Use caution and consider the following when mixing Fibre Channel (FC) and SATA drives in a single midrange array:
Managing array resources. Most midrange implementations don't allow administrators to control the allocation of controller resources for the disk drive types. As a result, cache, processor resources and internal ports are all managed as if the disk types were the same.
Controlling data placement. If controlling whether data gets placed on FC or SATA drives is a manual process, the administrative costs and risks associated with managing this process may be higher than just putting everything on more expensive FC disk drives.
Disk drive duty cycles. The reliability of FC and SATA disk drives is measured in very different ways. The failure rates of high-performance FC disk drives are generally benchmarked based upon 24-hour-a-day usage, while availability measurements for SATA drives are generally based on two to four hours of usage a day.
Disk drive processors. FC disk drives contain two processors -- one to handle the command processing for data access and to set up data transfer, and the other to control the servo mechanism to keep the head aligned with the data to ensure correct and optimal data transfer. SATA drives contain only one processor to handle the servo positioning, command processing and data transfer. Under heavy loads or multiple requests to the same disk for data, performance will be significantly impacted on SATA disks with only one processor to handle the multiple I/O requests.
About the author: Randy Kerns is a senior partner at the Evaluator Group.
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