How to use fdbbackup expire command

I am trying the fdbbackup expire command with following steps:

  1. Run backup command as following:
    fdbbackup start -z -s 120 -d file:///data/fdbbak/ -t testbak -C /etc/foundationdb/fdb.cluster

  2. Check the status of backup process:
    [root@fdbliyyue1 kvranges]# fdbbackup status -t testbak
    The backup on tag `testbak’ is restorable but continuing to file:///data/fdbbak/backup-2022-04-05-06-21-15.479382.
    BackupUID: f89545fd8bab5f0cc3d8e8f24bbc9363
    BackupURL: file:///data/fdbbak/backup-2022-04-05-06-21-15.479382
    Snapshot interval is 120 seconds. Current snapshot progress target is 68.93% (>100% means the snapshot is supposed to be done)

Details:
LogBytes written - 382369034
RangeBytes written - 5828416588
Last complete log version and timestamp - 7174762457697, 2022/04 /06.02:54:58-0400
Last complete snapshot version and timestamp - 7174698012972, 2022/04/06.02:53:53-0400
Current Snapshot start version and timestamp - 7174698119391, 2022/04/06.02:53:54-0400
Expected snapshot end version and timestamp - 7174818119391, 2022/04/06.02:55:54-0400
Backup supposed to stop at next snapshot completion - No

  1. Try expire command and encounter following error:
    [root@fdbliyyue1 kvranges]# fdbbackup expire -d file:///data/fdbbak/ --expire_before_timestamp “2022/04/06.02:54:58-0400”
    ERROR: Requested expiration would be unsafe. Backup would not meet minimum restorability. Use --force to delete data anyway.
    Fatal Error: Cannot expire requested data from backup without violating minimum restorability
    [root@fdbliyyue1 kvranges]# fdbbackup expire -d file:///data/fdbbak/ --expire_before_timestamp “2022/04/06.02:54:58-0400” --restorable_after_timestamp “2022/04/06.02:55:54-0400”
    ERROR: Requested expiration would be unsafe. Backup would not meet minimum restorability. Use --force to delete data anyway.
    Fatal Error: Cannot expire requested data from backup without violating minimum restorability
    [root@fdbliyyue1 kvranges]#

Anyone can tell me how to use the expire command for fdbbackup? What is the root cause of the error message? Thanks!

And I also found the expire command does not delete any file from disk:
I have tried to use --expire_before_version option, it seems expire command does not delete files from disk:

[root@fdbliyyue1 kvranges]# du -sm .
6206 .
[root@fdbliyyue1 kvranges]# ls -l | wc -l
745
[root@fdbliyyue1 kvranges]# fdbbackup expire -d file:///data/fdbbak/ --expire_before_version 000007180588091855
Describing backup…
All data before version 7180588091855 has been deleted.
[root@fdbliyyue1 kvranges]# du -sm .
6206 .
[root@fdbliyyue1 kvranges]# ls -l | wc -l
745

Anyone can help me to answer above 2 questions? Thanks!