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Sync Up Virtual Servers and Storage

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Migration of virtual machines
VMs can be deployed easily, moved around and then deleted when you're done with them. But it's that simplicity of deployment that can cause problems (see "Boot from SAN in a virtualized world," below).

VMware's VMotion enables the movement of VM images from one physical server to another for maintenance, to create new VMs and for business continuity.

Boot from SAN in a virtualized world
One of the virtues of connecting servers to shared storage is the ability to boot those servers from the Fibre Channel or IP (iSCSI) SAN. Server virtualization is proliferating, and 70% to 80% of virtual machines are attached to shared network storage that they increasingly rely on for many of their capabilities.

Boot from SAN technology evolved from the remote boot capability of Linux, Unix and NetWare NAS environments. Remote or network boots--in which the system boots over the LAN from a remote boot server--have been used for a number of years.

In the case of boot from SAN, the SAN replaces the LAN and remote boot server; via host bus adapters, the server communicates with the SAN and boots the system. When blade servers were introduced,

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and with them diskless blades, the need for SAN booting heightened.

Boot from SAN capability is more complex in pre-Windows Server 2003 environments. Prior to that release, Microsoft Corp. required a boot disk that was attached directly to the server with the operating system always in communication with the boot volume.

The same capability can be applied to guest operating systems running on virtualized physical servers, where it offers a variety of benefits, including the ability to consolidate servers and dispense with power-hungry disks, centralized management and speedy disaster recovery.

While there are numerous advantages to boot from SAN, there are also limitations in the Windows environment. Among them are the inability of Windows servers to share boot images--each server needs a dedicated LUN to boot and it's difficult to automate deployment of boot images across the network.

"We use VMotion not only for business-continuity purposes, but so we can do maintenance during the day," says Iannace at the Brattle Group. "We can transport the guests from one physical server to another. That has been a big plus for us since we can actually do maintenance during the day by moving the guests running on one physical host to another in real-time."

VMware recently announced that its Storage VMotion, a new feature in VMware Infrastructure 3.5, enables the nondisruptive migration of VM disk files from one data store to another.

This was first published in February 2008

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