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
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:
- The type of data to be backed up
- How your backup software uses tapes (interleaving, single stream, etc.)
- Media vaulting frequency
- Tape subsystem footprint constraints (high density tapes use less floor space per GB)
Note that cost is listed last as you should first establish what your requirements are, then look
at the price.
Cougias' answer to this question.
This was first published in August 2004