code: public void purgeLogsOlderThan(final long minTxIdToKeep) {
      synchronized (this) {
        // synchronized to prevent findbugs warning about inconsistent
        // synchronization. This will be JIT-ed out if asserts are
        // off.
        assert curSegmentTxId == HdfsConstants.INVALID_TXID || // on format this
                                                               // is no-op
            minTxIdToKeep <= curSegmentTxId : "cannot purge logs older than txid "
            + minTxIdToKeep + " when current segment starts at " + curSegmentTxId;
        try {
          journalSet.purgeLogsOlderThan(minTxIdToKeep);
        } catch (IOException ex) {
          // All journals have failed, it will be handled in logSync.
        }
  
      }
    }
docs: Archive any log files that are older than the given txid.