question: private boolean doFlush ( boolean applyAllDeletes ) throws IOException { if ( tragedy != null ) { throw new IllegalStateException ( "this writer hit an unrecoverable error; cannot flush" , tragedy ) ; } doBeforeFlush ( ) ; testPoint ( "startDoFlush" ) ; boolean success = false ; try { if ( infoStream . isEnabled ( "IW" ) ) { infoStream . message ( "IW" , "  start flush: applyAllDeletes=" + applyAllDeletes ) ; infoStream . message ( "IW" , "  index before flush " + segString ( ) ) ; } boolean anyChanges = false ; synchronized ( fullFlushLock ) { boolean flushSuccess = false ; try { long seqNo = docWriter . flushAllThreads ( ) ; if ( seqNo < 0 ) { seqNo = - seqNo ; anyChanges = true ; } else { anyChanges = false ; } if ( ! anyChanges ) { flushCount . incrementAndGet ( ) ; } flushSuccess = true ; } finally { docWriter . finishFullFlush ( this , flushSuccess ) ; processEvents ( false , true ) ; } } synchronized ( this ) { anyChanges |= maybeApplyDeletes ( applyAllDeletes ) ; doAfterFlush ( ) ; success = true ; return anyChanges ; } } catch ( AbortingException | VirtualMachineError tragedy ) { tragicEvent ( tragedy , "doFlush" ) ; return false ; } finally { if ( ! success ) { if ( infoStream . isEnabled ( "IW" ) ) { infoStream . message ( "IW" , "hit exception during flush" ) ; } } } }
target:  Returns true a segment was flushed or deletes were applied. 