Failed LTO drive assessment test

[peterpanny]

I have been getting many errors concerning read/write errors, dirty drive head (even though the drive has been cleaned) and vss associated errors from Backup Exec.  So I ran the HP Storageworks Library and Tape Tools on our drive (Ultrium 4-SCSI).   A sample of what was produced is below:

The write operation failed to complete

Byte2 0x3 SenseCode 0xc SenseQual 0x0 (Write error) ErrorCode 0x750E...

Overall drive margin: -3.8%

Worst-case margin (forward Direction): -100%

worst-case channel variatin margin (3.5 m/sec. tape speed): 99.8%

The Short Erase operation failed to complete

problems have been reported

The drive is no longer recommended for use

Please contact hp support for further assistance

The read write test failed as well as giving a warning of:

Please ensure a test cartridge is loaded into drive at address 5/7.7.7...

As far as I can tell there is already a tape in the drive.  Do the errors above indicate a drive failure? Could it be tape related?  I have yet to test this with a new tape since a tape shipment has not arrived yet.  The current set of 7 tapes is about 9 months old.  Could this also be firmware related?

Any help with what to do next would be greatly appreciated - Thanks.

[thomasr]

From what I know, it sounds like your tape head has reached its end of life.

The "margin" should be positive, and the higher the better.   Over time the movement of tape across the head will cause wear and tear and at some point, the head has been compromised to the point where it no longer reliably writes the data.

Your tapes -- assuming they have been stored in reasonable conditions -- are unlikely to be the problem; seven months of use is not much for a tape (unless they were being used 24x7.  Oh -- it's also possible that if the data is arriving very slowly so that the buffer continually empties, the constant start/write/stop/rewind/start/reposition/write cycle is causing excessive stretching of the tape media, and thus premature failure... but I'd guess it's the tape head, not the media, given your drive test.  

[peterpanny]

Thanks for you quick response

Concerning the speed of the data arriving, how slow is too slow? And how would I test this?

[thomasr]

I can't give you a simple number, but I'll tell you how to come close to figuring it.  How fast you need to get data to the tape drive depends on how compressible your data is.

First -- for incompressible data, LTO-4 runs full-speed at 120MB/second; it can slow down to 40MB/second sustained without entering a buffer underrun situation.   So if your data is incompressible, you need to keep the data coming to the drive at a minimum of 40MB/second.   If you're close to that, though, keep in mind that data isn't read of the disk at a steady rate -- big contiguous files (100MB and above) can be read very efficiently.  Small files (10K or less), or files with lots of fragmentation, will be read much slower -- even on fast servers, the maximum read speed for 1K files might be only 10-20MB/second.

Then multiply by the compressibility of the data.   The more compressible a data stream is, the faster the disk has to provide data to the tape drive to keep the tape drive streaming.  If your data is 2:1 compressible, the minimum speed doubles to 80MB/second, and the max to 240MB/s.    For 1.3:1 data, it's about 52MB/s min and 156MB/sec or more will ensure the tape drive is writing at its maximum potential.

You can get an idea of how fast your backup jobs are going by looking at the backup application logs.  They'll either tell you the average performance, or tell you when a job started and ended, as well as how many GB were written -- from which you can figure your average speed.

Compression is a harder beast to pin down.  You can look at a backup tape and the logs, and by comparing the bytes read to the space occupied on the tape get a pretty good idea (say that a full backup was 450GB, and the tape that backup is on shows 300GB used, your compression is just about 1.5:1  In that case, you'd want to feed the drive at a steady 60MB/second or more (more is always better!)

HP's free Library and Tape Tools can tell you how fast file-type data can be read off a disk:

                       http://www.hp.com/support/tapetools

HP also has the free TapeAssure that can monitor drive  and media health and will keep track of statistics such as performance, drive utilization, and actual compressibility of data.  See

                      http://www.hp.com/go/tapeassure