OT: determining bytes written to tape

[/begin rant]

We ran into a bit of a puzzle here.

It started innocently enough, with someone asking whether Oracle tablespaces that are largely empty are really being compressed before they are written to tape.

We use Veritas Enterprise NetBackup with a StorageTek tape silo. Our backups are done in parallel to multiple tape drives, and we back up many machines at once so these backups are all multiplexed and interleaved.

After weeks of discussion with the vendors, it is starting to look like the only way to determine the actual number of physical bytes written to tape has to be determined by backing up one tablespace file repeatedly on one tape, until the tape fills up, then infer (guesstimate) how many bytes were written.

The tape drives' hardware can be told to back up files in "native" mode instead of in "compressed" mode, but one cannot ask them to report how many bytes were written in "compressed mode" without the tape hardware "decompressing" the bytes written.

Veritas NetBackup doesn't know how many bytes were written to tape, it can only report how many bytes the original file had. It gets its values from the tape drives.

This took me by surprise, are we the only ones who want to have an exact reading of how many bytes are written to tape?

[/end rant]
Received on Wed Mar 19 2003 - 06:26:54 CST