We all saw the Facebook news article on using Blu-Ray for archival storage. This got me thinking and doing some research on Blu-Ray technology to see if it is really suitable for archiving things for long periods of time. Facebook seems to think it is a good idea, but I’m not sure, and here’s why – at least using standard technology:
The interface for Blu-Ray is either SATA or USB. USB is an extremely poor interface in terms of the high data reliability of the channel. The channel was never designed for data high reliability and has a much higher potential for silent data corruption than the SATA channel, which has a much higher potential for silent data corruption than SAS or Fibre Channel. So if you really care about your data you would be using the SAS or Fibre channel interface. And as Blu-Ray does not support these interfaces, Blu-Ray would be off my list for archival devices.
The Blu-Ray standard does not provide any means of doing end-to-end data protection. With SAS devices on disk you have the ANSI T10 PI/DIF standard, which is supported by all disk vendors. And I’m pretty sure all HBA vendors support it and a number of operating systems including Linux, AIX and Solaris for sure. Tape, which is the most used cold storage archival technology, has its own ANSI end-to-end data protection that ensures that each block is written correctly.
Hard Error Rates
Enterprise disk hard error rate is 1 sector in 10E16 bits; nearline drives with SAS interface (4TB drives today) have a hard error rate of 1 sector in 10E15 bits; LTO tape is 10E17th bits; and enterprise tape is even a few orders of magnitude higher. All of this data is published and well known.
What little I could find on Blu-Ray was 10E12, which means that assume the data is in 512 byte sectors, which is unclear from the document; that mean that every 9 TiB, I get a hard error =10000000000000/(1024*1024*1024*1024). (It should be noted that this was on the Panasonic web site but was removed. This is all I could find.)
If the optical industry wants to move toward the archive market they need a lot more reliability, end to end data protection and transparent easy to find information – no matter what Facebook thinks.
Labels: optical storage,data storage,Facebook
posted by: Henry Newman