Q

Assessing failure rates for tape

My question is this: How many tapes is it safe to use to complete a full backup? Obviously, the more tapes used, the greater the chances of failure. Using older backup technologies, backups can span three to four tapes before a full backup is complete. Would it be time to look into a new solution?
It is difficult to come up with the perfect balance when it comes to tape media capacity. Your statement is correct; the more tapes a backup spans across, the greater the chances of one of the tapes failing thus causing potential data loss. However, this is mostly true in the case of a large database backup for example, where you cannot necessarily restore only part of it. On the other hand, other types of backup such as file server data, allow partial restores. In this case, you must keep in mind that having all the backup data on one large capacity tape means that you loose it all should media failure occur.

Larger capacity tapes have also introduced a new challenge when it comes to off-site storage. Incremental backups take up much less space than full backups; if partially filled tapes are sent off site daily, data cannot be appended. In some cases, this has caused large capacity media users to complain about poor utilization and high cost.

To address the last part of your question, an eventual technology refresh is unavoidable in a field as dynamic as IT and looking at new solutions might not be a bad idea regardless. However, when it comes to selecting media capacity, your choice should be influenced by:

  1. The type of data to be backed up
  2. How your backup software uses tapes (interleaving, single stream, etc.)
  3. Media vaulting frequency
  4. Tape subsystem footprint constraints (high density tapes use less floor space per GB)
  5. Cost
Note that cost is listed last as you should first establish what your requirements are, then look at the price.

Read Dorian Cougias' answer to this question.

This was first published in August 2004
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