Virtualisation has great potential to simplify and reduce the cost of all manner of business applications. The ability to provision applications dynamically, and potentially add extra resources when demand requires it, allows companies to respond to needs more quickly.
But virtual applications, particularly those with high disk access requirements, can suffer from performance issues compared to more traditional ways of delivering IT.
Even if there are clear cost benefits at the basic provisioning level from virtualisation, because servers only need to be started up when needed and resources can be used elsewhere when not, these performance issues can make the benefits more moot.
In this feature, we look at how enhancing your virtualisation performance can ensure that the savings in provisioning are matched by the savings during everyday usage as well for every type of virtualised application.
Virtual vs traditional
One of the key differences between a virtualised environment and one delivered via traditional IT is that the storage will come from a centralised storage area network (SAN) pool rather than a local device. The ability of this storage pool to deliver data to virtual environments quickly is key to overall performance.
Typically, the overhead of running a virtual machine will be between 1 and 5 per cent compared to running the environment natively on 'bare metal', where the CPU is concerned, and memory performance can be 5 to 10 per cent lower, neither of which seem like a huge hit compared to the benefits of virtualisation.
But the disk performance difference can be much slower, because the SAN will be a single pool of disks connected to the CPU and memory via an external link. Configured for cost-effective capacity, there could also be one large mechanical hard disk serving many virtual machines, meaning it will have to serve many times more access calls than a disk that is reserved just for one environment.
More sensible configuration of the SAN will involve a greater number of smaller disks, but this is still likely to entail greater utilisation of each disk than would be the case if each virtual machine had its own dedicated hard disk, as would be the case in a traditional IT environment.
For this reason, applications which make frequent calls to the hard disk are considered not very good candidates for virtualisation. An application with high disk usage can not only experience slowness itself, but also effect other virtual machines sharing the same resources.
Whilst this can also be the case with high CPU and memory utilisation, adding more resources to combat this scaling issue can be achieved.
The same cannot be said of high hard disk utilisation. A greater capacity can be added, but the SAN pool will only be able to deliver a certain level of throughput and access time, beyond which it could be necessary to dedicate a SAN specifically to a particular virtualised application, which is beginning to get away from the benefits the virtualisation concept is supposed to deliver.
The alternative is faster SAN appliances, but there is a limit to the performance available from a SAN using conventional hard disks. The constant increase in data density on hard disk platters has continued, despite predictions more than a decade ago that the technology would soon hit a limit in magnetic grain size that would be impossible to surmount.
But this progress can't keep pace with needs of a virtualised environment, where data density means a disk can have the capacity to serve a large number of virtual machines, whilst its access time isn't many times greater. Configuring disks in a RAID can only provide some benefit, and mostly for lengthy delivery of sequential data rather than frequent random file access.
Just as Flash-based storage has begun taking over in desktops and notebooks for end users, it promises to solve the disk access problem for virtualised applications, as well. Where the throughput of a solid state disk (SSD) is only three or four times greater than that of a conventional hard disk, its access time can be hundreds of times quicker.
Overall, this will translate to IOPS (input-output operations per second) figures that are hundreds of times greater for small file access. This makes an SSD perfect for coping with multiple instances of virtual applications that require frequent file reads and writes. So switching your SAN to SSDs would seem like the answer.
However, the cost of SSDs will be at least three times greater than a conventional hard disk, and that's just for the lowest-end consumer-grade multi-level cell (MLC) drives.
Enterprise MLC drives will be more expensive still, and the premium single-level cell (SLC) drives can be more than ten times the price of hard disks with equivalent capacity. The capacity of single hard disks also still outstrips SSDs noticeably, with very large SSDs commanding a particularly high premium.
So, for the time being at least, Flash-based SANs will need to be used sparingly, when there is real need.
Fortunately, HP's 3PAR StoreServ can provide the best of both worlds. The StoreServ 7450 is specifically designed for Flash storage. It has processor, memory and software architecture that are all optimised for delivering the best performance available from SSDs.
A 4-node base with a pair of 2U enclosures supports up to 24 drives, but this can be expanded to 240 drives using extra enclosures, and SSD options range from low-cost consumer-grade MLC drives, to enterprise-grade MLC or SLC drives, with capacities up to 1.92TB per drive.
This makes the maximum raw capacity a very healthy 460.8TB. It also offers up to 24 Fibre Channel ports running at 8Gbits per second alongside up to eight 10Gbit Ethernet iSCSI ports.
On its own, the HP 3PAR StoreServ 7450 solves the performance problems that can reduce the cost efficiency of virtualised applications with high disk usage. But, although prices are reasonable, it would still be a costly choice for serving a mixed virtualisation infrastructure where some virtual applications don't rely so heavily on disk performance.
Used in tandem with other HP 3PAR StoreServe appliances using conventional hard disks, however, it can provide benefits with no downsides. The StoreServe 7450 can be configured as a Flash cache, so that applications needing speed are served by SSDs, whilst those with more modest requirements are served by conventional hard disks.
In this way, applications that need enhanced virtualised performance for more cost-effective usage get what they require, without the financial burden of moving your entire SAN infrastructure to Flash-based storage.