Tape drive startup and initialisation

[Richard Jordan_1]

I am currently developing an application that writes/reads data to/from an HP DDS DAT40i tape drive, however I think that my problem probably applies to tape drives in general.

As part of our requirements we have to be able to show the write/read data rate achieved. All we do is to simply time how long the write takes and from the amount of data written we can calculate the data rate.

However, the problem is that the first write session is always slower than any subsequent writes. This causes a problem for us because the first write fails the data rate requirement.

I suspect that maybe what is happening is the tape mechanism has to get ready, or that the tape performs some seek function before the data is written ?

Does anyone know what actually happens on the first write session and why this is slower. Is there any HP documentation available that explains what is happening within the tape drive.

Thanks, in anticipation.

[David Ruska]

By "first write session", if you are talking about the first write from load point, that write will take longer on most drives because the drive is rewriting the ID portion of the tape (which identifies which density the tape is written at).

I would caution against depending on a specific instantanteous data rate on a tape drive. If the drive needs to do physical retries (i.e. repositioning), any one write may get held off for a considerable time.