xfsprogs/xfsprogs-2.9.6/repair# ./xfs_repair /dev/mapper/backupdata-backup
Phase 1 - find and verify superblock...
Phase 2 - using internal log
        - zero log...
        - scan filesystem freespace and inode maps...
        - found root inode chunk
Phase 3 - for each AG...
        - scan and clear agi unlinked lists...
        - process known inodes and perform inode discovery...
        - agno = 0
<nothing more for quite a bit of time>

Attaching strace to the running xfs_repair gives only this (was attached for some time):
# strace -p 28353
Process 28353 attached - interrupt to quit
futex(0x8141c64, FUTEX_WAIT, 2, NULL



And xfs_repair has no cpu usage at that point, but uses some 750MB or so of memory...