HDFS Client的读取流是从DFSInputStream来获得的,外层做了不少包装。
从DFSInputStream读取数据有两种方式:
(1)seek(long targetPos) + read(byte buf[], int off, int len)
(2)read(long position, byte[] buffer, int offset, int length)
第一种适合顺序读取,比如hbase里面的scan请求或者hbase的compact读取。读取的数据量一般都比较大,所以打开预读可以减少磁盘的iops,预读可以见hdfs相关的jira
Add HDFS support for fadvise readahead and drop-behind
https://issues.apache.org/jira/browse/HDFS-2465
Enable fadvise readahead by default
https://issues.apache.org/jira/browse/HDFS-3697
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第二种是随机读取,适合读取少量的数据,比如hbase里面的get请求
这两种读取还有一些不同的地方:
同一个DFSInputStream可以有多个应用程序在使用,但是需要注意下面:
(1)同一个时候只能有一个应用程序在执行seek(long targetPos) + read(byte buf[], int off, int len)
(2)在一个应用程序执行seek(long targetPos) + read(byte buf[], int off, int len) 时,其他应用程序使用这个DFSInputStream执行read(long position, byte[] buffer, int offset, int length),并且同一时间可以有多个应用程序使用这个DFSInputStream执行read(long position, byte[] buffer, int offset, int length)
TestCase可以见hdfs的org.apache.hadoop.hdfs.TestPread
我们看下hbase是如何来使用的:
pread为true代表随机读取,当请求是get请求时会设置pread为true
0.90.x版本:
BoundedRangeFileInputStream
@Override public int read(byte[] b, int off, int len) throws IOException { if ((off | len | (off + len) | (b.length - (off + len))) < 0) { throw new IndexOutOfBoundsException(); } int n = (int) Math.min(Integer.MAX_VALUE, Math.min(len, (end - pos))); if (n == 0) return -1; int ret = 0; if (this.pread) { // 随机读取,第二种方式,可以任意多应用程序同时调用 ret = in.read(pos, b, off, n); } else { synchronized (in) { //顺序读取,第一种方式,同一个时候只能有一个应用程序调用,所以这个地方加了锁 in.seek(pos); ret = in.read(b, off, n); } } if (ret < 0) { end = pos; return -1; } pos += ret; return ret; }
0.94.x版本:
HFileBlock protected int readAtOffset(FSDataInputStream istream, byte[] dest, int destOffset, int size, boolean peekIntoNextBlock, long fileOffset, boolean pread) throws IOException { if (peekIntoNextBlock && destOffset + size + hdrSize > dest.length) { // We are asked to read the next block's header as well, but there is // not enough room in the array. throw new IOException("Attempted to read " + size + " bytes and " + hdrSize + " bytes of next header into a " + dest.length + "-byte array at offset " + destOffset); } if (pread) { // Positional read. Better for random reads. int extraSize = peekIntoNextBlock ? hdrSize : 0; int ret = istream.read(fileOffset, dest, destOffset, size + extraSize); // 随机读取,第二种方式,可以任意多应用程序同时调用 if (ret < size) { throw new IOException("Positional read of " + size + " bytes " + "failed at offset " + fileOffset + " (returned " + ret + ")"); } if (ret == size || ret < size + extraSize) { // Could not read the next block's header, or did not try. return -1; } } else { // Seek + read. Better for scanning. synchronized (istream) { //顺序读取,第一种方式,同一个时候只能有一个应用程序调用,所以这个地方加了锁 istream.seek(fileOffset); long realOffset = istream.getPos(); if (realOffset != fileOffset) { throw new IOException("Tried to seek to " + fileOffset + " to " + "read " + size + " bytes, but pos=" + realOffset + " after seek"); } if (!peekIntoNextBlock) { IOUtils.readFully(istream, dest, destOffset, size); return -1; } // Try to read the next block header. if (!readWithExtra(istream, dest, destOffset, size, hdrSize)) return -1; } } assert peekIntoNextBlock; return Bytes.toInt(dest, destOffset + size + BlockType.MAGIC_LENGTH) + hdrSize; }
为什么随机读取的时候可以任意多个应用程序使用呢:
(1)seek(long targetPos) + read(byte buf[], int off, int len)
第一种,seek很简单改变下pos,read(byte buf[], int off, int len) 里面:重要的是blockSeekTo(long target) ,blockSeekTo第一步做的操作就是看下当前blockReader是否为null,如果不为null那么就close这个BlockReader,
然后new一个BlockReader,向DataNode发送的请求里面包含当前开始读取的位置,以及长度(blk.getNumBytes() - offsetIntoBlock来获得,也就是说长度是当前块剩余可读的数据量), read(byte buf[], int off, int len) 可以多次调用,直到数据已经读完。在这里我们可以看到new 了一个BlockReader后,没有及时关闭,BlockReader关闭的时机是下次调用seek+read时发现上次new 的BlockReader还在,那么就关闭。 所以如果一个应用程序在调用一个DFSInputStream流的seek+read时,如果另外一个应用执行seek+read的话,后面这个应用程序会把前面那个应用程序的BlockReader关闭,导致前面一个应用程序读取不到数据了,所以同一时间只能有一个应用程序调用一个DFSInputStream流的seek+read,除非使用另外一个DFSInputStream流的seek+read
(2)read(long position, byte[] buffer, int offset, int length)
第二种,读取时每次都会new一个BlockReader,向DataNode发送的请求里面包含了读取开始的位置(position),以及长度length,读取完后就会close这个BlockReader
pread Use positional read instead of seek+read (positional is better doing random reads whereas seek+read is better scanning).
https://issues.apache.org/jira/browse/HBASE-2180
/proc/sys/net/ipv4/tcp_tw_recycle
Allow multiple readers per storefile
https://issues.apache.org/jira/browse/HBASE-7347
Non-pread DFSInputStreams should be associated with scanners, not HFile.Readers
https://issues.apache.org/jira/browse/HBASE-5979
HFileBlock.readAtOffset does not work well with multiple threads
https://issues.apache.org/jira/browse/HBASE-7336
Consider double-checked locking for block cache lock
https://issues.apache.org/jira/browse/HBASE-5898
http://markmail.org/thread/ulki4uccwb43ahdj
[89-fb] Using pread for non-compaction read request
https://issues.apache.org/jira/browse/HBASE-7266