目前在看HBase中跟Compaction相关的代码。
对于Compaction，我们最关心的一点，就是，何时会进行Compaction。因为这会直接影响我们HBase的读写性能。
环境
HBase rel-2.1.0
解析
HBase会在这三种情况下，进行Compact:
- MemStore flush时
- 检测线程检测到需要进行Compact时
- 主动调用
MemStore flush
第一种情况，我们可以简单看一下HRegion.internalFlushcache(WAL wal, long myseqid, Collection<HStore> storesToFlush, MonitoredTask status, boolean writeFlushWalMarker, FlushLifeCycleTracker tracker)的注释:
/**
* Flush the memstore. Flushing the memstore is a little tricky. We have a lot of updates in the
* memstore, all of which have also been written to the wal. We need to write those updates in the
* memstore out to disk, while being able to process reads/writes as much as possible during the
* flush operation.
* <p>
* This method may block for some time. Every time you call it, we up the regions sequence id even
* if we don't flush; i.e. the returned region id will be at least one larger than the last edit
* applied to this region. The returned id does not refer to an actual edit. The returned id can
* be used for say installing a bulk loaded file just ahead of the last hfile that was the result
* of this flush, etc.
* @param wal Null if we're NOT to go via wal.
* @param myseqid The seqid to use if <code>wal</code> is null writing out flush file.
* @param storesToFlush The list of stores to flush.
* @return object describing the flush's state
* @throws IOException general io exceptions
* @throws DroppedSnapshotException Thrown when replay of WAL is required.
*/
protected FlushResultImpl internalFlushcache(WAL wal, long myseqid,
Collection<HStore> storesToFlush, MonitoredTask status, boolean writeFlushWalMarker,
FlushLifeCycleTracker tracker) throws IOException {
......
}
这个方法的调用链，最终会检测，是否需要进行compact．
这段代码，在HStore.updateStorefiles(List<HStoreFile> sfs, long snapshotId)方法的最后:
/**
* Change storeFiles adding into place the Reader produced by this new flush.
* @param sfs Store files
* @param snapshotId
* @throws IOException
* @return Whether compaction is required.
*/
private boolean updateStorefiles(List<HStoreFile> sfs, long snapshotId) throws IOException {
......
return needsCompaction();
}
检测线程
具体的类是HRegionServer.CompactionChecker，它会每隔一段时间检测一下，是否有Region需要进行Compaction．默认是10s检测一次，当然，我们可以用hbase.server.thread.wakefrequency这个选项来控制．
主动调用
这种就没什么好说的了．无论是hbase shell里面，还是通过Admin API，我们都可以手动进行Compaction．而且，通常在性能调优时，都是更加推荐对于Major Compaction，使用手动的方式，而不是自动的方式．
