Error (failing assert) in xfs_repair

["Hans Kraus" <hans.w.kraus@xxxxxx>]

Hi,

I installed xfstools from the git repository on my debian 7 amd64
backup server. I got the following assertion error:
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root@elefant:/home/kraush# xfs_repair -L /dev/vg_stor1/lv_stor1
Phase 1 - find and verify superblock...
         - reporting progress in intervals of 15 minutes
Phase 2 - using internal log
         - zero log...
         - scan filesystem freespace and inode maps...
zeroing unused portion of secondary superblock (AG #20)
Metadata corruption detected at block 0x2b9e8a001/0x200
zeroing unused portion of secondary superblock (AG #16)
bad agbno 4274958142 in agfl, agno 16
freeblk count 1 != flcount 1806214135 in ag 16
bad agbno 131784061 for btbno root, agno 16
bad agbno 1628187110 for btbcnt root, agno 16
agf_freeblks 2414458438, counted 0 in ag 16
agf_longest 1482451932, counted 0 in ag 16
agf_btreeblks 4031360978, counted 0 in ag 16
bad agbno 1207336865 for inobt root, agno 16
agi_count 1835626108, counted 0 in ag 16
agi_freecount 952362526, counted 0 in ag 16
xfs_repair: scan.c:1579: scan_ag: Assertion `agf_dirty ||
agfbuf->b_error != 117' failed.
Aborted
root@elefant:/home/kraush#
-------------------------------------------------------------------
What shall I do to repair the file system (about 16 TB, 88% full,
backuppc storage medium).

Any help appreciated, Hans