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I have a pile of LTO 2 tapes, and both an LTO 2 drive (HP Ultrium 460e), and an autoloader with an LTO 3 drive in (Tandberg T24 autoloader, with a HP drive).

Performance of the LTO 2 tapes in the LTO 2 drive is adequate and consistent. HP L&TT tells me that the tapes can be read and written at 64 MB/s, which seems in line with the performance specifications of the drive. When I perform a backup (over the network) using Symantec Backup Exec, I get about 1700 MB/min backup and verify speeds, which is slower, but still adequate.

Performance of the LTO 2 tapes in the LTO 3 drive in the autoloader is a different story. HP L&TT tells me that the tapes can be read at 82 MB/s and written at 49 MB/s, which seems unusual at the write speed drop, but not the end of the world. When I perform a backup (over the network) using Symantec Backup Exec though, I get about 331 MB/min backup speed and 205 MB/min verify speeds, which is not only much slower, but also much slower for reads than for writes.

Notes: The comparison testing was done on the same server, SCSI card and SCSI cable, with the same backup data set and the same tape each time. The tape and drives are error-free (according to HP L&TT and Backup Exec). The SCSI card is a U160 card, which is not normally recommended for LTO 3, but we're not writing to LTO 3 tapes at LTO 3 speeds, and a U320 SCSI card is not available to me at the moment.

As I'm scratching my head to determine the reason for the performance drop, my first question is:

While LTO drives can write to the previous generation LTO tapes, does doing so normally incur a performance penalty?

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2 Answers 2

Yes. LTO drives can Write to 1 Generation earlier, and Read 2 Generations earlier. So your LTO3 drive should write 3 and 2 media, and read 1-3 media. But there are speed differences because of the different technology in the drive.

In an LTO-3 the drive's head is "smaller". It's not quite as simple as this, but the analogy is mostly true. The smaller LTO-3 drive head has to write twice to each "spot" on the LTO-2 tape in order to write data. The drive should read at the LTO-3 rate still as it only has to read any part of the LTO-2 tape's "spot" to determine the data.

As for the different rates in the software, that's very odd. Likely a driver or application level issue.

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You can use the older cartridges in newer drives. It part of the LTO standard. I do it in some environments.

However, much of this depends on what you're backing up and your source system. I don't see a big drop between using my LTO2 cartridges in an LTO2 drive versus LTO3. You do. The only big difference here is your autoloader. What's the make/model compared to your LTO2 (standalone?) drives?

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I have added the detail to the original question. Although the autoloader is a different manufacturer (Tandberg), the drives are both HP drives. –  hmallett Nov 13 '12 at 16:35

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