Monday, July 26, 2010

RAID Group Options and Considerations

When discussing storage solutions, one of the most frequently asked questions is when to use which RAID type. First the caveats. Some array manufacturers use a modified version of the standard RAID types or have an architecture that lends itself to one type or another. In general though, these guidelines are applicable.

The most common RAID configurations in use today are RAID 10, 5 and 6. Three factors need to be considered when determining which RAID type is most appropriate for your needs; Performance, Availability and Price.

RAID 10 – Mirroring and striping. Provides the best IOPS performance and is typically used for OLTP databases. Database vendors typically recommend RAID 10 because it provides the best IOPS performance. From an availability standpoint, RAID 10 can survive multiple drives failures as long as two disks from the same RAID 1 group are not lost. RAID 10 is the most expensive RAID configuration with 50% of the capacity being dedicated to protection.

RAID 5 – Block Level Striping with distributed parity. RAID 5 is the workhorse of most storage environments. Many storage vendors have made enhancements to their solutions such as dedicating a processor to perform parity calculations and using intelligent caching algorithms that perform full stripe writes whenever possible - minimizing the performance impact associated with parity calculations. RAID 5 configurations can withstand a single drive failure. As disk drive capacities increase there is more and more debate about the risk of data loss associated with RAID 5, in particular with slower SATA devices.

More information on this can be found in this article by Adam Leventhal.

From a cost perspective RAID 5 is very attractive since only one drive of capacity is used for protection. On HDS storage systems 7+1 RAID 5 groups are standard on their enterprise systems and 8+1 RAID 5 groups are most common on the midrange solutions.

RAID 6 – Block Level Striping with double distributed parity. RAID 6 incurs the biggest performance penalty from parity calculations. For write operations the performance impact is frequently between 25% and 30% compared to RAID 5. This penalty may prevent RAID 6 from consideration for environments requiring high levels of performance. A RAID 6 array group can withstand up to 2 simultaneous disk drive failures. The cost of RAID 6 depends on the size of the array group. Some maintain that since you can increase the number of data drives in the array group that the cost of RAID 5 and RAID 6 are the same. For example a RAID 5 8+1 array group is the same relative cost as a RAID 6 16+2 array group. This is of course manufacturer dependent and you need to understand how what you have actually works.

RAID 6 is highly recommended for SATA drives due to their large capacities, slower speeds and lower MTBF ratings.

So in short, here is what we recommend.
• RAID 6 for SATA drives
• RAID 10 for applications with the highest IOPS requirements
• RAID 5 for SAS\FC drives to support most workloads. As drive sizes increase

RAID 6 may become the preferred option.

A thorough overview of RAID technologies is provided on Wikipedia.


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