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 rebuilding is a process that must occur after a RAID-5 set experiences a disk failure. When a disk fails in a RAID-5 set, the missing data and parity contained on the failed drive must be regenerated on a replacement drive once the new working drive is inserted into the set or an existing hot spare is activated as the replacement drive target. Similar to initialization, the process of rebuilding requires that each data block on the system is read and the XOR computations are performed in order to obtain the absent data and parity blocks, which are then written onto the new disk. Often, during the process of reading all data from the disk to recompute the missing data and parity, bad sectors may be encountered, and it is no longer possible to rebuild the array. Depending on the size of the RAID group and the capacity of each drive, the rebuilding process is time consuming and may degrade the use of the drives in the RAID-5 set for normal activity. Both the initialization and the rebuild processes are ad ditional performance and reliability penalties of conventional RAID-5 implementations that will occur as a matter of normal operation.

Conventional RAID-5 Reliability Penalties

Based on the dominant approach to implementing RAID-5 at present, there are several discrete reliability problems that arise in common implementations. Many of these reliability concerns are generated by events like power failure, which can often set in motion a cascade of correlated failures. For instance, a power failure not only interrupts active writes, which can invalidate any parity that is in the process of being updated, but can also bum out disks with aging components. As a result, power failures can often cause data loss in many types of RAID implementations by destroying both the parity and data associated with a "parity stripe." Part of this is due to characteristics of the ATA platform itself, such as differences in assembly line quality control processes that have more tolerance for production variability. However a large part of the quality differential is due to ineffective strategies employed by the ATA RAID community using legacy RAID methodologies.

The most salient reliability problem in the ATA RAID arena is the nearly universal use of write back caching in all ATA implementations, even those driven by hardware RAID solutions. Write-back caching is a function that is enabled by the inclusion of small cache memory components within the disk drive electronics. By providing this additional memory, the drive is able to commit to write commands by buffering bursts of data in memory prior to the full completion of writing data onto the disk platter. When the drive signals that a write has been completed, the application moves on to its subsequent operation even if the data in question remains in the drive's write-back cache. Quicker completion of writes leads to faster application performance when disk latency is the primary performance limitation. Because of this, the logic behind making write-back caching a default strategy is straightforward: to increase the performance of the disk platform.