近一段在研究Impala文档，在讲述Parquest table分区性能调优方面提到了Xceivers设置。因此将该参数英文资料整理翻译如下：

介绍

dfs.datanode.max.xcievers参数对客户端有直接影响，他主要定义server端的线程数量，或者更详细说，数据连接的sockets。设置太小，当集群扩展时候，无法充分利用资源。以下部分帮助我们理解客户端和服务端工作机制，以及如何设置该参数大小。

问题：

当该参数设置太小，导致较少的资源提供给HBASE，意味着Server和client连接可能出现IOExceptions，如在RegionServer中出现的错误日志：

HDFS文件系统细节

客户端，HDFS库提供绝对调用路径，被HADOOP支持的文件系统类FileSystem class有一些implementation，其中一个DistributedFileSystem中DFSClient class，处理所有远程服务器交互。当客户端，如HBASE，打开一个文件，他调用FileSystem class中的open()或create()方法。

public DFSInputStream open(String src) throws IOException
public FSDataOutputStream create(Path f) throws IOException

返回的stream实例需要服务端的socket和线程，用于读写数据块数据，DFSOutputStream 或 DFSInputStream class处理所有NameNode交互，算出拷贝的数据块位置以及每个数据节点中每个数据块的数据通讯。

在服务端，数据节点DataXceiverServer为实际的类，读取以上配置值，当超过上限阀值将抛异常。当数据节点启动，它创建一个线程组，启动所谈到的DataXceiverServer如下：

this.threadGroup = new ThreadGroup(“dataXceiverServer”);
this.dataXceiverServer = new Daemon(threadGroup,
new DataXceiverServer(ss, conf, this));
this.threadGroup.setDaemon(true); // auto destroy when empty

DataXceiverServer 线程也在线程组内， 数据节点也有内部的类提取该组中活动的线程:

/** Number of concurrent xceivers per node. */
int getXceiverCount() {
return threadGroup == null 0 : threadGroup.activeCount();
}

客户端连接启动读写数据块，在握手完毕期间，一个线程被创建，注册到以上的线程组，使得每个活动的新线程读写操作被跟踪到服务端，如果在线程组中的线程数量超过配置，在数据节点日志中也将抛出异常记录。

if (curXceiverCount > dataXceiverServer.maxXceiverCount) {
throw new IOException(“xceiverCount ” + curXceiverCount
+ ” exceeds the limit of concurrent xcievers “
+ dataXceiverServer.maxXceiverCount);
}

客户端处理

客户端如何通过服务端的线程读写操作，我们在DataXceiver类加debug信息

LOG.debug(“Number of active connections is: ” + datanode.getXceiverCount());
…
LOG.debug(datanode.dnRegistration + “:Number of active connections is: “ + datanode.getXceiverCount());

下面图标显示 RegionServer的状态：

最重要信息是storefile=22，HBASE有许多文件要处理，还包括write-ahead log，我们知道至少有22个活动连接。启动HBASE，检验DataNode和RegionServer日志信息：

命令行:

$ bin/start-hbase.sh
…

DataNode Log:

2012-03-05 13:01:35,309 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 1
2012-03-05 13:01:35,315 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 2
12/03/05 13:01:35 INFO regionserver.MemStoreFlusher: globalMemStoreLimit=396.7m, globalMemStoreLimitLowMark=347.1m, maxHeap=991.7m
12/03/05 13:01:39 INFO http.HttpServer: Port returned by webServer.getConnectors()[0].getLocalPort() before open() is -1. Opening the listener on 60030
2012-03-05 13:01:40,003 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 1
12/03/05 13:01:40 INFO regionserver.HRegionServer: Received request to open region: -ROOT-,,0.70236052
2012-03-05 13:01:40,882 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 3
2012-03-05 13:01:40,884 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 4
2012-03-05 13:01:40,888 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 3
…
12/03/05 13:01:40 INFO regionserver.HRegion: Onlined -ROOT-,,0.70236052; next sequenceid=63083
2012-03-05 13:01:40,982 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 3
2012-03-05 13:01:40,983 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 4
…
12/03/05 13:01:41 INFO regionserver.HRegionServer: Received request to open region: .META.,,1.1028785192
2012-03-05 13:01:41,026 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 3
2012-03-05 13:01:41,027 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 4
…
12/03/05 13:01:41 INFO regionserver.HRegion: Onlined .META.,,1.1028785192; next sequenceid=63082
2012-03-05 13:01:41,109 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 3
2012-03-05 13:01:41,114 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 4
2012-03-05 13:01:41,117 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 5
12/03/05 13:01:41 INFO regionserver.HRegionServer: Received request to open 16 region(s)
12/03/05 13:01:41 INFO regionserver.HRegionServer: Received request to open region: usertable,,1330944810191.62a312d67981c86c42b6bc02e6ec7e3f.
12/03/05 13:01:41 INFO regionserver.HRegionServer: Received request to open region: usertable,user1120311784,1330944810191.90d287473fe223f0ddc137020efda25d.
…

