Twin Studies

Nexenta Systems [1] claims its storage platform provides 80 percent of the cost savings offered by heavyweights such as IBM or EMC. Different models of DIY and preconfigured storage devices are possible, all with one thing in common: They are based on NexentaOS [2], which links the kernel of the OpenSolaris successor Illumos to GNU userland tools and Debian packaging. The core element is the modern and powerful ZFS filesystem, with its snapshots and integrated RAID levels, unlimited capacity and file size, and block-level mirroring.

Differences

NexentaOS (also known as Nexenta Core Platform, NCP) is free software that continues to exist today under the Illumos umbrella. NexentaStor also calls itself "Open Storage," but the term only means that it uses open standards and protocols. The product is commercially licensed. NexentaStor [3] distinguishes between the Community Edition and the Enterprise Edition. The Community Edition is free, but not intended for production systems (which the EULA explicitly forbids), is limited to a maximum of 18TB, and lacks advanced features such as active/active HA clusters, the Virtual Machine DataCenter plugin for integrating various hypervisors, auto sync, or the ability to use WORM storage for archiving. However, for home use or in an academic environment, the community edition offers an unbeatable, low-budget solution. If you opt for the Enterprise Edition, on the other hand, you will find it in prebuilt appliances by Nexenta partners, such as Boston, Racktop, Thomas Krenn, CDW, or transtec.

Netgear [4] takes a third approach: Without being a certified Nexenta partner, it develops its own storage operating system, ReadyDATAOS, that is based on the free Core platform. In this review, I compared Netgear ReadyDATA 5200 [5] with the Nexenta Community Edition.

Volumes and Shares

With the Netgear appliance, you don't need to install the operating system – this is done by the manufacturer – but the Community Version of Nexenta is installed quickly and easily. Things could become more difficult if you need to integrate external disk storage, which the underlying Nexenta Illumos kernel does not recognize (e.g., for lack of a matching driver). Otherwise, half an hour and the answers to a handful of simple questions are all you need to serve up some initial shares on the network.

When setting up volumes and shares, the differences are a bit more obvious. Netgear ReadyDATAOS scores its first points by letting users select the disks for a volume they want to create with just a few clicks in a graphical device overview (Figure 1). This is practical and facilitates orientation. Understandably, Nexenta, which is not tied to a specific device, but supports a very wide range of hardware, does not automatically provide a view: Here, you have to make do with the less intuitive Solaris device names.

Figure 1: A graphical view shows the slots in which the disks of a volume reside (dark gray, not assigned; light gray, assigned; ocher, cache; green, spare disk; blue, belongs to selected volume).

However, NexentaStor comes with a text console (Nexenta Management Console, NMC; Figure 2) in addition to the graphical user interface (Nexenta Management View, NMV). This tool is useful, for example, if you want to automate certain processes with your own scripts. Although admins can also enable SSH access for Netgear, it does not give you the special commands for creating, viewing, managing, and deleting storage objects that the NMC offers. NexentaStor loses points in the GUI, but it makes them up at the command line.

Figure 2: The Nexenta Management Console lets you automate administrative tasks through scripting.

RAIDZ

Besides mirroring (RAID 1), Nexenta supports the RAID-Z levels that are integrated into ZFS and refers to them in true ZFS style as Redundancy Groups: RAID-1, RAID-Z1 (like RAID-5), RAID-Z2 (like RAID-6), and RAID-Z3 (with triple parity). Netgear uses these modes, too, but calls them RAID-5 and RAID-6, following the classic model, because it considers these the common names (although this is not quite correct). Compared with the Nexenta Community Edition, however, Netgear does not offer RAID-Z3 triple parity, which can be an advantage with very large volumes.

After creating a volume, you can assign more disks to act as a cache with both Nexenta and Netgear. SSDs are advisable for this purpose. Both candidates can use both read and write cache; ZFS also supports Hybrid Storage Pools, which combine DRAM and flash drives.