Sometimes you'll need to balance this against your available resources. If you have a system that has four drives and you're emulating four servers on that machine, you can place each of the virtual drives for the three most commonly-accessed servers on separate physical drives. The least-used machine can share the same hard drive as the host operating system. This cuts down on the amount of contention between the guest operating systems and the host operating systems.
If you have the money and the drive bay space, it doesn't hurt to always dedicate a drive to a given virtual server whenever possible—in other words, to link a virtual disk to a physical disk. Keep in mind that this is not for system disks, but for data disks; you can't run a virtual server's OS files from a linked disk.
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This was first published in May 2006