Why does RBD snap id start from 4?

Hello everyone, I'm a newbee Ceph developer, and recenlty reading code of snapshots. From pg_pool_t::add_unmanaged_snap, it's obvious that the first RBD snapshot id should start from 2, but in reality, it starts from 4, I wonder whether there are some organisms in RBD snap, which increments snap_seq, could anyone help me? Thanks in advance! Below is the code of pg_pool_t::add_unmanaged_snap. void pg_pool_t::add_unmanaged_snap(uint64_t& snapid) { ceph_assert(!is_pool_snaps_mode()); if (snap_seq == 0) { // kludge for pre-mimic tracking of pool vs selfmanaged snaps. after // mimic this field is not decoded but our flag is set; pre-mimic, we // have a non-empty removed_snaps to signifiy a non-pool-snaps pool. removed_snaps.insert(snapid_t(1)); snap_seq = 1; } flags |= FLAG_SELFMANAGED_SNAPS; snapid = snap_seq = snap_seq + 1; } Best regards, ghost