tape recycling looses capacity

[digidax]

in reference to: https://bugs.bareos.org/view.php?id=1341

I'm trying to find out, if the mt - command procedure is the problem or the backup software (bareos a bacula fork) it self.
After doing the mt commands and starting a backup, a "medium volume full" occurs by using of only around 650 GB from the 6 TB of a LTO-7 tape:

Mount the tape into drive an then:

[root@bareos ~]# mt -f /dev/st0 rewind
[root@bareos ~]# mt -f /dev/st0 weof
[root@bareos ~]# mt -f /dev/st0 rewind
[root@bareos ~]# mt -f /dev/st0 status
SCSI 2 tape drive:
File number=0, block number=0, partition=0.
Tape block size 0 bytes. Density code 0x5c (no translation).
Soft error count since last status=0
General status bits on (41010000):
BOT ONLINE IM_REP_EN

In bconsole, labeling and add to the pool:

Sending label command for Volume "Audio-001" Slot 0 ...
3000 OK label. VolBytes=64512 Volume="Audio-001" Device="LTO7-Tape-Drive" (/dev/nst0)
Catalog record for Volume "Audio-001", Slot 0 successfully created.
Requesting to mount LTO7-Tape-Drive ...
3001 Mounted Volume: Audio-001
3001 Device "LTO7-Tape-Drive" (/dev/nst0) is already mounted with Volume "Audio-001"

check the tape:

bls -V Audio-001 /dev/nst0

bls: stored/butil.cc:291-0 Using device: "/dev/nst0" for reading.
22-Apr 08:53 bls JobId 0: Ready to read from volume "Audio-001" on device "LTO7-Tape-Drive" (/dev/nst0).
22-Apr 08:53 bls JobId 0: End of Volume at file 1 on device "LTO7-Tape-Drive" (/dev/nst0), Volume "Audio-001"
22-Apr 08:53 bls JobId 0: End of all volumes.
0 files and directories found.
22-Apr 08:53 bls JobId 0: Releasing device "LTO7-Tape-Drive" (/dev/nst0).

btape /dev/nst0

*cap
Configured device capabilities:
EOF BSR BSF FSR FSF FASTFSF !BSFATEOM EOM REM !RACCESS AUTOMOUNT !LABEL !ANONVOLS !ALWAYSOPEN MTIOCGET
Device status:
OPENED TAPE !LABEL !MALLOC APPEND !READ !EOT !WEOT !EOF !NEXTVOL !SHORT
Device parameters:
Device name: /dev/nst0
File=0 block=0
Min block=0 Max block=524288
Status:
Bareos status: file=0 block=0
Device status: BOT ONLINE IM_REP_EN file=0 block=0
Device status: TAPE BOT ONLINE IMMREPORT. ERR=

*eod
btape: stored/btape.cc:620-0 Moved to end of medium.

*cap
Configured device capabilities:
EOF BSR BSF FSR FSF FASTFSF !BSFATEOM EOM REM !RACCESS AUTOMOUNT !LABEL !ANONVOLS !ALWAYSOPEN MTIOCGET
Device status:
OPENED TAPE !LABEL !MALLOC APPEND !READ !EOT !WEOT EOF !NEXTVOL !SHORT
Device parameters:
Device name: /dev/nst0
File=1 block=0
Min block=0 Max block=524288
Status:
EOF Bareos status: file=1 block=0
Device status: EOD ONLINE IM_REP_EN file=1 block=-1
Device status: TAPE EOF EOD ONLINE IMMREPORT. ERR=

*readlabel
btape: stored/btape.cc:528-0 Volume label read correctly.

Volume Label:
Id : Bareos 2.0 immortal
VerNo : 20
VolName : Audio-001
PrevVolName :
VolFile : 0
LabelType : PRE_LABEL
LabelSize : 183
PoolName : AudioBackup
MediaType : LTO-7
PoolType : Backup
HostName : bareos.123.com
Date label written: 22-Apr-2021 08:50

Starting a 629 GB backup job:

22-Apr 14:05 bareos-dir JobId 246: Start Backup JobId 246, Job=BackupAudio.2021-04-22_14.05.44_15
22-Apr 14:05 bareos-dir JobId 246: Connected Storage daemon at 192.168.130.211:9103, encryption: TLS_CHACHA20_POLY1305_SHA256 TLSv1.3
22-Apr 14:05 bareos-dir JobId 246: Using Device "LTO7-Tape-Drive" to write.
22-Apr 14:05 bareos-dir JobId 246: Probing client protocol... (result will be saved until config reload)
22-Apr 14:05 bareos-dir JobId 246: Connected Client: bareos-fd at localhost:9102, encryption: TLS_CHACHA20_POLY1305_SHA256 TLSv1.3
22-Apr 14:05 bareos-dir JobId 246: Handshake: Immediate TLS 22-Apr 14:05 bareos-dir JobId 246: Encryption: TLS_CHACHA20_POLY1305_SHA256 TLSv1.3
22-Apr 14:05 localhost-fd JobId 246: Connected Storage daemon at 192.168.130.211:9103, encryption: TLS_CHACHA20_POLY1305_SHA256 TLSv1.3
22-Apr 14:05 localhost-fd JobId 246: Extended attribute support is enabled
22-Apr 14:05 localhost-fd JobId 246: ACL support is enabled
22-Apr 14:05 bareos-sd JobId 246: Wrote label to prelabeled Volume "Audio-001" on device "LTO7-Tape-Drive" (/dev/nst0)
22-Apr 14:05 bareos-sd JobId 246: Spooling data ...
22-Apr 15:36 bareos-sd JobId 246: Committing spooled data to Volume "Audio-001". Despooling 629,102,376,675 bytes ...
22-Apr 17:01 bareos-sd JobId 246: Error: stored/block.cc:803 Write error at 29:11710 on device "LTO7-Tape-Drive" (/dev/nst0). ERR=Input/output error.
22-Apr 17:01 bareos-sd JobId 246: Error: stored/block.cc:822 Write error on fd=6 at file:blk 29:11710 on device "LTO7-Tape-Drive" (/dev/nst0). ERR=Input/output error.
22-Apr 17:01 bareos-sd JobId 246: Error: Error writing final EOF to tape. This Volume may not be readable.
backends/generic_tape_device.cc:403 ioctl MTWEOF error on "LTO7-Tape-Drive" (/dev/nst0). ERR=Input/output error.
22-Apr 17:01 bareos-sd JobId 246: End of medium on Volume "Audio-001" Bytes=586,124,688,384 Blocks=1,117,944 at 22-Apr-2021 17:01.
22-Apr 17:01 bareos-sd JobId 246: Job BackupAudio.2021-04-22_14.05.44_15 is waiting. Cannot find any appendable volumes.

[exuvo]

The only case i have seen with that little data written was due to not using sensible block sizes when writing the data.
Example chaining tar to mbuffers but not giving mbuffers a block size to use nor --tapeaware: https://www.reddit.com/r/homelab/comments/d9g0j1/lto5_capacity_problem/ .