Original URL: http://www.theregister.co.uk/2011/11/21/virtualisation_storage/

Virtualised storage: the perfect space-saving solution

Make the most of your disks

By Trevor Pott

Posted in Network Futures, 21st November 2011 10:30 GMT

In case you have been living under a rock for the past month or so, we are in the middle of a hard drive shortage. Now that we can’t simply add more drives at will, storage utilisation has suddenly become important again.

It should not have taken this crisis to shine a light on this topic; we are reminded at regular intervals that enterprise storage utilisation generally sits at around 50 per cent.

Utilisation below 50 per cent means that your storage is occupying double the amount of physical space required to get the job done. That’s double the drives, double the systems to house them, double the power infrastructure and double the power draw.

This requires additional cooling and additional network ports to support what is essentially wasted space.

Inevitable demise

As well as these easily measurable costs, there are more nebulous issues to consider. Storage dies. Mechanical magnetics or flash SSDs: all storage eventually comes to an end. The more drives are in play, the more you have to replace every month.

Replacing drives means technician time. It also means processing RMAs and verifying new media is free of defects. Every drive you swap exacts an annoyance penalty on those who maintain the hardware.

Drive swaps trigger RAID array rebuilds.

This is bad in a couple of different ways. The most obvious is that your data is in a more vulnerable state during an array rebuild than when it is fully protected.

Array rebuilds also place a great deal of additional stress on all drives in the array and can easily push other borderline drives over the edge.

Depending on your RAID level, the loss of two drives can lead to catastrophic data loss.

Letting go

It is time to explore storage virtualisation. Just as virtualisation of operating systems brings consolidation and management bonuses to various compute workloads, storage virtualisation is all about making the best use of your available disks.

You are probably already familiar with the basics of storage virtualisation. All storage is done centrally and is handed off to relevant servers over protocols such as iSCSI, FCoE, NFS or even CIFS.

These drawbacks are a small price to pay for the utilisation benefits

There is overhead involved: shared, networked storage is rarely as fast as dedicated, local storage. Implementation and management take some training, as they certainly aren’t as easy as simply plugging in another drive to a physical server.

These drawbacks are a small price to pay for the utilisation benefits that storage virtualisation brings. First and foremost is free space consolidation.

By pooling all your drives together, you are not only pooling the data but the free space. If you have enough free space, you can take some disks – or entire filers – off the line until they are needed again.

Enviably slim

Centralised filers can make use of other technologies to wring even more space out of existing disks.

Thin provisioning allows you to over commit your storage. Let’s say you assign a large block of potential storage to a system. Instead of creating a massive reserved block equal to that assignation, you actually commit to disk only the storage that the system in question has actually used.

Most filers can also do deduplication. Blocks of identical data are not duplicated on the disk, resulting in potentially huge savings, depending on the applications involved.

Virtual desktop infrastructure (VDI) is an excellent example of how centralised, virtualised storage can offer serious utilisation benefits.

In a VDI scenario, you have virtual hard drives each containing an operating system and set of applications.

Avoiding waste

In nearly all cases, there are no more than a handful of different “master" images from which all virtual desktops are descended. If each virtual desktop were to have a reserved block of storage equal in size to the virtual hard drive it has been provisioned there would be a great deal of duplication.

Deduplication looks at all the blocks occupied by the various virtual hard drives and writes only one copy of any identical pieces to disk. Since most of the files on a given set of virtual desktops are identical, only one copy really needs to exist.

Thin provisioning reduces the storage consumption by ensuring that virtual hard drives are not reserving storage blocks for empty space.

Combined, the various storage virtualisation technologies can help you make the best use of your storage infrastructure. This is an important consideration, whether you are uncertain of your ability to source new storage on demand or simply want to make the most of your money. ®