Gopal V created HDFS-4070:
-----------------------------
Summary: DFSClient ignores bufferSize argument & always performs
small writes
Key: HDFS-4070
URL: https://issues.apache.org/jira/browse/HDFS-4070
Project: Hadoop HDFS
Issue Type: Bug
Components: hdfs client
Affects Versions: 1.0.3, 2.0.3-alpha
Environment: RHEL 5.5 x86_64 (ec2)
Reporter: Gopal V
Priority: Minor
The following code illustrates the issue at hand
{code}
protected void map(LongWritable offset, Text value, Context context)
throws IOException, InterruptedException {
OutputStream out = fs.create(new
Path("/tmp/benchmark/",value.toString()), true, 1024*1024);
int i;
for(i = 0; i < 1024*1024; i++) {
out.write(buffer, 0, 1024);
}
out.close();
context.write(value, new IntWritable(i));
}
{code}
This code is run as a single map-only task with an input file on disk and
map-output to disk.
{{# su - hdfs -c 'hadoop jar /tmp/dfs-test-1.0-SNAPSHOT-job.jar
file:///tmp/list file:///grid/0/hadoop/hdfs/tmp/benchmark'}}
In the data node disk access patterns, the following consistent pattern was
observed irrespective of bufferSize provided.
{code}
21119 read(58, <unfinished ...>
21119 <... read resumed>
"\0\1\0\0\0\0\0\0\0034\212\0\0\0\0\0\0\0+\220\0\0\0\376\0\262\252ux\262\252u"...,
65557) = 65557
21119 lseek(107, 0, SEEK_CUR <unfinished ...>
21119 <... lseek resumed> ) = 53774848
21119 write(107,
"\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 65024
<unfinished ...>
21119 <... write resumed> ) = 65024
21119 write(108,
"\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux\262\252ux"...,
508 <unfinished ...>
21119 <... write resumed> ) = 508
{code}
Here fd 58 is the incoming socket, 107 is the blk file and 108 is the .meta
file.
The DFS packet size ignores the bufferSize argument and suffers from suboptimal
syscall & disk performance because of the default 64kb value, as is obvious
from the interrupted read/write operations.
Changing the packet size to a more optimal 1056405 bytes results in a decent
spike in performance, by cutting down on disk & network iops.
h3. Average time (milliseconds) for a 10 GB write as 10 files in a single map
task
||timestamp||65536||1056252||
|1350469614|88530|78662|
|1350469827|88610|81680|
|1350470042|92632|78277|
|1350470261|89726|79225|
|1350470476|92272|78265|
|1350470696|89646|81352|
|1350470913|92311|77281|
|1350471132|89632|77601|
|1350471345|89302|81530|
|1350471564|91844|80413|
That is by average an increase from ~115 MB/s to ~130 MB/s, by modifying the
global packet size setting.
This suggests that there is value in adapting the user provided buffer sizes to
hadoop packet sizing, per stream.
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