Using LTO 3 Tapes In an LTO 2 Drive

Published: July 23, 2008

Hey, Joe:

Do you know if we'll be able to read tapes created from a new Linear Tape Open Ultrium drive (400 GB native format) on our old Ultrium LTO 2 drives? We want to take tapes from a scientific vessel at sea that may purchase the LTO 3 drive and read the tapes back in the office on our LTO 2 drive.

--Anne

When comparing LTO 2 and LTO 3 tape drives, the rule is that an LTO drive can usually read tapes that were created at a lower tape drive level, but not tapes that were created at a higher tape drive level. So if your ship has an LTO 3 drive and your office has an LTO 2 drive, the LTO 3 drive will be able to read LTO 2 tapes, but the LTO 2 drive will not be able to read LTO 3 tapes. So without adjustments, your office won't be able to read the tapes created on the scientific vessel.

This should hold true even if you're formatting the LTO 3 tapes at their native 400 GB uncompressed capacity. True, the LTO 2 drives create and read tapes at 400 GB, but their 400 GB format is a compressed format, while the 400 GB put out by the LTO 3 drive is an uncompressed native format. So they're not quite the same thing.

It's also worth noting that this disparity between the drives exists whether you are using LTO 3 and LTO 2 drives with an iSeries or any other type of computer.

However, if you're running your LTO drives on companion i5/OS or OS/400 machines, there is a way to format the tapes so that you can write a tape on an LTO 3 and then read it on an LTO 2. (You should also be able to do this for LTO drives attached to different platforms, but I just work with the LTOs on iSeries, System i, and Power i boxes.) Here's how you pull off that trick under i5/OS.

There is a parameter on the Initialize Tape command (INZTAP) called Tape Density (DENSITY). By default, DENSITY is set to *DEVTYPE, which will automatically format the tape for the highest density your tape drive will support. This means that a tape initialized on an LTO 3 will be formatted to back up either 400 GB per tape in native mode or 800 GB per tape with 2:1 compression. The LTO 2 drive won't be able to read the LTO 3 tapes because it uses a different tape, one that can only be formatted to read and write 200 GB of data in native mode and 400 GB in compressed mode.

However, your LTO 3 can format, read, and write to LTO 2 tapes if you initialize the tapes correctly. If you want the LTO 3 to produce tapes compatible with your LTO 2 drive, follow these steps:

1. Put an Ultrium LTO 2 tape into your LTO 3 drive.

2. Initialize the tape using an INZTAP command with the following parameters:

INZTAP DEV(TAPxx) DENSITY(*ULTRIUM2)

When DENSITY is set to *ULTRIUM2, the drive will set the recording format on the tape to an Ultrium LTO 2 format, rather than the LTO 3 format that is normally used with the drive.

3. Save your files to that tape. By default, all of the i5/OS and OS/400 SAV commands write data to the media format already loaded into your tape drive. So even though you have an LTO 3 drive, it can still write and read tapes as if it were an LTO 2.

The downside of this technique is that you're not using your LTO 3 to its full capacity. Your shipboard tape drive is basically losing 200 GB to 400 GB of usable backup space for every tape that it writes. But since compatibility is an issue with your ship-to-shore setup, this is one way to get around it.

There are a couple of other thoughts I had on this issue. First, this may not be that big a deal if the tape drives will only be mismatched in the short term. If you're buying an LTO 3 tape for the ship, and you're going to buy another companion LTO 3 drive for the office sometime in the near future, then it's a short term problem and it's possible to perform the initialization work-around.

Second, does your scientific ship need the LTO 3? If it turns out that they only need an LTO 2 for storage, then you could conceivably ship them your LTO 2 and keep the LTO 3 for yourself. You may want to think about upgrading the home office, and because LTO 2 tapes are automatically readable in LTO 3 drives, there wouldn't be a need to have the ship change its INZTAP parameters.

Finally, if you able to establish that your ship can get away with using an LTO 2, what if you go out and buy a used LTO 2 and have both of your locations running the same format until you're ready to update? It would be a cheaper way to go, and you wouldn't have to always worry about properly formatting your tapes.

--Joe

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