Should RAID 5 configurations include different disk volumes?

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ANSWERED June 2009:
A. To get your preferred layout, you would need to create a 3+1 RAID 5 volume with three disks' worth of capacity available for storage and one disk's worth of capacity used for the parity calculations. Using a separate volume for the operating system (OS) is recommended for some servers, but this depends a lot on the workload of the application that will reside on the host.

In your situation, however, you're constrained by the number of drives available. I would never recommend creating an OS drive that has no resilience -- this would force you to use two of your available four drives for the OS in a mirrored (RAID 1) configuration. This would leave only two drives remaining for data, which would again need to be configured as RAID 1, leaving you unable to utilise your preferred RAID 5 configuration and having only ¼ of the available disk space for data.

My recommendation would be to create a 3+1 RAID 5 RAID group and then put the OS and data on this volume.