Your library has LTO3 and LTO4 drives. If you use LTO4 tape with LTO3 tape drive then it will not write more than 800GB. If your library supports then you can do a partition such that LTO3 drive could not access the LTO4 tape slots.

Otherwise you can get LTO3 drive replaced with LTO4.

LTO3 native capacity is 400GB.  So if you get 800GB when volume is filled it means your average compression is 2:1.  If with LTO4 you get 800GB too it probably means compression is not used.

800 gig is the native capacity for LTO4. Check for compression (most likely this is not enabled)  The 1.6 TB is based on a 2:1 compression ration.

Good luck,

of course the drives are LTO4

the LTO3 tapes show in the volume database as 800GB written and full

the same is for LTO4 tapes written in LTO4 drives

how do i check if the HW compression is on if the drives are FC connected to a Netapp?

NW doesn't know what your volume is type, but it assumes it is the same type as device.  In your case, as you state, this is LTO4.

The only way to see how much really lands on your tape is to check volume summary in GUI, mminfo for tape and sum totalsize or grep daemon log for messages where NW says how mush have been written to tape before marked full.

If your device path ends up with a (for NetApp) you should use compression then.

i am little bit closer to resolution

the problem is that the netapp filer gives me only nrst4l device when autoscanned for devices but i would need to use nrst4a instead to be able to write with compression enabled.

HP      Ultrium 4-SCSI

    rst4l  -  rewind device,        format is: LTO-2(ro)/3 2/400GB

    nrst4l -  no rewind device,     format is: LTO-2(ro)/3 2/400GB

    urst4l -  unload/reload device, format is: LTO-2(ro)/3 2/400GB

    rst4m  -  rewind device,        format is: LTO-2(ro)/3 4/800GB cmp

    nrst4m -  no rewind device,     format is: LTO-2(ro)/3 4/800GB cmp

    urst4m -  unload/reload device, format is: LTO-2(ro)/3 4/800GB cmp

    rst4h  -  rewind device,        format is: LTO-4 800GB

    nrst4h -  no rewind device,     format is: LTO-4 800GB

    urst4h -  unload/reload device, format is: LTO-4 800GB

    rst4a  -  rewind device,        format is: LTO-4 1600GB cmp

    nrst4a -  no rewind device,     format is: LTO-4 1600GB cmp

    urst4a -  unload/reload device, format is: LTO-4 1600GB cmp