How many tape drives?

[James M. Dunn]

All,

How many tape drives can hange off one SCSI card?

Thanks,

[A. Clay Stephenson · Accepted solution]

For SE-SCSI, 7; for HVD/LVD 15. That being said, there is a big difference between "hanging" and being an active tape drive. For example, one active DLT-7000 or 8000 will all but saturate an HVD/FWD bus but the same bus could handle 2 DLT-4000's and keep them both streaming. So you need to change your question to indicate the type of SCSI bus and the types of drives and the maximum number that you plan to have active at any one time.

[James M. Dunn]

A. Clay,

This is what I thought....you see it all the time but the brain wanders.

Do you know how may an FC card can handle?

Thanks

J

[A. Clay Stephenson]

That too depends upon the type of tape drives, and the bus topology. For example, are you going from an FC card to Multiplexer? In that case, the multiplexer might have 4 ports which could in turn could each support 15 tape drives. Each of those ports and their load would be analyzed as though it were a single SCSI bus. Then those four buses at their maximum anticipated load would be analyzed against the throughput of the Fibre Channel adapter itself. The basic answer for FC is "a lot" but from the perspective of the host computer, splitting the i/o across more FC adapters is a good thing eventhough you might be below the capacity of a single FC.

[Bill Hassell]

And to amplify a bit on what Clay said, are you planning on using all of them simultaneously? That changes the question considerably. You can rewind a dozen tape drives with no problems, but trying to keep four Ultrium 460 drives busy (on the same SCSI bus) is not very likely. The reason is that these drives need 30 Mb/sec each to run full speed (four drives = 120 Mb/sec), a bus rate that will be extremely difficult to achieve. Even with a fibre interface, getting 120 Mb/sec from opening and reading files will be quite a task.

The high end tape drives have run into these data starvation problems so much that they offer a data rate matching. In English, this means that the drive will stream at a slower speed, good because the drive doesn't have to resync over and over, but total backup time is increased because the data is too slow from the computer.

[A. Clay Stephenson]

What we are both trying to tell you is that you have to greatly narrow your question to the types of tape drive, the bus topology, and the anticipated load. The backup software is important as well; a hi-speed backup package like DataProtector is all but essential. Even reading files from disk fast enough to keep several hi-speed drives happy is a far from trivial task. Nevertheless, the more separate i/o channels (to disk and tape) that you have, the better.