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Where are the “Holes” in Virtualisation?

LONDON February 10, 2011

– Optimising Lean Storage Utilisation and Maximum I/O Performance

Intense demands for network efficiency and lower operating costs are driving the phenomenal growth of virtualization. Yet despite outstanding virtualization technologies, the ability to effectively share resources in virtual environments runs into three barriers:

     1. I/O bottlenecks and performance degradation created by accelerated        fragmentation - the action of breaking up a file into pieces and        writing it to scattered locations on the disk - in virtual platforms.      2. Virtual disks that are set for dynamic growth don't shrink again when        data is deleted. They waste the free space instead.      3. Virtual machines compete for shared I/O resources and their use is not        effectively prioritized across the platform.

David Marshall

The host virtualization server’s file system – VMFS for VMware ESX Server, or NTFS for Hyper-V – has to be able to contain all the VM files for every VM that is hosted. A VM file can be preconfigured to its full size from the start. This is called Thick Disk in VMware’s terms and Fixed Disk by Microsoft; it’s the default and the most popular setting. But this setting automatically eats up each file’s limit in storage space, even if the file is only filled with data to a quarter of its capacity.

The alternative is a better option for saving space: use of the Thin or Dynamic Disk setting, which sets the space consumption of the file to the actual file size. Files can grow or can be added as needed. The only problem with Thin/Dynamic disks is that deleted files aren’t really deleted; they’re simply marked as space that is unavailable for writes. This is why the Thin or Dynamic disk will only grow, and never shrink – unless it has some help.

Better performance and better storage space utilization are both elements of the efficiency promised by virtualization. Sharpening efficiency in both of these areas can result in off-the-charts delivery on the virtualization promise.

SOURCE Diskeeper Corporation

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