private int calculateMaxLogFiles(Configuration conf, long logRollSize) {
    checkArgument(logRollSize > 0,
      "The log roll size cannot be zero or negative when calculating max log files, "
        + "current value is " + logRollSize);
    Pair<Long, MemoryType> globalMemstoreSize = MemorySizeUtil.getGlobalMemStoreSize(conf);
    return (int) ((globalMemstoreSize.getFirst() * 2) / logRollSize);
  }
A log file has a creation timestamp (in ms) in its file name ({@link #filenum}. This helper
method returns the creation timestamp from a given log file. It extracts the timestamp assuming
the filename is created with the {@link #computeFilename(long filenum)} method.
@return timestamp, as in the log file name.
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/wal/AbstractFSWAL.java