2012-03-05 13:01:41,246 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 6
2012-03-05 13:01:41,248 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 7
…
2012-03-05 13:01:41,257 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 10
2012-03-05 13:01:41,257 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 9
…
12/03/05 13:01:41 INFO regionserver.HRegion: Onlined usertable,user1120311784,1330944810191.90d287473fe223f0ddc137020efda25d.; next sequenceid=62917
12/03/05 13:01:41 INFO regionserver.HRegion: Onlined usertable,,1330944810191.62a312d67981c86c42b6bc02e6ec7e3f.; next sequenceid=62916
…
12/03/05 13:01:41 INFO regionserver.HRegion: Onlined usertable,user1361265841,1330944811370.80663fcf291e3ce00080599964f406ba.; next sequenceid=62919
2012-03-05 13:01:41,474 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 6
2012-03-05 13:01:41,491 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 7
2012-03-05 13:01:41,495 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 8
2012-03-05 13:01:41,508 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 7
…
12/03/05 13:01:41 INFO regionserver.HRegion: Onlined usertable,user1964968041,1330944848231.dd89596e9129e1caa7e07f8a491c9734.; next sequenceid=62920
2012-03-05 13:01:41,618 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 6
2012-03-05 13:01:41,621 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 7
…
2012-03-05 13:01:41,829 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 7
12/03/05 13:01:41 INFO regionserver.HRegion: Onlined usertable,user515290649,1330944849739.d23924dc9e9d5891f332c337977af83d.; next sequenceid=62926
2012-03-05 13:01:41,832 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 6
2012-03-05 13:01:41,838 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 7
12/03/05 13:01:41 INFO regionserver.HRegion: Onlined usertable,user757669512,1330944850808.cd0d6f16d8ae9cf0c9277f5d6c6c6b9f.; next sequenceid=62929
…
2012-03-05 14:01:39,711 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 4
2012-03-05 22:48:41,945 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 4
12/03/05 22:48:41 INFO regionserver.HRegion: Onlined usertable,user757669512,1330944850808.cd0d6f16d8ae9cf0c9277f5d6c6c6b9f.; next sequenceid=62929
2012-03-05 22:48:41,963 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 4

从以上的日志可以看出，活动连接数从来没有达到22，仅仅达到10，为什么？为了明白这个，我们将看HDFS文件如何映射到服务端DataXceiver的实例。

Hadoop深层挖掘

DFSInputStream和DFSOutputStream是普通的stream内容，客户端-服务端是标准的Java接口，内部路由到所选的DataNode,拷贝当前一个数据块。根据需要打开和关闭连接，当客户端读HDFS文件，客户端库类透明从数据节点到数据节点切换，因此也是根据需要打开个关闭连接。

以上两个读写数据块需要线程保留socket，依赖于客户端做什么，你将看到许多连接在当前HDFS文件数量访问数上下进行浮动。

回到上面HBASE：你没有看到22个连接的原因是连接所需要的数据是HFILE的数据块信息，该数据块能活动每个文件的重要信息，然后关闭连接，这意味着服务端的资源很快就释放。保留四个连接很难确定，可以使用JStack dump DataNode所有线程，显示如下：

“DataXceiver for client /127.0.0.1:64281 [sending block blk_5532741233443227208_4201]” daemon prio=5 tid=7fb96481d000 nid=0x1178b4000 runnable [1178b3000]
java.lang.Thread.State: RUNNABLE
…

“DataXceiver for client /127.0.0.1:64172 [receiving block blk_-2005512129579433420_4199 client=DFSClient_hb_rs_10.0.0.29,60020,1330984111693_1330984118810]” daemon prio=5 tid=7fb966109000 nid=0x1169cb000 runnable [1169ca000]
java.lang.Thread.State: RUNNABLE
…

这仅仅是DataXceiver记录，因此线程组的数量有点误导。DataXceiverServer 后台进程本身也被计算在内。和其他两个活动连接，实际上是三个活动线程。日志显示4个线程，实际上市一个活动的线程log计数将完成，所以实际上是三个，与启动时候3个线程匹配。
内部helper类，如PacketResponder占用另一个线程，JSTACK输出如下：

