很多网上转载解决如下：
修改hdfs-site.xml:添加
<property>
<name>dfs.datanode.du.reserved</name>
<value>214748364800</value>
<description>Reserved space in bytes per volume. Always leave this much space free for non dfs use.200G </description>
</property>
不同磁盘空间大小，hadoop将数据平均写入，hadoop并不会自动将数据写到有更大空闲空间的磁盘中，还是会将之前的小磁盘写满，小磁盘写满会使mapreduce产生的临时文件没有空间写，而导致mapreduce执行失败。所以需要小磁盘留有一定的空闲空间，查看hadoop资料，设置 dfs.datanode.du.reserved配置项可以使每个磁盘保留相应的磁盘空间，单位使用bytes，但是我设置之后起作用了，总体容量下降，依然往小盘上写数据，艹艹，我使用的hadoop版本是cloudera的cdh4.6。
后续继续设置
<property>
</property>
测试后依然如此。
观察源代码：
./hadoop-hdfs/src/main/java/org/apache/hadoop/hdfs/server/datanode/fsdataset/impl/FsVolumeImpl.java
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long getCapacity() {
long remaining = usage.getCapacity() - reserved;
return remaining > 0 ? remaining : 0;
}
@Override
public long getAvailable() throws IOException {
long remaining = getCapacity()-getDfsUsed();
long available = usage.getAvailable();
if (remaining > available) {
remaining = available;
}
return (remaining > 0) ? remaining : 0;
}
long getReserved(){
return reserved;
}
./hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/fs/LocalDirAllocator.java
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static class AllocatorPerContext {
private final Log LOG =
LogFactory.getLog(AllocatorPerContext.class);
private int dirNumLastAccessed;
private Random dirIndexRandomizer = new Random();
private FileSystem localFS;
private DF[] dirDF;
private String contextCfgItemName;
private String[] localDirs;
private String savedLocalDirs = "";
public AllocatorPerContext(String contextCfgItemName) {
this.contextCfgItemName = contextCfgItemName;
}
private synchronized void confChanged(Configuration conf)
throws IOException {
String newLocalDirs = conf.get(contextCfgItemName);
if (!newLocalDirs.equals(savedLocalDirs)) {
localDirs = StringUtils.getTrimmedStrings(newLocalDirs);
localFS = FileSystem.getLocal(conf);
int numDirs = localDirs.length;
ArrayList<String> dirs = new ArrayList<String>(numDirs);
ArrayList<DF> dfList = new ArrayList<DF>(numDirs);
for (int i = 0; i < numDirs; i++) {
try {
File tmpFile = tmpDir.isAbsolute()
? new File(localFS.makeQualified(tmpDir).toUri())
: new File(localDirs[i]);
DiskChecker.checkDir(tmpFile);
dirs.add(tmpFile.getPath());
dfList.add(new DF(tmpFile, 30000));
} catch (DiskErrorException de) {
LOG.warn( localDirs[i] + " is not writable\n", de);
}
} else {
LOG.warn( "Failed to create " + localDirs[i]);
}
} catch (IOException ie) {
LOG.warn( "Failed to create " + localDirs[i] + ": " +
ie.getMessage() + "\n", ie);
} //ignore
}
localDirs = dirs.toArray(new String[dirs.size()]);
dirDF = dfList.toArray(new DF[dirs.size()]);
savedLocalDirs = newLocalDirs;
// randomize the first disk picked in the round-robin selection
dirNumLastAccessed = dirIndexRandomizer.nextInt(dirs.size());
}
}
public synchronized Path getLocalPathForWrite(String pathStr, long size,
Configuration conf, boolean checkWrite) throws IOException {
confChanged(conf);
int numDirs = localDirs.length;
int numDirsSearched = 0;
//remove the leading slash from the path (to make sure that the uri
//resolution results in a valid path on the dir being checked)
if (pathStr.startsWith("/")) {
pathStr = pathStr.substring(1);
}
Path returnPath = null;
if(size == SIZE_UNKNOWN) { //do roulette selection: pick dir with probability
//proportional to available size
long[] availableOnDisk = new long[dirDF.length];
long totalAvailable = 0;
//build the "roulette wheel"
for(int i =0; i < dirDF.length; ++i) {
availableOnDisk[i] = dirDF[i].getAvailable();
totalAvailable += availableOnDisk[i];
}
// Keep rolling the wheel till we get a valid path
Random r = new java.util.Random();
while (numDirsSearched < numDirs && returnPath == null) {
long randomPosition = Math.abs(r.nextLong()) % totalAvailable;
int dir = 0;
while (randomPosition > availableOnDisk[dir]) {
randomPosition -= availableOnDisk[dir];
dir++;
}
dirNumLastAccessed = dir;
returnPath = createPath(pathStr, checkWrite);
if (returnPath == null) {
totalAvailable -= availableOnDisk[dir];
availableOnDisk[dir] = 0; // skip this disk
numDirsSearched++;
}
}
} else {
while (numDirsSearched < numDirs && returnPath == null) {
long capacity = dirDF[dirNumLastAccessed].getAvailable();
if (capacity > size) {
returnPath = createPath(pathStr, checkWrite);
}
dirNumLastAccessed++;
dirNumLastAccessed = dirNumLastAccessed % numDirs;
numDirsSearched++;
}
}
if (returnPath != null) {
return returnPath;
}
//no path found
throw new DiskErrorException("Could not find any valid local " +
"directory for " + pathStr);
}
对配置都有判读，咋无效呢？无语，有空再深究把。
其他配置选项依旧不能保证小磁盘写入少，哎~~~~~~~~~
还是以FAQ上为准把：
http://wiki.apache.org/hadoop/FAQ#On_an_individual_data_node.2C_how_do_you_balance_the_blocks_on_the_disk.3F
3.12. On an individual data node, how do you balance the blocks on the disk?
Hadoop currently does not have a method by which to do this automatically. To do this manually:
1. Take down the HDFS
2. Use the UNIX mv command to move the individual blocks and meta pairs from one directory to another on each host
3. Restart the HDFS
对于1）停止hdfs，只需要停止datanode，使用命令$HADOOP_HOME/bin/hadoop-daemon.sh stop datanode
对于2）必须是dfs.data.dir目录下current目录的子目录 mv path-to-data-dir/current/finalized/subdir11/* path-to-data-dir/current/finalized/subdir11
对于3）$HADOOP_HOME/bin/hadoop-daemon.sh start datanode
参见这个：http://search-hadoop.com/m/fSof91EYbe9
#disk
#hdfs
#hadoop
#linux
