Limitations of conventional RAID-5 on the ATA platform inhibit promise of ATA in the enterprise - Tape/Disk/Optical Storage

Computer Technology Review, June, 2003

RAID-5 specifically is a methodology for achieving redundancy of data on a group of drives without sacrificing half of the available capacity as mirroring (RAID-1) and its variations (i.e., RAID-10) do. RAID-5 achieves this storage efficiency by performing a parity calculation on the data written to disk and storing this parity information on an additional drive. Should a disk drive fail, the data can be recovered by computing the missing data using the parity and data blocks in the remaining drives. RAID-5 is an especially popular methodology for achieving redundancy because it is more economical than RAID-1 insofar as more disk-drive capacity can be rendered usable from a group of active drives. It has been estimated that RAID-5 accounts for 70 percent of all drive volumes shipped into RAID configurations (the actual percentage of RAID-S per discrete RAID configuration is lower, given the popularity of striping and mirroring with OLTP). This would be sensible given that RAID-5 is typically associated with f ile serving and similar workloads, which account for significantly more capacity usage on a global basis than higher intensity OLTP workloads, for which RAID-5 is rarely used.

The attractiveness of RAID-5 to the ATA storage opportunity is even more pronounced. Given the great volumetric density advantages of the ATA platform versus SCSI and Fibre Channel, ATA is ideally suited for larger capacity storage installations. The capacity efficient RAID Level 5 is functionally allied with this focus on maximum capacity per dollar of storage cost. Though some have speculated that the high density advantage of the ATA platform will result in a willingness of end users to employ mirroring given a surplus of raw capacity, the fundamental laws of technology would seem to argue against this. The sharp and continuous rise in the processing power of the Intel chip, for instance, has not been accompanied by an increase in the sales of 4-way or 8-way servers--quite the reverse is true, with one- and two-way processor servers today dominating most application usages on the market. In the storage market, given its long evidenced storage elasticity, greater volumetric densities will be accompanied by a growth in the desire to maximize capacity as well as prevent disruption from drive failure. In this view data protection based on parity strategies, as opposed to redundancy ones, will be maximally appealing--provided that they pose no crippling obstacles in their implementation.

Today, even for expensive solutions on SCSI and Fibre Channel platforms, there are obstacles to the universal ascendance of RAID Level 5 and the foremost among these is speed. For instance, one reason that RAID-5 is rarely used for OLTP application storage is because of its low performance for such workloads. As a tradeoff to its storage efficiency benefits, RAID-5 imposes additional computational as well as I/O burdens on the underlying magnetic disk storage. These additional burdens in many cases result in the general characterization that RAID-5 is slower than other types of RAID. And, in fact, with many commercial RAID controller technology--both hardware and software-- RAID-5 is often the slowest performing configuration, especially when compared to straight striping (RAID-0), mirroring (RAID-1) or striping mirroring (RAID-10). In some cases--for instance, software RAID from vendors like VERITAS--the difference in performance between RAID-S and RAID-0 is as much as lox.