ZFS on FreeNAS

Courtesy: FreeNAS


While FreeNAS™ is available for both 32-bit and 64-bit architectures, you should use 64-bit hardware if you care about speed or performance. A 32-bit system can only address up to 4GB of RAM, making it poorly suited to the RAM requirements of ZFS. If you only have access to a 32-bit system, consider using UFS instead of ZFS.


The best way to get the most out of your FreeNAS™ hardware is to install as much RAM as possible. If your RAM is limited, consider using UFS until you can afford better hardware. ZFS typically requires a minimum of 6 GB of RAM in order to provide good performance; in practical terms (what you can actually install), this means that the minimum is really 8 GB. The more RAM, the better the performance, and the Forums provide anecdotal evidence from users on how much performance is gained by adding more RAM. For systems with large disk capacity (greater than 6 TB), a general rule of thumb is 1GB of RAM for every 1TB of storage.

NOTE: by default, ZFS disables pre-fetching (caching) for systems containing less than 4 GB of usable RAM. Not using pre-fetching can really slow down performance. 4 GB of usable RAM is not the same thing as 4 GB of installed RAM as the operating system resides in RAM. This means that the practical pre-fetching threshhold is 6 GB, or 8 GB of installed RAM. You can still use ZFS with less RAM, but performance will be affected.

If you are installing FreeNAS™ on a headless system, disable the shared memory settings for the videocard in the BIOS.

Compact or USB Flash

The FreeNAS™ 8 operating system is a running image. This means that it should not be installed onto a hard drive, but rather to a USB or compact flash device that is at least 2 GB in size. A list of compact flash drives known to work with FreeNAS™ can be found here on the .7 wiki. If you don’t have compact flash, you can instead use a USB thumb drive that is dedicated to the running image and which stays inserted in the USB slot. While technically you can install FreeNAS™ onto a hard drive, this is discouraged as you will lose the storage capacity of the drive: that is, the operating system will “take over” the drive and will not allow you to store data on it, regardless of the size of the drive.

The FreeNAS™ installation will partition the operating system drive into two ~ 1GB partitions. One partition holds the current operating system and the other partition is used when you upgrade. This allows you to safely upgrade to a new image or to revert to an older image should you encounter problems. However, it also means that the device it is installed into becomes “dedicated” to FreeNAS™. You will not be able to use that device to store data. For example, if you were to install FreeNAS™ onto a hard drive, the entire hard drive would become inaccesible, regardless of its size.

ZFS Overview

While ZFS isn’t hardware (it is a filesystem), an overview is included in this section as the decision to use ZFS may impact on your hardware choices and whether or not to use hardware RAID.

If you’re new to ZFS, the Wikipedia entry on ZFS provides an excellent starting point to learn about its features. These resources are also useful to bookmark and refer to as needed:


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