DLT 8000 maximum tape I/O

[Mark Blonde]

We are presently tuning our Veritas Netbackup software and are trying to figure out what value to assign to the data buffer size parameter. However, to do so, we would need to know the maximum tape I/O size supported by a DLT 8000 tape drive. Does anyone know what this value is?

Thanks in advance.

[Vincent Farrugia]

Hello,

What do you mean by maximum tape IO size? If you are referring to speed, it is 6Mb/s native, 12Mb/s compressed. If you are referring to block size, the recommended size is 32kbytes.

HTH,
Vince

[Mark Blonde]

I'm not exactly sure what it means. It's not a spec I've heard of when dealing with tape drives. But here is the statement from Veritas:

"Because the data buffer size equals the tape I/O size, the value specified in SIZE_DATA_BUFFERS must not exceed the maximum tape I/O size supported by the tape drive or operating system. This is usually 256K or 128K bytes."

Their recommendation is to set it to 256K provided the tape drive and/or O/S can handle it.

Hope this helps.

[Michael Lampi · Accepted solution]

Your real question should be "What is the maximum tape I/O size supported by my system when using Netbackup software and writing to a DLT8000?".

By "I/O size supported" you really mean the amount of data that your system is reliably able to aggregate and present to the tape drive so that the drive can stream.

If your system is unable to provide data to the tape drive at its streaming rate, then the tape must be repositioned to a point just after the last block successfully written in order to write the next block of data.

If your system is slow to provide data, then it is better to write smaller blocks and use the extra space between these blocks to provide time for your system to generate more data to be written. Yes, the overall tape capacity decreases somewhat, but your backups and restores will be faster overall than they otherwise would be, as any time the tape is repositioned there is a significant delay.

If your system is fast, then increasing the block size improves the overall throughput as well as the amount that will actually fit on a given tape.

A DLT8000 drive is capable of reading and writing tape block sizes of megabytes. The HP-UX tape driver is also capable of such tape block sizes.

So, after this long-winded response, the answer to your question is "Try different sizes (32KB, 64KB, 128KB, 256KB, 512KB), and see which works best for you.".

[Mark Blonde]

Thanks Michael, what you say makes sense.