为了提高索引的速度，利用缓存的方式，达到一定量的时候flush。
lucene2.2版本代码这样写：（部分代码）
IndexWriter fsWriter = new IndexWriter(fsDir,analyzer, true);
addDocument(fsWriter, s); //添加一条信息到Document
if (fsWriter.ramSizeInBytes() > IParaConf.MAXMEMERY) {// 重点：判断内存使用量，大于指定的flush到硬盘。
System.out.println("flush...");
fsWriter.flush();
}

看了lucene2.3的API发现多了一个方法：
public void setRAMBufferSizeMB(double mb)

Determines the amount of RAM that may be used for buffering added documents before they are flushed as a new Segment. Generally for faster indexing performance it's best to flush by RAM usage instead of document count and use as large a RAM buffer as you can.

When this is set, the writer will flush whenever buffered documents use this much RAM. Pass in DISABLE_AUTO_FLUSH to prevent triggering a flush due to RAM usage. Note that if flushing by document count is also enabled, then the flush will be triggered by whichever comes first.

The default value is DEFAULT_RAM_BUFFER_SIZE_MB


于是乎我想既然有了这个方法，我就可以设定好需要的内存，不用再如上那样判断内存使用量了。 可是替换完发现，make index 速度奇慢无比。看一下索引目录，原来在不停的写索引归并文件。 看来这个方法并没有起作用。 无奈，又替换回lucene2.2版本了。