@Override
    public boolean truncatePrefix(long firstIndexKept) {
        this.readLock.lock();
        try {
            checkState();
            final long startIndex = getFirstLogIndex();
            final boolean ret = saveFirstLogIndex(firstIndexKept);
            if (ret) {
                setFirstLogIndex(firstIndexKept);
            }
            truncatePrefixInBackground(startIndex, firstIndexKept);
            return true;
        } catch (DatabaseException e) {
            LOG.error("Fail to truncatePrefix {}.", firstIndexKept, e);
        } finally {
            this.readLock.unlock();
        }
        return false;
    }
First log index and last log index key in configuration column family.
jraft-extension/bdb-log-storage-impl/src/main/java/com/alipay/sofa/jraft/storage/impl/BDBLogStorage.java