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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7055) Add tracing to DFSInputStream
Date Fri, 03 Oct 2014 18:00:35 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14158254#comment-14158254
] 

Colin Patrick McCabe commented on HDFS-7055:
--------------------------------------------

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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