上一篇RocketMQ设计之同步刷盘
异步刷盘方式：在返回写成功状态时，消息可能只是被写入了内存的PAGECACHE，写操作的返回快，吞吐量大；当内存里的消息量积累到一定程度时，统一触发写磁盘操作，快速写入
RocketMQ默认采用异步刷盘，异步刷盘两种策略：开启缓冲池，不开启缓冲池
CommitLog的handleDiskFlush方法：
public void handleDiskFlush(AppendMessageResult result, PutMessageResult putMessageResult, MessageExt messageExt) {
// Synchronization flush
if (FlushDiskType.SYNC_FLUSH == this.defaultMessageStore.getMessageStoreConfig().getFlushDiskType()) {
final GroupCommitService service = (GroupCommitService) this.flushCommitLogService;
if (messageExt.isWaitStoreMsgOK()) {
GroupCommitRequest request = new GroupCommitRequest(result.getWroteOffset() + result.getWroteBytes());
service.putRequest(request);
boolean flushOK = request.waitForFlush(this.defaultMessageStore.getMessageStoreConfig().getSyncFlushTimeout());
if (!flushOK) {
log.error("do groupcommit, wait for flush failed, topic: " + messageExt.getTopic() + " tags: " + messageExt.getTags()
+ " client address: " + messageExt.getBornHostString());
putMessageResult.setPutMessageStatus(PutMessageStatus.FLUSH_DISK_TIMEOUT);
}
} else {
service.wakeup();
}
}
// Asynchronous flush
else {
if (!this.defaultMessageStore.getMessageStoreConfig().isTransientStorePoolEnable()) {
flushCommitLogService.wakeup();
} else {
commitLogService.wakeup();
}
}
}不开启缓冲池：默认不开启，刷盘线程FlushRealTimeService会每间隔500毫秒尝试去刷盘。
class FlushRealTimeService extends FlushCommitLogService {
private long lastFlushTimestamp = 0;
private long printTimes = 0;
public void run() {
CommitLog.log.info(this.getServiceName() + " service started");
while (!this.isStopped()) {
boolean flushCommitLogTimed = CommitLog.this.defaultMessageStore.getMessageStoreConfig().isFlushCommitLogTimed();
//每次Flush间隔500毫秒
int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushIntervalCommitLog();
//每次Flush最少4页内存数据（16KB）
int flushPhysicQueueLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogLeastPages();
//距离上次刷盘时间阈值为10秒
int flushPhysicQueueThoroughInterval =
CommitLog.this.defaultMessageStore.getMessageStoreConfig().getFlushCommitLogThoroughInterval();
boolean printFlushProgress = false;
// Print flush progress
long currentTimeMillis = System.currentTimeMillis();
if (currentTimeMillis >= (this.lastFlushTimestamp + flushPhysicQueueThoroughInterval)) {
this.lastFlushTimestamp = currentTimeMillis;
flushPhysicQueueLeastPages = 0;
printFlushProgress = (printTimes++ % 10) == 0;
}
try {
if (flushCommitLogTimed) {
Thread.sleep(interval);
} else {
this.waitForRunning(interval);
}
if (printFlushProgress) {
this.printFlushProgress();
}
long begin = System.currentTimeMillis();
CommitLog.this.mappedFileQueue.flush(flushPhysicQueueLeastPages);
long storeTimestamp = CommitLog.this.mappedFileQueue.getStoreTimestamp();
if (storeTimestamp > 0) {
CommitLog.this.defaultMessageStore.getStoreCheckpoint().setPhysicMsgTimestamp(storeTimestamp);
}
long past = System.currentTimeMillis() - begin;
if (past > 500) {
log.info("Flush data to disk costs {} ms", past);
}
} catch (Throwable e) {
CommitLog.log.warn(this.getServiceName() + " service has exception. ", e);
this.printFlushProgress();
}
}
// Normal shutdown, to ensure that all the flush before exit
boolean result = false;
for (int i = 0; i < RETRY_TIMES_OVER && !result; i++) {
result = CommitLog.this.mappedFileQueue.flush(0);
CommitLog.log.info(this.getServiceName() + " service shutdown, retry " + (i + 1) + " times " + (result ? "OK" : "Not OK"));
}
this.printFlushProgress();
CommitLog.log.info(this.getServiceName() + " service end");
}
@Override
public String getServiceName() {
return FlushRealTimeService.class.getSimpleName();
}
private void printFlushProgress() {
// CommitLog.log.info("how much disk fall behind memory, "
// + CommitLog.this.mappedFileQueue.howMuchFallBehind());
}
@Override
public long getJointime() {
return 1000 * 60 * 5;
}
}- 判断是否超过10秒没刷盘了，如果超过强制刷盘
- 等待Flush间隔500ms
- 通过
MappedFile刷盘 - 设置
StoreCheckpoint刷盘时间点 - 超过500ms的刷盘记录日志
- Broker正常停止前，把内存page中的数据刷盘
开启缓冲池：
class CommitRealTimeService extends FlushCommitLogService {
private long lastCommitTimestamp = 0;
@Override
public String getServiceName() {
return CommitRealTimeService.class.getSimpleName();
}
@Override
public void run() {
CommitLog.log.info(this.getServiceName() + " service started");
while (!this.isStopped()) {
//每次提交间隔200毫秒
int interval = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitIntervalCommitLog();
//每次提交最少4页内存数据（16KB）
int commitDataLeastPages = CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogLeastPages();
//距离上次提交时间阈值为200毫秒
int commitDataThoroughInterval =
CommitLog.this.defaultMessageStore.getMessageStoreConfig().getCommitCommitLogThoroughInterval();
long begin = System.currentTimeMillis();
if (begin >= (this.lastCommitTimestamp + commitDataThoroughInterval)) {
this.lastCommitTimestamp = begin;
commitDataLeastPages = 0;
}
try {
boolean result = CommitLog.this.mappedFileQueue.commit(commitDataLeastPages);
long end = System.currentTimeMillis();
if (!result) {
this.lastCommitTimestamp = end; // result = false means some data committed.
//now wake up flush thread.
flushCommitLogService.wakeup();
}
if (end - begin > 500) {
log.info("Commit data to file costs {} ms", end - begin);
}
this.waitForRunning(interval);
} catch (Throwable e) {
CommitLog.log.error(this.getServiceName() + " service has exception. ", e);
}
}
boolean result = false;
for (int i = 0; i < RETRY_TIMES_OVER && !result; i++) {
result = CommitLog.this.mappedFileQueue.commit(0);
CommitLog.log.info(this.getServiceName() + " service shutdown, retry " + (i + 1) + " times " + (result ? "OK" : "Not OK"));
}
CommitLog.log.info(this.getServiceName() + " service end");
}
}RocketMQ申请一块和CommitLog文件相同大小的堆外内存来做缓冲池，数据会先写入缓冲池，提交线程CommitRealTimeService也每间隔500毫秒尝试提交到文件通道等待刷盘，刷盘最终由FlushRealTimeService来完成，和不开启缓冲池的处理一致。使用缓冲池的目的是多条数据合并写入，从而提高io性能。
- 判断是否超过200毫秒没提交，需要强制提交
- 提交到
MappedFile，此时还未刷盘 - 然后唤醒刷盘线程
- 在Broker正常停止前，提交内存page中的数据
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