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https://issues.apache.org/jira/browse/HDFS-7055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14158254#comment-14158254
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Colin Patrick McCabe edited comment on HDFS-7055 at 10/3/14 6:05 PM:
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bq. I agree but still see 280 spans on .005 patch.
Thanks for testing this, Masatake. Unfortunately, I don't see a good way to
reduce the number of spans you're seeing.
Are you tracing every read? I guess my intention here was that people who
wanted fewer spans would set tracing to {{ProbabilitySampler}}, not
{{AlwaysSampler}}.
Most of the spans shown in that picture are {{DFSInputStream#byteArrayRead}}.
But this is something that needs to be traced when tracing is on. It's one of
the entry points to the HDFS read path. If we don't trace this entry point, we
have no way of associating any trace spans underneath with the HDFS read that
caused them. In the context of HBase, this would also mean we didn't know
where HBase's spans ended and HDFS's began.
Can we discuss this in a follow-on? It seems like we can keep refining the
trace point locations as we do more and more testing. I've been planning on
doing some system-wide testing on a 4-node cluster, but I've been distracted by
some other things.
was (Author: cmccabe):
bq. I agree but still see 280 spans on .005 patch.
If you trace every read (sampler set to Sampler.ALWAYS), then yes, you will see
a lot of spans. If you want a smaller number of spans, using the
{{ProbabilitySampler}} would be a better choice.
Most of the spans shown in that picture are {{DFSInputStream#byteArrayRead}}.
But this is something that needs to be traced when tracing is on. It's one of
the entry points to the HDFS read path. If we don't trace this entry point, we
have no way of associating any trace spans underneath with the HDFS read that
caused them. In the context of HBase, this would also mean we didn't know
where HBase's spans ended and HDFS's began.
Let's get this in and then we can continue tweaking trace points.
> Add tracing to DFSInputStream
> -----------------------------
>
> Key: HDFS-7055
> URL: https://issues.apache.org/jira/browse/HDFS-7055
> Project: Hadoop HDFS
> Issue Type: Sub-task
> Components: datanode, namenode
> Affects Versions: 2.6.0
> Reporter: Colin Patrick McCabe
> Assignee: Colin Patrick McCabe
> Attachments: HDFS-7055.002.patch, HDFS-7055.003.patch,
> HDFS-7055.004.patch, HDFS-7055.005.patch, screenshot-get-1mb.005.png,
> screenshot-get-1mb.png
>
>
> Add tracing to DFSInputStream.
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