ceph_osd_reqid
ceph_start_encoding(&p, 2, 2, sizeof(struct ceph_osd_reqid));
memset(p, 0, sizeof(struct ceph_osd_reqid));
p += sizeof(struct ceph_osd_reqid);
sizeof(struct ceph_osd_reqid)];
sizeof(struct ceph_osd_reqid); /* reqid */