[HDFS-8696] Make the lower and higher watermark in the DN Netty server configurable

• Type: Improvement
• Status: Resolved
• Priority: Major
• Resolution: Fixed
• Affects Version/s: 2.7.0
• Fix Version/s: 2.8.0, 3.0.0-alpha1
• Component/s: webhdfs
• Hadoop Flags: Reviewed
• Release Note: Introduced two new configuration dfs.webhdfs.netty.low.watermark and dfs.webhdfs.netty.high.watermark to enable tuning the size of the buffers of the Netty server inside Datanodes.
• Assignee: Xiaobing Zhou
• Reporter: Xiaobing Zhou
• Created: 30/Jun/15 00:26
• Updated: 30/Aug/16 01:31
• Resolved: 29/Sep/15 22:12

Description

There is an issue that appears related to the webhdfs server. When making two concurrent requests, the DN will sometimes pause for extended periods (I've seen 1-300 seconds), killing performance and dropping connections.

To reproduce:
1. set up a HDFS cluster
2. Upload a large file (I was using 10GB). Perform 1-byte reads, writing
the time out to /tmp/times.txt

i=1
while (true); do 
echo $i
let i++
/usr/bin/time -f %e -o /tmp/times.txt -a curl -s -L -o /dev/null "http://<namenode>:50070/webhdfs/v1/tmp/bigfile?op=OPEN&user.name=root&length=1";
done

3. Watch for 1-byte requests that take more than one second:
tail -F /tmp/times.txt | grep -E "^[^0]"

4. After it has had a chance to warm up, start doing large transfers from
another shell:

i=1
while (true); do 
echo $i
let i++
/usr/bin/time -f %e curl -s -L -o /dev/null "http://<namenode>:50070/webhdfs/v1/tmp/bigfile?op=OPEN&user.name=root";
done

It's easy to find after a minute or two that small reads will sometimes
pause for 1-300 seconds. In some extreme cases, it appears that the
transfers timeout and the DN drops the connection.