private boolean loadExistsFile(final SegmentFileOptions opts) {
        this.writeLock.lock();
        try {
            if (!mmapFile(false)) {
                return false;
            }
            if (!tryRecoverExistsFile(opts)) {
                return false;
            }
            this.readOnly = !opts.isLastFile;
            return true;
        } finally {
            this.writeLock.unlock();
        }
    }
Clear data in [startPos, startPos+64).

@param startPos the start position(inclusive)
jraft-core/src/main/java/com/alipay/sofa/jraft/storage/log/SegmentFile.java