LTO6 performance poor on first write pass?

Hi all,

This isn't specifically a Bacula question but I know a bunch of you are 
running LTO drives and I hope you'll grant me the wisdom of your experience!

I'm seeing an unusual (to me) issue with a new-old-stock IBM drive and 
tapes. They came as a package deal. The drive has about 600 power-on 
hours and about 40 media-movement hours.

What I'm seeing is that tapes don't write at full speed on their first 
pass through the drive. I can hear the drive stopping and starting as it 
writes to the tape.

Sometimes the drive partially ejects the tape and reloads it.


The data flow is:

   * NVME disk (hdparm -tT speed = 2254MB/s buffered, cached ~10x faster)
   * zstd -T0 --fast
   * mbuffer -t -M 2G -L -s 262144 -P 80 -f -o /dev/nst0


When the drive stalls, Mbuffer reports that the buffer is full to 
capacity and a zero write rate to the drive -- it seems like the drive's 
buffer is full and it isn't actively writing.

If I pull the error correction logs with smartctl on the sg device, I 
see 25 write errors corrected in about 35GB of data.


I figured I had some bad tapes, but here's where it gets weird.

If I write to the same area I just wrote, the tape writes at about 
85MBytes/sec. Not the 150-160MByte/sec the drive should be giving, but 
still better than the 35MB/s average from the first pass.

I've tried a cleaning tape, this made no difference. The drive isn't 
requesting cleaning either.

The tapes are Quantum branded Barium Ferrite tapes, which I gather are 
the same as the IBM branded BaFe tapes I'm using in another drive.


Does anyone have any idea what's going on here, and is it expected 
behaviour for the first write pass on a new-old-stock tape?

Is the time these tapes -- or the drive -- spent on the shelf likely to 
be relevant? That is to say, if I bought a box of brand new tapes from a 
channel distributor, are they likely to behave any better?


I should note that the tapes I've written read back fine, the drive 
doesn't report any read errors that I've noticed.

Thanks.
-- 
Phil.
[email protected]
https://www.philpem.me.uk/