A warning on the pitfalls of virtualization

While server and storage virtualization continues to make its way into corporate data centres, one independent software vendor says companies need to be aware of virtualization’s impact on disk fragmentation and its resulting affects on system performance.

In a recent white paper entitled Virtualization and Disk Performance, Burbank, Calif.-based Diskeeper discusses the pitfalls of server virtualization, including the fragmentation of the hard drive.

“Fragmentation causes the disk head to go to more spots on the disk to pull back one file rather than just going to one space on the disk to pull back a file,” said Michael Materie, Diskeeper product manager and author of the white paper.

“It’s the difference between you reading a paper versus reading a shredded paper. Because hard drives are slower than CPU and memory, it really bottlenecks the whole system.”

Materie added that the CPU and memory are working at electronic speeds whereas the disk is a physical device with a mechanical head moving around the disk platter, making it “ions slower than the CPU and memory on any system.”

As a way to alleviate the problem of disk fragmentation, Materie recommends running advanced automatic defragmentation on host and all guest operating systems.

“A lot of times businesses buy expensive hardware and say, ‘This OS is using 10 per cent of the CPU, we’re wasting 90 per cent that’s going unused,’” he said. “This is a way for them to put more OS on the same hardware and get better usage of CPU without having 17 boxes, buy cooling systems and pay for power to run multiple servers. It’s a way to consolidate as well.”

Another recommendation Materie makes in the white paper is to pre-allocate virtual disks. But that’s a practice which Patrick Lin, director of product management at VMware, finds questionable.

“Pre-allocation of virtual disks so that they are physically contiguous on disk is fine for locally attached SCSI disks,” he said. “This may reduce disk seek and disk head latency time if the assumption is that one virtual machine (VM) at a time will run and execute disk I/O requests and the disk driver does not do some sort of intelligent I/O sorting.”

But if multiple VMs are issuing interleaving I/O requests, Lin added, then pre-allocation could in fact make matters worse. “Moreover, we would question if space pre-allocation would help even in the case of no VM interleaved requests when using a SAN array,” he said.

Lin does agree, however, with a few of the paper’s suggestions such as using high performance disks. “This is the best advice,” he said. VMware’s “EXS Server recommends the use of high performance local SCSI or SAN arrays.”

Diskeeper also suggests keeping the host paging file on a physical disk separate from those of the virtual disks. According to Lin, this is another EXS best practice recommendation.

Gordon Haff, an industry analyst at Nashua, N.H.-based Illuminata, said although there is a clear gap between disk performance, memory performance and CPU performance, “disk defragmentation is far from being a cure for system performance in a virtualized world.”

“There may be circumstances where it does help disk performance some what and if you can help disk performance than that may very well be a good thing for an IT department to spend some money on,” said Haff.

“It may be a ROI justifiable expense for an IT department, but it’s very far from solving big performance problems in data centres,” he added.

But according to Frederick Broussard, a research manager at IDC, the issue of disk defragmentation is very relevant to server stability.

“Anything that’s going to help keep the system stable, up and running and being responsive is going to be helpful to any computer,” said Broussard. “That’s where the Diskeeper solution comes in.”

Without going into detail, Materie said Diskeeper is currently partnered with Microsoft on its virtual server technology, specifically the Windows Longhorn server expected to be released in 2008 or later.

He added that VMware is another potential technology pairing for Diskeeper. “I do envisage us doing something with them,” said Materie. “VMware is one of the early advocates of defragmenting, they have different architectures, both guest host and hypervisor.”

But, he added, Diskeeper is still needed within an operating system. “We’re not competing directly with them (VMware), we’re more of an add-on.”

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