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MSL4048 with three Ultrium 920 drives

abcxyz
Frequent Advisor

MSL4048 with three Ultrium 920 drives

Hi,

I'm connecting a MSL4048 with three Ultrium 920 drives which are daisy chained.

The Device Analysis under Library and Tape Tools shows "The Drive has Detected Errors consistent with cabling issues"
Please check your cabling and connections between the host and the Drive"

What could be wrong!
14 REPLIES
Rasheed Tamton
Honored Contributor

Re: MSL4048 with three Ultrium 920 drives

Hi,

Just a tip.

If this particulars model requires termination, then you need to do the termiantion.

Please check the hardware manual came with the device.

Regards,
Rasheed Tamton.
abcxyz
Frequent Advisor

Re: MSL4048 with three Ultrium 920 drives

Thanks for the reply !

The Last device is terminated on the SCSI Bus.

The Manual does not offer much help on multi-drive connectivity.Infact have tried swapping the cables as well but still the Library and Tape tools still report errors.

Any help is highly appreciated.



Richard Bickers
Trusted Contributor

Re: MSL4048 with three Ultrium 920 drives

L&TT uses the internal logs to detect SCSI cabling errors (amongst many other things). In this case it is picking up SCSI parity errors so there really is something wrong with your cabling.

Is it with just one drive or is it with all three?

We don't recommend daisy chaining these drives but that's more a matter of performance than cabling reliability - though you'll have quite a few connectors involved and latest SCSI speeds are very high and more sensitive to cabling issues than they used to be.

If it's not termination (remember - just one terminator at the end) then try reducing the number of drives to see if the problem goes away.

Another thing you could try is the L&TT device performance test. That runs the drives at full speed (one at a time) so you can see what sort of peformance you get from each one independent of the data source and backup app. You might find that one drive goes more slowly if you have cabling issues.

Some things to try at least.

The warnings are not repeated (though you can find them again in the support ticket under drive history->device analysis) so if they continue to be reported it indicates that you are continuing to have problems.

I assume you have a supported HBA with the correct drivers etc.

Good luck. Cabling issues are hard to track down - so start simple and work your way up.
It's more interesting when it's gone wrong
abcxyz
Frequent Advisor

Re: MSL4048 with three Ultrium 920 drives

Hi Richard,

Thanks for your reply and I'll check with one drive at a time to Narrow down.

But,Can the error be because of Daisy Chain.As you said the drives are not recommended to be daisy chained.

Many Thanks.
Richard Bickers
Trusted Contributor

Re: MSL4048 with three Ultrium 920 drives

SCSI should daisy chain OK but you have to be careful to stay in spec with cable length etc. Also the connectors drop a little signal strength so will only make matters worse with a few in line.

We advise 1 HBA and cable per drive because of performance - these drives are getting fast. It *is* also more reliable but that doesn't mean your setup shouldn't work. You might find it works with 2 rather than 3 for instance.

You'll have to experiment a bit to find the problem. I'd still recommend going to 2 HBAs - a daisy chain of 3 is pushing it.

It's more interesting when it's gone wrong
Curtis Ballard
Honored Contributor

Re: MSL4048 with three Ultrium 920 drives

Three 920 drives daisy chained is going to be a problem. At Ultra 320 speeds it is possible to daisy chain devices but the signal loss in each connection is enough that the most I have ever been able to get working is 2 devices on one bus and often even that causes the bus to drop to 160 MBps instead of 320 MBps.

The cable lengths that are quoted in the specs are for single cables with no connections.

I have one working bus with 3 devices and it is only capable of 40MBps. Often you will start seeing parity errors with 3 devices on the bus.

We don't recommend it but with Ultrium 920 drives you can daisy chain 2 drives if you have short cables and very clean connections. I would not recommend daisy chaining 3 drives.
abcxyz
Frequent Advisor

Re: MSL4048 with three Ultrium 920 drives

Thanks for all the help.