”PacketResponder 0 for Block blk_-2005512129579433420_4199″ daemon prio=5 tid=7fb96384d000 nid=0x116ace000 in Object.wait() [116acd000]
java.lang.Thread.State: TIMED_WAITING (on object monitor)
at java.lang.Object.wait(Native Method)
at org.apache.hadoop.hdfs.server.datanode.BlockReceiver$PacketResponder \
.lastDataNodeRun(BlockReceiver.java:779)
- locked (a org.apache.hadoop.hdfs.server.datanode.BlockReceiver$PacketResponder)
at org.apache.hadoop.hdfs.server.datanode.BlockReceiver$PacketResponder.run(BlockReceiver.java:870)
at java.lang.Thread.run(Thread.java:680)

该线程状态目前是TIMED_WAITING，这就是为什么log中没有包含这些线程。如果客户端发送数据，活动线程数将立马上升，另一个注意地方是线程不需要另外的连接或者socket，PacketResponder仅仅是server端的一个线程，接收数据块数据，在写管道中，将数据发送到另一个DataNode。
Hadoop fsck命令，报告目前打开写的文件：

$ hadoop fsck /hbase -openforwrite
FSCK started by larsgeorge from /10.0.0.29 for path /hbase at Mon Mar 05 22:59:47 CET 2012
……/hbase/.logs/10.0.0.29,60020,1330984111693/10.0.0.29%3A60020.1330984118842 0 bytes, 1 block(s),OPENFORWRITE: ………………………………..Status: HEALTHY
Total size: 2088783626 B
Total dirs: 54
Total files: 45
…

他不会立马占用server端线程，通过block ID分配，打开数据块进行写。命令将显示实际文件盒Block ID:

$ hadoop fsck /hbase -files -blocks
FSCK started by larsgeorge from /10.0.0.29 for path /hbase at Tue Mar 06 10:39:50 CET 2012

…
/hbase/.META./1028785192/.tmp

…
Status: HEALTHY
Total size: 2088788703 B
Total dirs: 54
Total files: 45 (Files currently being written: 1)
Total blocks (validated): 64 (avg. block size 32637323 B) (Total open file blocks (not validated): 1)
Minimally replicated blocks: 64 (100.0 %)
…

从输出可以看到两件事：
第一，在命令运行时，有一个文件打开，与-openforwriting匹配；
第二，block列表与文件线程名匹配，例如Block_id=“blk_5532741233443227208_4201″ 从server端发到客户端，该数据块属于HBASE .META表，在fsck命令中显示，结合JStack和fsck可以取代lsof命令来查看打开文件
JStack报告DataXceiver线程，在fsck命令没有出现该数据块列表。这是因为该数据块没有完成，所以没有提供到输出报告中，fsck仅仅报告完成数据块

回到HBASE

在server端打开所有region不需要很多资源，如果扫描整个HBASE表，强迫HBASE读所有HFILE数据块：
HBase Shell:

hbase(main):003:0> scan ‘usertable’
…
1000000 row(s) in 1460.3120 seconds

DataNode Log:

2012-03-05 14:42:20,580 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 6
2012-03-05 14:43:23,293 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 7
2012-03-05 14:43:23,299 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 8
…
2012-03-05 14:49:24,332 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 11
2012-03-05 14:49:24,332 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 10
2012-03-05 14:49:59,987 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 11
2012-03-05 14:51:12,603 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 12
2012-03-05 14:51:12,605 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 11
2012-03-05 14:51:46,473 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 12
…
2012-03-05 14:56:59,420 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 15
2012-03-05 14:57:31,722 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 16
2012-03-05 14:58:24,909 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 17
2012-03-05 14:58:24,910 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 16
…
2012-03-05 15:04:17,688 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 21
2012-03-05 15:04:17,689 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 22
2012-03-05 15:04:54,545 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 21
2012-03-05 15:05:55,901 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: DatanodeRegistration(127.0.0.1:50010, storageID=DS-1423642448-10.0.0.64-50010-1321352233772, infoPort=50075, ipcPort=50020):Number of active connections is: 22
2012-03-05 15:05:55,901 DEBUG org.apache.hadoop.hdfs.server.datanode.DataNode: Number of active connections is: 21

活动连接数就基本达到22.

所有意味着什么？

因此需要多少xcievers？如果使用HBase，仅仅简单监控storefiles+加上一些中间数量所需要使用比例+write ahead 日志文件。
以上例子输出是在单节点运行，如果是集群，需要将总的storefile除以DataNode，例如存储文件数量是1000,10个节点datanode，缺省256个xcievier线程数是OK。

最差情况是包含了所有读和写的数量，这很难事先确定，你也许很想建立十几个保留。因为写进程需要额外的，另外需要一些更短活动线程给PacketResponder，也不得不计算。因此合理的简单公式如下：
xceivers数量=（活动的写数量*2+活动的读数量）/Datanode数量

所以每个节点dfs.datanode.max.xcievers数量可以设置

对于纯的HBASE设置，我们评估该公式如下：

因为一些参数很难估算，我们用最大值来代替，替换公式如下：