Data Tape Constraints

By Henry Newman (Send Email)
Posted Sep 14, 2006


I was asked recently if there were limits on the number of times tapes can be written, read and reused. I didn't know the answer, nor had I seen much written on the topic, and a quick search provided little in the way of technical answers. Like any other medium, data tapes have limits and finite lifespans. To prevent data loss, such parameters must be followed.

So began my quest to get some answers, but first, what are the real issues? Do tapes wear out? Is this true for high-end tapes such as IBM 3952 and midrange tapes such as LTO-3? If tapes really do wear out, why don't disk drives wear out? What in a tape wears out? Most importantly, if tapes do wear out, what should you do, and what are the questions that should be asked?

Disk vs. Tape

Tapes and disks are both mechanical devices. A disk drive is designed to have the hardware required to write and read the storage built into the package, while a tape must be loaded into a tape drive to be written or read. Enterprise and SATA disk drives are designed to be powered on, kept at a wide designed temperature range (approximately 5 to 55 C for enterprise drives, and from 0 to 60 C for SATA drives).

You cannot use disk like tape, and you cannot use tape like disk and apply the same usage patterns and specifications to each other. The devices are made from different types of technology and the design of each dictates different usage.

Hard drives are designed to be read and written continuously. While this may be true for tape drives, the design of the tape device itself does not lend itself to be used in the same way. The tape media itself is not placed on a hard metal surface like disk drives, but on a thin piece of plastic. Anyone who expects the behavior of tape and the behavior of disk to be the same for environmental specification and usage need look no further than the materials used to make both. I am not suggesting the abandonment of tapes in favor of disk drives. What I am saying is that disk drives should run within their environmental ranges, and tape drives should be run within their usage ranges. Follow the manufacturer's specifications.

The bottom line, according to my research, is that all tapes have a recommended number of times they should be written end-to-end, and a number of times that an area of tape should be written or read.

Vendors that sell tape products test under extreme conditions, writing and reading the tape thousands of times in the same spot, end-to-end, and every which way possible. Just because vendors do these extreme types of testing does not mean that that usage is recommended. These specifications are no different than a disk drive company testing drives under extreme temperature, humidity and other environmental factors not within the written specification for the disk drive.

You cannot use disk like tape, and you cannot use tape like disk and apply the same usage patterns and specifications to each other. The devices are made from different types of technology and the design of each dictates different usage.

And Now for the Numbers

I turned to Imation, one of the leading manufacturers of tapes and media, and Jim Goins, one of their media experts. When I asked Jim the question about tape life and usage, he immediately sent me his public spreadsheet on the topic (see below). I should have asked him in the first place instead of wasting time with Google. Now this sheet does not cover every tape type and it applies only to Imation tapes, but the concepts are the same for tape from any vendor.


Tape Usage and Durability Chart
Click to view Tape Usage and Durability Chart (courtesy of Imation)

Clearly, tapes have a life expectancy and should be replaced when those limits are reached. I was unable to find a consolidated list of specifications on tape end-to-end passes and full file passes from other vendors. Bits and pieces of information are out there on the Internet but not a detailed list with all of the information in one place. Ask your vendor or supplier for the information on each tape type used in your environment.

What It All Means

This is all pretty important, given the potential for data loss. If you use tape outside the recommended specifications, you are risking data loss. I don't think manufacturers make up specifications to sell you more hardware. The market is far too competitive for that. Storage manufacturers stay up nights worrying about data loss. Vendors go to extremes to understand how their products work, how to maintain the products and what conditions affect their product — all for the sake of ensuring that what you write is valid and what you read is the same as what you wrote.

TEXTIf you lose data with recycled tapes, then you will get little sympathy from anyone, especially the tape manufacturers, since they pretty much wash their hands when you buy products from third parties that recycle tapes.

One of the big problems I see is the use of recycled types, given that we now know that tapes cannot be used forever. So why do companies sell recycled tapes? If you are not in control of the tape, you have no idea how many times it has been used and whether you are within the manufacturer's specification. If you lose data with recycled tapes, then you will get little sympathy from anyone, especially the tape manufacturers, since they pretty much wash their hands when you buy products from third parties that recycle tapes. So if you plan on using recycled types, you know the risk.

Until I researched this issue, I was unaware that tapes had a lifespan and could not be rewritten forever. I'm surprised this information is not widely published and distributed. Perhaps tape manufacturers are worried that publishing this information would affect sales, or perhaps users just don't think to ask.

I had known for years that recycled tapes had reliability issues and that tape vendors were always telling customers not to use recycled tapes. Now I understand the reasons behind the high failure rates. After this, I will never recommend recycled tapes for data that any of my customers care about.

This article was originally published on Enterprise Storage Forum.

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