Tape - What is a tape media "pass" ?

Asset ID:	1-71-1012600.1
Update Date:	2011-05-06
Keywords:

Solution Type Technical Instruction Sure

Solution 1012600.1 : Tape - What is a tape media "pass" ?

Applies to:

Sun Storage LTO Tape Drive
Sun StorageTek L180 Tape Library
Sun StorageTek L40 Tape Library
Sun Storage L500 Tape Library
All Platforms
Checked for relevance on 6-Msy-2011.

Goal

Description
What does a "pass" refer to in the context of tape backup media?

Tape backup media are frequently listed as being "good for 1 million passes" however there is no definition of a "pass" supplied with the media. There have been occasions when a "pass" has been confused with the number of times that a piece of media has been mounted (loaded) in a tape drive. This document addresses that confusion.

Solution

Steps to Follow
One tape pass is defined as any point on the tape passing the head in either direction. The head is the only point of contact in the tape path for the recording side of the media.
This definition is the industry standard. You can find it documented in this white paper: http://www.dlttape.com/NR/rdonlyres/73CD2358-29E9-46AA-A67E-1170248B7126/0/DurabilityWhitePaper.pdf

For 9x40, DLT and LTO media, this definition applies.

There are some LTO technology primer flash videos which show this in action

http://www.ultrium.com/newsite/html/about_tech.html

HP also publish a whitepaper on LTO-3 (their Ultrium 960):

ftp://ftp.compaq.com/pub/products/storageworks/whitepapers/5983-0148EN.pdf

From this you can see that LTO-3 has a 16 channel head and in LTO-3 format there are 704 tracks. As such, to fill all 704 tracks you would take 44 full passes to fill an entire tape. That means that 1 pass is approximately 9GB (native mode) of data for each pass.

For example, writing a 23Gb chunk of data to an LTO-3 tape and then rewinding the tape amounts to 4 passes:

pass 1 (9GB native) +
pass 2 (9GB native) +
pass 3 (6GB native) +
pass 4 (rewind).

As you can see, a "pass" is an event which occurs many times during reading or writing data, and is not the same thing as a tape load or mount event.

lto, dlt, 9x40, 9x80, pass, media, mount, count, definition
Previously Published As 82839

Attachments

This solution has no attachment