Storing the virtual world

Storage virtualisation has become another major piece of the virtual infrastructure – and iSCSI appliances look set to dominate this market.

Virtual infrastructures are all the rage. Businesses need to be able to respond flexibly to rapidly changing market conditions, while at the time saving money and increasing their green credentials.

Consolidating data centres and switching to virtual servers to increase server utilisation is a big part of the story, but it starts to fall apart when it takes several minutes to copy a virtual disk from one machine to another, or from one network-attached LUN to another.

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That's where storage virtualisation comes in. Instead of defined LUNs (logical unit numbers) and physical volumes, all your storage hardware can be treated as a single pool of storage, which can be used a single storage utility. Storage can be partitioned as required, and quickly moved between servers, using user-friendly storage management tools. Applications than need LUN-based access can work with virtual LUNs, while protocols like iSCSI give mixed block-and-file access to your stored data.

Virtualised storage is an important piece of any virtual infrastructure. It allows you to quickly move virtual machines from physical server to physical server, simply by moving the pointers for virtual LUNs or volumes. It also lets you use the same techniques to quickly failover from one VM to another, keeping down time to a minimum and increasing service reliability.

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With virtual storage in demand, it's no surprise that storage vendors are buying start-ups. Dell bought EqualLogic, while HP recently purchased LeftHand Networks. Even the NAS stalwarts like NetApp have brought out storage virtualisation products.

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Virtual storage and virtual infrastructure

In the past, servers and storage went together hand-in-hand. Each server had its own storage, whether it was a set of directly attached disks or a SAN on the end of some fibre. Even file-based NAS just looked like another storage volume that could be mounted and dismounted as required.

That model doesn't work very well for virtual infrastructures. Server consolidation means that multiple virtual servers run on a single physical server. While each physical server can be connected to enough physical storage to handle the requirements of each guest server, it's appropriate to consider how storage can be consolidated along with the servers. It's an approach that saves space in the data centre (along with reduced power and cooling demand), as well as simplifying the network connections to each server.

You can use your virtual servers with block network storage solutions like NAS, but it's not really suitable for the dynamic approach to storage required by tools like VMware's Virtual Infrastructure, where storage needs to be mapped between servers on demand. Different problems arise with SANs, where LUN migration can be a slow and complex process.

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IP networks have a level of flexibility that fibre finds hard to match though fibre's speed gives it an edge in other arenas. The alternative, iSCSI, is a relative new-comer to the field, but allows system administrators to mix file and block-based storage on a single piece of equipment. But iSCSI-based solutions scale well. Dell's EqualLogic systems are shipping with the ability to scale up to 192TB. While the 16TB entry-level systems use SATA drives, the higher-end solutions use the much faster SAS drive format.

You can link iSCSI management tools to your virtual infrastructure, either taking advantage of any built-in integration points or using APIs to build out your own management tools. LeftHand is exposing much of its management interface using WMI which means it should be accessible using tools like Microsoft's PowerShell.

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