Wierd DLT Behaviour

[Scott Hanson_1]

We have a Surestore 4/40 DLT Library using HP Omniback. We have noticed that one of the drives (SCSI ID 1) is significantly slower than the other three drives. Has anyone else seen this behaviour? It is most noticable when using raw device backups.

[Jan Klier]

There are a whole number of possible causes for such behaviour. Has it always been slower, and you just noticed it now, or is this a new situation?

The issue could be related to your bus configuration (are multiple drives sharing a bus?).

Or it could be that this particular drive experiences some problems writing and has to retry some of the data. There may potentially be a need to clean this drive.

The data source for this drive may be slower than the other data sources - you mentioned raw device backups.

The way your backup schedule is setup, this drive may always run in parallel to other backups as well.

Try changing your backup schedule to swap jobs this drive backups up with another drive and see if the speed issue follows the jobs or the drive.

And you can download HP's diagnostics tool HP Library & Tape Tools (http://www.hp.com/support/tapetools) and run the read/write test on each one of the drives - you must supply a scratch tape for the test. Compare the results between the slow drive and the remaining drives.

[Stefan Saliba · Accepted solution]

Has your drive been always slow, or has it suddenly become slow ?

I have a 4/40 and experienced the same problem. A 1/2 hour backup takes 8 hours. What I suggest is clean the drive and try a different tape. If problem persists change the drive.

In my case it was the only solution. I changed the drive and speed was back to normal.

What comes also in mind for the 4/40 is to check your firmware versions for the drive and library. I think the drive is v59 and the library is 1.33. I think there is a new version for the library which is still new though.

Usually what happens is that the drive will be doing alot of retries and thus prolong the read/write time.

Be sure that you take the time to clean the drives every now and then, and try to get rid of old tapes. They tend to deposit more on the heads and wear the drives faster.



Good luck

Stefan

[Jan Klier]

Please use caution with this last advice: DLT drives are not supposed to be cleaned proactively, as this wears the heads of the drive unnecessarily. DLT drives are only supposed to be cleaned if the drive requests such an action.

This is different from other tape drives, such as DDS/DAT which require regular cleaning for proper operation.

Please always consult the technical documentation of a tape drive to see what the proper cleaning procedure and schedule is.

[Scott Hanson_1]

Jan,

Thanks. We just let Omniback manage the cleaning of the drives.

[Chiabudini]

Disk drives on the same SCSI bus as the tape drive
Be very careful when mixing different types of SCSI devices on the same bus. The chances are that newer hard drives will be a LVD device, and when attached with an older tape devices it's performance can be halved.

In addition, having the Tape device and a hard drive on the same SCSI bus can potentially cause problems, as both will contend for bandwidth.

Each device on the SCSI bus has its own unique number (Click here for more information on SCSI ID). These are numbered either 0 to 7 or 0 to 16, depending on the type of SCSI used. The priority that a device has on the SCSI bus is based on its ID number. For the first 8 IDs, higher numbers have higher priority, so 7 is the highest and 0 the lowest. For Wide SCSI, the additional IDs from 8 to 15 again have the highest number as the highest priority, but the entire sequence is lower priority than the numbers from 0 to 7. So the overall priority sequence for wide SCSI is 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8.

The priority levels are used to determine which device can have control of the SCSI bus. If more than one device wants control at the same time, the higher-priority device will "win", while the lower-priority device will have to wait.

Hard drives tend to monopolize the bus as they are always being accessed. If the hard drive has a high priority then it may effectively deny the tape drive the information that it needs causing performance and read/write issues.

The disk could be set to a lower priority SCSI ID such as 0 or 1 but even then the issues with priority may cause problems with the tape drive. A high priority hard drive will generally result in a low performance tape backup and a high priority tape drive will result in a low performance system, especially during backups.

The best solution, in all cases, is to put the tape drive onto its own SCSI bus with its own host bus controller.