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Virtual desktops and storage

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The VDI storage challenge

Unlike traditional hypervisors, Parallels' Virtuozzo Containers virtualization product is architected to minimize storage requirements. Any newly created virtual container references the underlying OS, and it's only as the OS and applications within

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each virtual container change that changes are stored within each container's virtual disk. "We can create a 50 Gig virtual disk, but the actually used space may only be a few megabytes and [it] grows as files change," explained Jamie Moore, senior sales engineer in the enterprise group at Parallels. Because of its efficient use of storage, Virtuozzo works well with both networked and direct-attached storage (DAS), making it very attractive for smaller environments.

Both Citrix Systems and VMware realized that for their VDI offerings to succeed they had to overcome the detriment of having to store and manage full VM images for each virtualized desktop, and they did. But unlike Parallels, they implemented their solutions outside the hypervisor.

Citrix Systems has solved the problem within its Provisioning Server. It supports creating one or several golden virtual machine images, as well as VM templates that define resource configurations such as memory, disk space and I/O devices. Once a golden VM image is created, it can be used for multiple virtual desktop users. Virtual desktop changes that occur during a user session are stored and retained in a so-called write-cache. "You can get hundreds of virtual desktops running off the same image, with literally 100% savings on disk space," noted Citrix Systems' Hsu. Different from Parallels, the write-cache feature doesn't work with DAS but requires shared storage.

VMware has addressed the storage issue with its VMware View Composer, a new component in VMware View 3. View Composer uses VMware linked clone technology to create desktop images that share virtual disks with a master image to conserve disk space. Individual desktop VM images are linked to a master virtual machine image and, as a result, can simply be patched or updated by updating the shared master image, all without affecting user-specific settings, data or apps. During runtime, VMware View Composer combines the master image with changes the user made. "View Composer reduces storage needs and costs by up to 70% while simplifying desktop management," VMware's Sakac said.

This was first published in August 2009

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