Flush操作是将HBase中的数据存到硬盘上的过程,具体的flush的流程图如下,本文主要简要分析flush的过程相关源码。
Flush 任务提交
每当HRegion完成数据插入的操作的时候,就会进行检查此时是否需要进行一次flush,flush是将HRegion缓存的数据存储到磁盘的过程:
long addedSize = doMiniBatchMutation(batchOp);
long newSize = this.addAndGetGlobalMemstoreSize(addedSize);
if (isFlushSize(newSize)) {
requestFlush();
}
本文主要分析flush的过程以及涉及到得相关数据结构,在requestFlush内部调用:
this.rsServices.getFlushRequester().requestFlush(this);
实际是调用了MemStoreFlusher
具体执行flush的操作:
public void requestFlush(HRegion r) {
synchronized (regionsInQueue) {
if (!regionsInQueue.containsKey(r)) {
// This entry has no delay so it will be added at the top of the flush
// queue. It‘ll come out near immediately.
FlushRegionEntry fqe = new FlushRegionEntry(r);
this.regionsInQueue.put(r, fqe);
this.flushQueue.add(fqe);
}
}
}
MemStoreFlushRequeter有两个数据结构管理者需要flush的任务,private BlockingQueue<FlushQueueEntry>flushQueue
Map<HRegion, FlushRegionEntry> regionsInQueue
flushQueue相当于需要flush的工作队列,而regionsInQueue则是于保存已经在队列中的region的信息,上面的一段代码表示当当前请求flush的region没有记录在flushQueue中的时候则加入。其中FlushRegionEntry是一个flushQueue的单元数据结构
到这里flush request的请求就提交结束了,接下来等待MemStore中的FlushHander线程取出region并执行flush的任务。
Flush的任务执行前期准备
1.FlushHandler从flushQueue中取出FlushRegionEntry并执行
flushRegion(final FlushRegionEntry fqe)
这里首先判断当前region中是否含有过多的storefile的文件,如果是的话,需要首先进行storefile的合并操作(这里有必要解释一下HRegion中的数据组织),然后重新加入队列,否则的话直接对region执行flush操作:
isTooManyStoreFiles(region)
this.server.compactSplitThread.requestSystemCompaction(
region, Thread.currentThread().getName());
this.flushQueue.add(fqe.requeue(this.blockingWaitTime / 100));
else
return flushRegion(region, false);
2.flushRegion函数内部的主要执行逻辑如下,首先notifyFlushRequest只是进行一些flush线程数量的统计,region.flashcache具体负责flush的工作。执行完之后会根据返回值进行相关的辅助操作
notifyFlushRequest(region, emergencyFlush);
HRegion.FlushResult flushResult = region.flushcache();
boolean shouldCompact = flushResult.isCompactionNeeded();
// We just want to check the size
boolean shouldSplit = region.checkSplit() != null;
if (shouldSplit) {
this.server.compactSplitThread.requestSplit(region);
} else if (shouldCompact) {
server.compactSplitThread.requestSystemCompaction(
region, Thread.currentThread().getName());
}
if (flushResult.isFlushSucceeded()) {
long endTime = EnvironmentEdgeManager.currentTime();
server.metricsRegionServer.updateFlushTime(endTime - startTime);
}
Flush的任务执行过程
flushcahe内部调用 FlushResult fs = internalFlushcache(status);实际执行flush操作,StoreFlushContext的实现为StoreFlusherImpl,为每个HStore建一个StoreFlusherImpl,它为对应的HStore执行着具体非flush的操作。flush的具体实现包括三个步骤:
1.快照
public void prepare() {
this.snapshot = memstore.snapshot();
this.cacheFlushCount = snapshot.getCellsCount();
this.cacheFlushSize = snapshot.getSize();
committedFiles = new ArrayList<Path>(1);
}
2.将memestore中的数据写入.tmp文件中
public void flushCache(MonitoredTask status) throws IOException {
tempFiles = HStore.this.flushCache(cacheFlushSeqNum, snapshot, status);
}
3.将.tmp文件写入对应的cf下面的对应的文件中去,并用StoreFile保存相应的HFile的文件信息
public boolean commit(MonitoredTask status) throws IOException {
if (this.tempFiles == null || this.tempFiles.isEmpty()) {
return false;
}
List<StoreFile> storeFiles = new ArrayList<StoreFile>(this.tempFiles.size());
for (Path storeFilePath : tempFiles) {
try {
storeFiles.add(HStore.this.commitFile(storeFilePath, cacheFlushSeqNum, status));
} catch (IOException ex) {
LOG.error("Failed to commit store file " + storeFilePath, ex);
// Try to delete the files we have committed before.
for (StoreFile sf : storeFiles) {
Path pathToDelete = sf.getPath();
try {
sf.deleteReader();
} catch (IOException deleteEx) {
LOG.fatal("Failed to delete store file we committed, halting " + pathToDelete, ex);
Runtime.getRuntime().halt(1);
}
}
throw new IOException("Failed to commit the flush", ex);
}
}
for (StoreFile sf : storeFiles) {
if (HStore.this.getCoprocessorHost() != null) {
HStore.this.getCoprocessorHost().postFlush(HStore.this, sf);
}
committedFiles.add(sf.getPath());
}
HStore.this.flushedCellsCount += cacheFlushCount;
HStore.this.flushedCellsSize += cacheFlushSize;
// Add new file to store files. Clear snapshot too while we have the Store write lock.
return HStore.this.updateStorefiles(storeFiles, snapshot.getId());
}
至此HBase的flush的操作过程就完成了。