Analysis

posted on 23 February 2008 11:09

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Many storage suppliers say that their virtualised storage does for storage what VMware does for servers. It does not, at least in one critical respect.

Server virtualisation is operating system multi-tasking visited on the operating system itself. It turns out that operating systems are so bad at multi-tasking applications it is more efficient to multi-task operating systems (plus an application). That's what VMware and other hypervisors do; they share the same physical hardware between multiple virtual machines (VMs) which are switched in and out of memory as needed.

Server virtualisation, used in this context and not in the context of RAID, means that multiple storage arrays are aggregated into a single virtual pool of storage. Instead of having individual servers with their own independent and direct-attached storage (DAS), many servers access a single, consolidated pool of storage, a storage area network (SAN) which is abstracted by a virtualisation layer between the servers and the drive arrays.

There may also be grouped storage arrays from a single supplier, such as 3PAR or Pillar which are not configured as a SAN but instead function as a single, large data store with individual LUNs apportioned to particular server/application combinations. The LUNs can be based on multiple separate drive arrays all presented as a single logical - or virtual - pool.

A benefit is that stored data can be moved from one array to another without shutting applications down. Another is that storage array utilisation can be increased.

An additional extra virtualisation layer can tell applications in operating systems that must have a fully-assigned lump or LUN of storage that they do have it. In fact, they don't. The full LUN is provisioned in smaller portions so that only the storage really needed to hold written data is assigned. This is termed thin provisioning and its benefit is that actual disk capacity purchases can be delayed until needed. Disk capacity utilisation increases still more.

Virtualisation so described works at the block level. But filers can also be virtualised into, say, a single name space.

Currently storage virtualisation generally refers to a combination of SAN multiple array aggregation and thin provisioning and also filer virtualisation.

Server virtualisation refers to the sequential re-use of the same physical hardware by multiple virtual (server) machines, Storage virtualisation refers to the elimination of empty disk capacity by driving up utilisation through consolidating disparate storage islands and thinly-provisioning applications. There is no hardware reuse involved.

This no storage virtualisation can be the equivalent of server virtualisation.

Storage virtualisation can work well with server virtualisation by, for example, very quickly and/or automatically provisioning storage resources for an application at the same time as you provision virtual server resources for it. But that is a different matter, although it is a very beneficial thing to do in terms of reducing storage provisioning time and easing the IT administration burden.