Virtual machines need defragging too

Weighing in on a seemingly perennial question asked by system administrators, IT research firm Osterman Research is recommending that virtual machines be defragmented just as often as physical drives. 

“The need for defragmentation is even more acute in virtual environments,” stated a newly posted white paper co-authored by Osterman and disk management software vendor Diskeeper. “Physical hardware in a virtualized storage environment must support more operating systems and so can undergo even more disk access and more stress than in a non-virtualized environment.”

Disk fragmentation occurs when data is continually written and rewritten to a disk, which can cause data to be scattered across different physical sectors of the physical drive when there is not enough contiguous space to write a file in one uninterrupted block. As a result, when a file is called, the disk must do more work recollecting the bits of the file from different locations, slowing performance. 

Windows, as well as third-party software firms, offer defragmenters to reassemble fragmented files. Fragmentation is not as large of a problem on Unix systems, due to the way that the OS writes files to disk. 

According to the paper, virtual disks can become just as fragmented as physical disks. For instance, a single file on a virtual disk may be split into four fragments, and the virtual disk itself may be split into another three fragments on the physical disk itself. The cumulative effect of fragmentation on both layers reduces system performance considerably. 

“Disk I/O in one virtual machine has a cascading effect on disk I/O in other virtual machines, and so the problem of excessive disk I/O in virtual machines is, in fact, even worse than what would be experienced in a physical disk environment,” the paper states. 

Not surprisingly, Diskeeper offers virtual disk optimizer software, V-locity, which can prevent virtual machines from becoming too fragmented. But both Microsoft and VMware have recommendeddefragmentation in various support forums as well.