I have noticed another thing,the Master Drive shows SCSI ID 4,the Second Drive shows SCSI ID 5 and the third drive again shows SCSI ID 4 again.

Is this correct.

After connecting the drives individually the Assessment test passed on all the three but again the Backup was not able to initiate on the topmost drive.Swapped the Drives but result was same.

Any thoughts!!
Richard Bickers
Trusted Contributor

Re: MSL4048 with three Ultrium 920 drives

All devices on a single bus must have different SCSI IDs! Your setup can't work as the two ID 4 drives will be competing with each other. Try again with unique IDs.

We still recommend reducing the length of the chain as per Curtis' findings.
It's more interesting when it's gone wrong
Curtis Ballard
Honored Contributor

Re: MSL4048 with three Ultrium 920 drives

Oops I forgot about the ID's, good catch. This library is design for a maximum of two half height drives to be daisy chained and no full height drives to be daisy chained.

The default ID's are in the MSL G3 family in each half height location are 4/5 then they repeat at 4/5 for the next two half height locations. All full height drives will get the same ID and can not be daisy chained without changing the ID's. Only two half height drives can be daisy chained without changing ID's.
abcxyz
Frequent Advisor

Re: MSL4048 with three Ultrium 920 drives

Thanks Curtis,

So will the below config be correct:

Drive1 (Master Drive) - ID 4
Drive 2 - ID 5
Daisy chain the above two drives

Drive 3 - ID 4
Connect directly to HBA

The Manuals do not mention these points.

Appreciate your assistance.
abcxyz
Frequent Advisor

Re: MSL4048 with three Ultrium 920 drives

Another Question,

Incase the Backup is started on all the three drives simultaneously in this configuration using Data protector Express,Should it work.I mean would the backup start immediately on all the three drives.What happens here is the Disk Agent goes into a Wait state for long on one of the drive and has to be aborted.

The User wants the backup to happen simultaneously on all the drives.
Curtis Ballard
Honored Contributor

Re: MSL4048 with three Ultrium 920 drives

The documentation does talk about how the SCSI bus should be configured in multiple places. One of them is in section 8a of the getting started guide.

That section states that only one 920 drive should be on a bus. The way you have it configured now with two 920 drives on a bus will probably work but I would never recommend it. Make certain you run L&TT and pull drive support tickets then check the negotiated transfer rate to make certain the drives can actually run Ultra 320 with this configuration.

Is there something about this section that is confusing? We could improve it in future releases. I have copied it here:

Follow these general guidelines when planning the parallel SCSI configuration:
â ¢ The Ultrium 960 and 920 tape drives are Ultra320 SCSI devices. Only one Ultrium 960 or 920 tape drive should be put on an Ultra320 SCSI bus. Putting an Ultrium 960 or 920 tape drive on a slower bus or putting two drives on the same bus may degrade performance.
â ¢ The Ultrium 448 tape drive is an Ultra160 SCSI device. Only one Ultrium 448 tape drive should be put on an Ultra160 bus. Up to two Ultrium 448 tape drives can be on an Ultra320 bus.
â ¢ Avoid putting the Library on the same SCSI bus as a disk drive or SE device. The Library has an LVDS SCSI bus. It is NOT compatible with a standard differential (Diff) or High-
Voltage (HVD) SCSI bus.
â ¢ The default SCSI ID for all full-height tape drives is 4.
â ¢ The default SCSI ID for half-height drives is 4 or 5, depending on the drive bay as shown below.
Curtis Ballard
Honored Contributor

Re: MSL4048 with three Ultrium 920 drives

On your other question about starting a backup on all of the drives at once:

Yes - DP should be able to start the backup on all drives at once. I'm not a DP expert though and can't say what might be configured wrong. That question might need posted to the DP forum.
abcxyz
Frequent Advisor

Re: MSL4048 with three Ultrium 920 drives

Thanks Curtis !

I had not gone through this documentation.I will be checking on all aspects and would Post the results here.