Archive Reliability, Tape vs. Disk

I was recently having dinner with an IT executive for an internet service provider, when the conversation turned to the so-called “Tape Revival.”  “Tape?” he said, “c’mon!”  He had heard about tape’s killer archive application but he held onto the common misconception that tape is not as reliable as disk for archiving purposes.  Because tape is playing an increasingly important role in both archive and backup, and especially in active archiving applications, I decided to present the case for tape.

To begin with, I explained that the archival shelf life of today’s modern tape formats such as LTO Ultrium is up to 30 years based on accelerated life testing.  (Not really a stretch by any means, I still have some party mix audio tapes from school that still work 27 years later!)  Disk on the other hand, is typically rated at about 3 to 5 years.  And I admitted that even if tape drives aren’t around after 30 years, best practices call for a migration at least every 7 to 10 years to keep up with hardware/software changes and take advantage of faster and higher capacity generations of media.

“Yeah, okay, but how about error rates?” inquired my dining companion.  I explained that the numbers here also look better for today’s tape than they do for disk.  The generally published numbers for hard error rates are as follows:

LTO Tape: 1 bit in 1 x 10E17 bits

EnterpriseTape: 1 bit in 1 x 10E19 bits

Desktop SATA: 1 sector in: 1 x 10E14 bits

EnterpriseSATA: 1 sector in: 1 x 10E15 bits

EnterpriseFC/SAS: 1 sector in 1 x10E16 bits

I had to remind him that the error rates for disk are the number of bits read before the failure of a sector, whereas the error rates for tape are the number of bits read before the failure of a bit.  As someone who deals with massive amounts of data, he wanted to know what this meant when archiving terabytes or petabytes of data.  Simply put, I said, it means you have less risk associated with tape archives, especially if the applications involve encryption or data compression, as the loss of a single bit can render the data unrecoverable.  Finally, to drive the point home, I told him that expert analysis based on these error rates in large archives, does show that LTO and Enterprise tape can achieve higher reliability ratings than disk.

I think my internet friend was satisfied with the explanation.  He, like many other IT professionals, just wasn’t up to speed on the latest developments in tape today compared to the legacy formats of the 80’s and 90s, but these facts have been the case for more than five years now.

“Reliability is just part of the tape story however,” I continued.  There are numerous other benefits associated with today’s tape including improved capacity and lower cost per/GB, improved transfer speeds, energy efficiency, and applications like active archiving that improve accessibility.  So stay tuned to the active archive blog for more….. I promised to save that part of the conversation for the next meal as long as he was buying!