Tapes report full before their Native Capacity is reached

Problem

Backup jobs are reporting media full, long before the rated native capacity of the tape cartridge in use.  For example, an LTO3 is rated for approx. 400 GB uncompressed, but Backup Exec shows the tape as full, but only containing approx. 300 GB uncompressed.  Often the achievable capacity will be close to 75% of the rated native capacity

Cause

The tape drive may have a clogged tape head, which reduces the drive’s capacity. When this occurs only limited tape alerts or errors are reported by the drive, which results in Backup Exec being unable to return specific diagnostic information (errors) and limits the troubleshooting that can be done from Backup Exec.

Vendor specific diagnostic tools will be required to confirm the diagnosis. When a tape head is clogged, the tape cartridge is only able to store 50% of the capacity when writing in one direction. It still is able to get 100% capacity in the other direction. The 50% and 100% average out to 75% of rated capacity. Symantec has confirmed this behaviour with multiple tape hardware vendors.

Note 1: Backup Exec writes to a tape until the tape drive tells Backup Exec to stop writing due to the tape being full. Therefore the capacity numbers shown in Backup Exec are for the customer’s information only; they do not affect how much data can actually be put on the tape.

Note 2: There are other possible explanations for not being able to obtain the native capacity of a tape drive:

Data expansion during compression (if data affected by this is encountered, disable the use of compression)
Rewrites, often seen as excessive error counts (if this is seen, clean the drive, replace the media or use vendor diagnostics to troubleshoot)
Damaged media (replace the media)

Solution

Troubleshoot to confirm the issue by performing the following steps:

• Ensure that the drive has been cleaned their tape drive multiple times, if possible using a fresh cleaning tape.
• Run a test job with compression disabled that will overwrite the tape and fill it.  Note: This is needed as the compressible value for a given set of data is difficult to determine, making only the native capacity of the cartridge an official, testable number for the media.
• Contact the tape drive (or tape library) vendor for further hardware troubleshooting.