LTO 8 Slow write speeds

Hi all, I'm having issues with write speeds to our LTO drive. Copy jobs 
aren't going over 100 MB/sec, and on average it's around 80 MB/sec.

Using btape speed I was benchmarking the performance on different tapes 
(HW compression on/off, HW encryption on/off), and it reached 180-220 
MB/sec, depending on the test. Still, it's nowhere near the theoretical 
maximum of the LTO 8 specification. But even reaching the benchmark 
speeds would mean that our job copy time would be halved.

Our Storage Daemon stores all full and incremental backups on a file 
volume. This volume resides on a ZFS pool. I've also benchmarked the ZFS 
pool, and it easily reads continuously over 300 MB/sec. Once a month, we 
issue a Copy Job to the LTO 8 tape.

Bacula DIR and SD run on the same machine, with a Quantum LTO-8 HH 
drive, using Quantum MR-L8MQN-01 tapes.

bacula-dir.conf

Storage {

   Name = ultrium

   Address = bacula

   SDPort = 9103

   Password = <redacted>

   Device = ultrium

   Media Type = LTO

   AllowCompression = no         # LTO drive does the compression

   Maximum Concurrent Jobs = 1   # Prevent shoeshining

}

bacula-sd.conf

Device {

   Name = FileStorage

   Media Type = File

   Archive Device = /mnt/backup/tapes  # Zfs pool

   Label Media = yes;                  # let Bacula label unlabeled media

   Random Access = Yes;

   AutomaticMount = yes;

   RemovableMedia = no;

   AlwaysOpen = no;

}

Device {

   Name = ultrium

   Media Type = LTO

   Device Type = Tape;

   Archive Device = /dev/nst0

   AutomaticMount = yes;

   AlwaysOpen = no;

   RemovableMedia = yes;

   RandomAccess = no;

   Maximum File Size = 5GB

   Device Type = Tape;

   Volume Poll Interval = 5s;

   Close on Poll = Yes;

   Maximum Concurrent Jobs = 1; # Prevent shoeshining

}

tapestat

Tape:     r/s     w/s   kB_read/s   kB_wrtn/s  %Rd  %Wr  %Oa    Rs/s    Ot/s

st0         0    1536        0.0k       94.5M   0%  41%  41%       0       0

Am I'm missing some important properties in the resources? Any help is 
appreciated.

- Gilles

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