On VDI and storage architecturesby Robin Birtstone, theregister.co.uk
April 13th 2011 9:00 AM
One of the most complex aspects of desktop virtualisation is working out what to do with all the users' data. Sizing and planning storage infrastructures to support hundreds or thousands of remotely-hosted desktops is a daunting prospect, especially to those who have relied heavily on local PC hard drives in the past. What is the best approach?
"Thirty-five to 40 per cent of the cost of VDI lies in enterprise storage," estimates Andy Goddard, practice leader at Computacenter. "When you have to provide a lot of VDI for lots of users, it ramps up quickly. An average Windows user might have 10Gb of space." But there are ways to get the costs down.
Understanding where to store virtual machine data is the first challenge. "Typically you'll store it on a storage area network (SAN), and then share the virtual machine data out," says Joe Mayhew, CTO at Calyx, a managed services provider.
The speed of SAN
Getting the data to and from the SAN to the server quickly enough can be a challenging task. Servers need low-latency data access for performance purposes. Typically, SANs have connected to servers using the specialist high-speed Fibre Channel storage networking protocol.
However, Fibre Channel is typically a higher-cost system, and depending on the type of applications that you're running, the performance gain may not always warrant the cost. In addition to the extra capital expenditure, it requires more specialist management skills, which can bump up your operational costs too.
ISCSI is an alternative SAN connectivity protocol, which unlike original Fibre Channel uses Ethernet as a transport layer. It is typically cheaper than Fibre Channel, and with 10Gbit Ethernet connections now available, it can offer relatively high performance. That said, there are downsides. In particular, iSCSI is a lossy protocol although this can be mitigated somewhat by putting it on a separate network segment.
The alternative is Fibre Channel over Ethernet, although this requires an upgrade to Converged Enhanced Ethernet (CEE) to provide the lossless transport layer that Fibre Channel requires.
"There are vendors doing hybrids - Brocade does FCoE and iSCSI," says Mayhew. "You plug it all into the same switch, so that you have converged network adaptors that talk both protocols." These hybrid systems are still fairly new, though.
Storage networking isn't the only place where system designers need to think smarter; the storage devices themselves must be designed to reduce costs. Organisations will want to balance performance and price.
So Solid Crew
At the high-performance end, some solid-state storage can boost performance in critical areas. One approach is to store user data on lower-cost hard drives, while storing virtual machines themselves on solid state drives that offer higher performance.
Much of the smarts come with good software. A storage resource management package is crucial if you are to make the best use of your storage infrastructure. This will effectively manage the allocation of disk space in a virtualised storage system, and generally carries a number of benefits (depending on the package you choose). For example, automated storage tiering can help to minimise the capital outlay on hard drives, too.
Administrators can use automated tiering to put 'disposable' and file swap data on cheaper, slower disks. David Chalmers, CTO of HP, recommends simplifying automated rules for tiering. "Having that automated, rather than having storage analysts moving it around, is where you get a big benefit," he says. The simplest rule focuses on time; if a file is accessed infrequently enough, then it trickles down from a set of SAS drives to SATA storage instead, for example.
It also pays to consider deduplication, says Calyx's Mayhew. Storage resource management software now frequently includes deduplication capabilities that will strip out duplicate data, sometimes at byte level, to minimise storage impact. What other measures can be taken to reduce storage overhead?
The way that desktops are provisioned can have a significant effect, says David Angwin, European marketing director at Wyse. "You're not picking up someone's whole environment and dropping it into the data centre as-is", he says. Instead, desktop images will be drawn from a pool, and the user profile will be built on the fly using streamed applications.
Ultimately, effective storage in a virtualised desktop architecture involves a mixture of hardware and software expertise.
Choosing the right devices and the right networking equipment for the job will give you a foundation for optimal performance, but without proper storage management, you could still see costs skyrocketing.
The key here is automation, so that administrators already tasked with learning new ways to manage a virtualised desktop environment aren't overtaxed by the huge pools of data that they suddenly find under their control. ®
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