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I have an HP StorageWorks Ultrium 1840 LTO-4 external tape drive attached to a machine running Ubuntu 18.04.4 LTS (kernel 4.15.0-99-generic) using an LSI20320IE PCI-X x4 SCSI adaptor with a VHDCI to HD cable.

Writing to the tape drive is slow and involves a lot of shoe-shining. It maxes out at about 36MB/s. That is an order of magnitude slower than it should be capable of.

I have tried connecting the tape drive to an older machine using an Adaptec 29160 PCI SCSI adaptor and an HD to HD cable. The write speed of the tape drive was pretty much exactly the same (36MB/s).

The hard drives are usually able to transfer data between each other in excess of 150MB/s. But just to check that it wasn't a hard drive or SATA bus limitation, I fed the tape drive from /dev/urandom via buffer and it was still maxing out at about 36MB/s.

I suspect the problem is either with the tape drive itself or at OS level, not the SCSI adaptor or cable. Perhaps there is an mt command to change a setting on the tape drive. How can I diagnose and fix the problem?

Edit: The HP L&TT diagnostic test produces the following output:

    - Firmware rev B56D is out-of-date for Ultrium 4-SCSI as of Thu May 26 19:00:00 2011; update to rev B63D.
    - Rule 19.0 (Performance & Config./Scsi configuration):
    - The current SCSI configuration is likely to be limiting the performance of the drive.
    - Please check that your HBA is the correct type for the drive and that the cabling is good.
    - Current SCSI configuration: narrow   Recommended: wide
    - Current SCSI transfer rate limited to: 40 MB/sec.   Recommended: 320 MB/sec. or better
    - There were 20 rules and 141 subrules checked.
    - Device Analysis completed, and warnings have been reported.

So it looks like the adaptor is detecting the link as narrow instead of wide. That would explain the speed problems. But what could cause that?

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You don’t have differential cabling so HBA is switching into UltraWide mode. Get proper cable in between your HBA and SCSI device.

https://ask.adaptec.com/app/answers/detail/a_id/11007/~/scsi-standards-%2F-transfer-rates-%2F-bus-width-%2F-cable-length

(mixing LVD and SE is what’s limiting your performance)

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