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Maximum number of drives in a RAID-5 set

What is the maximum number of physical drives recommended in a RAID-5 set? Does the drive architecture (SATA vs. SCSI) impact this recommendation?

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The maximum number of drives in a RAID-5 set will vary by vendor implementation. Some vendors support a dozen or more drives, while others support a fewer. Having more drives in a RAID-5 set lets you spread I/Os across more spindles in a stripe, which can improve performance and reduce the storage overhead tax. The storage overhead tax is associated with having extra disks for mirroring and data protection, including parity. Thus, the more disks that are sharing parity drive, the lower your storage overhead tax for data protection.

The downside to a wide RAID-5 set is that you can place more overhead on parity drives even though a rotating parity scheme is being used. This can impact drive rebuilds in a wide stripe, as performance is degraded using normal drive rebuilds and exasperated with many drives competing for parity resources. You want to strike a balance between having too few drives and too many. Confer with your vendor for their specific recommendations.

As to your question of the drive architecture's impact on RAID, there are two things to consider. One is the type of disk (single or dual processor), which can have an impact on performance; the other factor is the type of interface and how it is configured. SATA disk drives typically utilize single processors, while SCSI disks often have dual processors. Thus, for performance and I/O intensive environments, SCSI and Fibre Channel disks can provide better performance, while SATA is good for storage intensive storage applications.

This was first published in October 2004

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