having is that, if run it thru the usb I see the program loading but at the end the dos screen just disappear and nothing. Also with the command List Disk I can see my thumb-drive listed but bigger external does show.

  11. Ante
    December 5th, 2010 at 14:39 | #11


    I’m thinking about building home NAS for dvd files and media to stream to my wdtv live.

    I was wondering, could I expand my raid5 or raidz array with additional disks? As I can see so far, no adding of disks to existing array is possible.

    Is it true? how can I get expendability of storage and relative safety of my data?

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