Implementing RAID in Enterprise Environments Page 2


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Storage-centric RAIDs

I use the term storage-centric because these devices primarily depend not on the cache but rather on the underlying storage and architecture. These devices have the following features:

  • Good reliability, but nowhere near as reliable as the cache-centric devices
  • Smaller caches (usually 2GB to 8GB)
  • RAID-5 support with up to 20 disks in a RAID-5 LUN (Logical Unit Number)
  • RAIDs allowing cache mirroring can often be turned off, wherein significantly improved write performance is realized
  • Far less software for management and maintenance
  • Far less storage in a single box than enterprise boxes support
  • Support for streaming I/O with large blocks (which is not supported well by Enterprise RAIDs)
  • Support for high IOPs, but with the smaller caches, most I/O has to be to disk
  • More back-end bandwidth than front-end bandwidth (more channel bandwidth from cache to disk than from cache to the servers)
  • Much lower cost per MB for storage than cache-centric RAIDs

Examples of storage-centric RAID vendors and products include:

  • EMC CLARiiON CX line (Dell resells EMC)
  • LSI 5600 (OEM'ed by many other vendors)
  • Sun T3 and S1
  • Hitachi Data Systems 9500
  • DotHill
  • DataDirect Networks
  • Ciprico
  • Many others
There are many vendors in this market area as it is much easier to develop products than for the cache-centric devices given the lower reliability requirement. Additionally, vendors do not have to create mainframe interfaces and the market has a lower expectation for software. Most vendors in this market space are now looking at options that use ATA drives instead of SCSI drives. They are doing this to provide greater data density and lower price points than are available from SCSI drives. Of course, you are trading reliability as bit error rates for Fibre Channel or SCSI drives are an order of magnitude greater than those for ATA drives (according to the Seagate website). RAID devices of this type are far more prevalent and usually have two or more times more bandwidth to disk than to the front-end servers.

Page 3: So Which Is Better?

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