History 2001: StorageTek Tape Array

Tape arrays or RAIT (redundant arrays of independent tapes) are by now an old story that never seems to lead anywhere.

Data General was the first to offer the devices in 1992. Companies such as Land-5 or Ultera Systems are currently specialists in the matter, but their sales remain confidential.

RAITs nevertheless do offer 2 advantages:
1) thanks to a parity tape drive, backup is much more secure;
2) since the drives work at the same time, total transfer rates are more or less equivalent to the combined total of the transfer times for each drive.

Even so, 2 major criticisms can be advanced about the technology:
1) since the backup takes place simultaneously on several drives, all cartridges must be kept together, and never mixed up (effectively, you have room to lose one of them, since the content can then be recovered by parity calculation);
2) more than anything, the one big drawback appears if one of the drives fails or if a cartridge is in poor condition during backup, since when a tape drives stops, or must rewind in order to start over, the transfer rate of the entire RAIT system drops to zero, which is obviously devastating for a subsystem that is supposed to recover data at very high speed.

StorageTek has clearly understood all this, since it is poised to unveil before the end of the year its own RAIT. The tape controller will be housed in one of its libraries, which means that cartridge transfers no longer pose any problems.

Moreover, planned configurations are for 5 drives plus 2 for parity, or even 8 drives and 3 for parity.

The additional spare drive, in the latter case, will in fact serve as an automatic substitute to any drive that fails, or report a defective cartridge, in order to ensure perfect continuity for backup operations.

This particular formula nevertheless involves an additional cost that will likely limit it to heavy users only. StorageTek also expects to release a simpler RAIT-1 (mirroring) before offering its heavy guns.

This article is an abstract of news published on issue 161 on June 2001 from the former paper version of Computer Data Storage Newsletter.