Microsoft (and many others) recommends having a discrete disk pair for the OS, another for transaction logs (email or other database) and a RAID 5 array for the database itself. This recommendation is based on a flawed analysis. However, while performance is enhanced when drives are added and dedicated to these tasks, it would be even greater if the array card was allowed to manage the load across all disk drives. And, since Windows systems rarely demand equal access to all drives, generally one or more pairs will be idle while the other is overloaded.
To determine how many drives you need to achieve the desired level of performance, group them in a RAID 1.0 array. On average, the system will be much faster in everyday use while single-threaded tasks (like disk defragmentation, disk repair, backup and restore) will use the full drive set, not a single drive.
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This was first published in June 2007