Introduction to RAID

Henry Newman

I am sure that over the years, many of you have seen a great deal written about RAID hardware, software, and a myriad of related topics. In this column, I will look at the whole topic of RAID from a slightly different perspective.

I divide RAID devices into two categories: cache-centric and storage-centric. You may see different terminology used to describe the same thing. Some people call these RAID types "enterprise" and "mid-range", for example. Whatever you call them, there are major architectural differences between these two device types.

Cache-Centric RAID

I use the term cache-centric because RAID devices in this category depend significantly on data residing in cache to ensure good performance. Cache-centric devices generally have feature sets such as:

• Very high reliability (dual everything, virtually no downtime)
• Large caches (e.g., 64 GB or greater)
• Designed emphasis on using RAID-1
• Software that allows snapshots, hot upgrades, and many other features
• If RAID-5 is supported, generally a smaller number of devices supported stripe sizes (e.g., 4+1 4 data disks plus 1 parity drive as compared to 8+1 configurations available on mid-range products)
• Cache is always mirrored
• Large number of front-end connections
• Support for many types of remote mirroring (e.g., dark fibre, IP)
• Smaller block sizes
• Huge amounts of storage managed in a single box (e.g., 100 TB)
• Per-component reliability testing
• Error monitoring for all hardware, including disk monitoring
• Designed for I/O processors (IOPs) not streaming I/O
• Far more bandwidth from cache to servers than from cache to disk (I call this front-end bandwidth and back-end bandwidth)
• Very high cost per MB of storage compared with storage-centric RAID

Cache-centric RAID vendors include:

• EMC Symmetrix
• Hitachi Data Systems 99xx series
• IBM Shark

Most of these products can run both on UNIX servers and on IBM mainframes.