I have an LTO4 drive that when given even new LTO4 tapes, only writes around 460GiB to the tape even though the tape's uncompressed capacity is supposed to be 800GB.
I have noticed that in one direction the drive writes close to the stated 80MB/sec, however in the reverse direction it only manages close to 40MB/sec.
The drive is a supported HP model and needed a firmware upgrade which I have done, however the problem remains. The drive's self test fails when run through HP Tape Tools, however there are no error messages just warnings on the lines where it shows the amount of data written at each different tape speed.
What could be causing this problem? The tape motors sound like they are running at the same speed in both directions, so I am wondering whether writing at barely 40MB/sec means the drive is encountering errors and having to write the data again, causing the apparent loss of write speed and capacity. Is this possible?
Does anyone else know what would cause similar problems but have the HP Tape Tools drive assessment come back as otherwise OK?
EDIT: I've tried with encryption on (462GB) and off (467GB) and the data I'm writing is not very compressible so the drive's compression doesn't do much.
EDIT2: @Mark I tried with an LTO3 tape and instead of the expected 400GB it only writes 296GiB. Watching the write amounts, it seems to write 9GiB on the forward wrap but only 4.4GiB on the reverse wrap, but the motors run at the same speed. So looks like your theory is right, and the drive is getting a lot of verify-after-write errors and having to rewrite quite a lot of data. It's a bit weird that a reason for this doesn't show up in any of the Tape Tools tests, they all say the head life is excellent and there are no issues, other than there's an issue with the amount of data written (approx 2GB during the test, but I only know from other tests with other drives this figure is normally around 6GB.)