private long getTotalSize(Collection<HStoreFile> sfs) {
    return sfs.stream().mapToLong(sf -> sf.getReader().length()).sum();
  }
Write out current snapshot. Presumes {@code StoreFlusherImpl.prepare()} has been called
previously.
@param logCacheFlushId flush sequence number
@return The path name of the tmp file to which the store was flushed
@throws IOException if exception occurs during process
hbase-server/src/main/java/org/apache/hadoop/hbase/regionserver/HStore.java