private boolean saveFirstLogIndex(final long firstLogIndex) {
        this.readLock.lock();
        try {
            final byte[] vs = new byte[8];
            Bits.putLong(vs, 0, firstLogIndex);
            checkState();
            this.db.put(this.confHandle, this.writeOptions, FIRST_LOG_IDX_KEY, vs);
            return true;
        } catch (final RocksDBException e) {
            LOG.error("Fail to save first log index {} in {}.", firstLogIndex, this.path, e);
            return false;
        } finally {
            this.readLock.unlock();
        }
    }
Save the first log index into conf column family.
jraft-core/src/main/java/com/alipay/sofa/jraft/storage/impl/RocksDBLogStorage.java