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Why are LTO autoloaders significantly more expensive with SCSI interface vs SAS?

Posted on 2010-11-10
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I was looking at prices on an HP LTO 4 Autoloader (HP), and the one with the SCSI LVD/SE interface is nearly $1000 more than the one with the SAS interface. Is there a reason for this?

We already have one LTO 3 autoloader with the SCSI interface, is it a bad idea to have one autoloader on SCSI and one on SAS on the same backup server, or is it best to have them both on SCSI?
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Question by:Haunty
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by:balasundaram_s
balasundaram_s earned 200 total points
ID: 34104502
Parallel SCSI (LVD/SE) is faster than Serial SCSI ( SAS), so its expensive.
It depends on the environment, if data being copied from one tape drive to another, then I would prefer to have both same technology.
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by:Haunty
ID: 34105990
The interface on the autoloader I was looking at is Ultra-320 SCSI which is only slightly faster than SAS 3Gbps according to the specs, it doesn't seem to justify such a large cost difference.
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SelfGovern earned 300 total points
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I'm confused by balasundaram's comments, since you can have 6Gb SAS on many, many devices, but you'll be hard-pressed to find much that's 640MB SCSI (and they have essentially equal performance).  See http://en.wikipedia.org/wiki/Parallel_SCSI#Ultra-640

Parallel SCSI (pSCSI) became too complex of an implementation, because it requires that you get 32 signals from point A to point B within a very short time of each other, and without corruption or crosstalk... at about Ultra 320, it became clear that it would be easier to implement faster speeds over serial-attached SCSI (SAS) where you had one very, very fast signal without any of the crosstalk or synchronization issue.  For what it's worth, the same thing has happened inside your computer, where buses went from parallel (PCI) to serial (PCIe).

In answer to your question -- there's no issue with mixing pSCSI and SAS libraries in the same server.  With HP Libraries and autoloaders, I do recommend you load HP's free LIbrary and Tape Tools, and make sure your firmware is up-to-date periodically.  See http://www.hp.com/support/tapetools
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