Have you set the config parameters? there is a tunning guide that goes into details of all tuning parameters
create following file:
Try value of 524288 or 104857
create folowing two files:
Try value of 128 or 256 for NUMBER_DATA_BUFFERS
For Lt08 tape try value of 262144 or 524288 for SIZE_DATA_BUFFERS
You will need to set parameters and perform a backup and restore. Then adjust parametyers till you get the best results
Here a SWAG for you environment
NUMBER_DATA_BUFFERS = 256
SIZE_DATA_BUFFERS = 524288
NET_BUFFER_SZ = 524288
NET_BUFFER_SZ does not have a usage in modern versions of netbackup so you can leave that setting out
You are getting a low value because you can't keep the LTO8 streaming. LTO has variables writing speed - but there is a lower boundary for each generation, where the drive will start doing start/stop operations (aka shoe shining). For LTO3 this lower boundary is 40 MB/sec, for LTO8 it is 112 MB/sec.
1: Set up buffer tuning as above. NUMBER_DATA_BUFFERS should be 256 or higher, SIZE_DATA_BUFFERS should at least be 262144. Be carefull getting the SIZE_DATA_BUFFERS right - a typo will hurt performance a lot.
2: Be careful of how may drives you but behind each controller, I recommend a 1:1 ratio
3: Make sure network is 10GB, 1GB will not do it with LTO8
4: No swapping on the master/media server - what so ever.
5: If client backup speed is low, consider to use disk to disk to tape, where backup data is written to disk on Netbackup server during night time and written to tape during day time. Do not mix read and write workload to the disk storag unit, as this will impact performance a lot.
Yes it is likely to be a (not) streaming issue. If the Media Server is Unix or Linux (or a NetBackup Appliance) I suggest trying GEN_DATA per https://www.veritas.com/support/en_US/article.TECH75213 to test the path from Media Server memory direct to tape. You can use this to rule out many variables including client I/O speed and network bandwidth.
Be aware this WILL create real backup images so should only be used for testing and the resulting backups should expire quickly.
Good luck, Andrew