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

Deactivation of write-back caching prevents the most severe of the ATA RAID corruption problems. The tradeoff for RAID-5, however, involves even lower performance. As discussed in the previous section, the legacy methodologies for RAID-5 impose a significant performance limitation on this type of RAID, one that is partially addressed by vendors through the default use of write-back caching. Unfortunately, deactivating write-back caching usually has a dire effect on performance.

And yet, there is a further dilemma. Since ATA vendors are not currently certifying the recovery of drives that deactivate write-back caching, it is possible that drives operating without this function will have greater failure rates. So, while vendors do achieve the goal of preventing an obvious source of data corruption, they run the risk of increasing drive failure.

The other showstopper problem posed by disk failure in ATA RAID-5 solutions is the parity recalculation problem. If the system crashes during the middle of a write process, the parity calculation that applied to the active data write may be inconsistent. As a result, when the system is powered back on, it is necessary to regenerate this parity and write it to disk. Since the system will not be able to determine where the last active write was in progress, one solution is to recalculate all of the parity on the RAID-5 group. This recalculation process takes time and every sector of each participating RAID group must be scanned. Based on various leading system implementations currently available, the parity recalculation process can take between 45 minutes for a standard RAID-5 group of five or six drives to several hours for larger sets.

Currently, the parity recalculation problem is a significant drawback of software RAID-5 solutions. There is no easy way to avoid this penalty when using the traditional read-modify-write approach to RAID-5. Some RAID-5 solutions in the ATA universe do avoid this limitation, however, through the use of "pointers" that records the positions of the in-place updates. These pointers are stored either on another disk or within a small NVRAM component. This technique is called "dirty region logging." If the pointer is stored on. another disk, it generates an additional I/O step that will further degrade performance. Nonetheless, it will deliver a performance benefit by avoiding the need to recalculate all parity upon power failure; however, it does not eliminate the associated reliability problem since, in the event of a crash, some parity will still be left in an inconsistent state until recovery can be performed. If dirty region logging is combined with write-back-caching, the original reliability problem caused by a power failure or power spike event will result in inconsistent or corrupt data. Another solution is to log the data and parity to a separate portion of the disks before responding to the write request; the logged data and parity are then copied to the actual RAID stripe. In the event of a failure, the data and parity can be copied back to the RAID stripe. This approach, while much more reliable than dirty region logging, imposes additional disk latency and makes RAID-5 writes significantly slower.