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From "Colin Patrick McCabe (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7055) Add tracing to DFSInputStream
Date Wed, 01 Oct 2014 01:12:34 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7055?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14154161#comment-14154161

Colin Patrick McCabe commented on HDFS-7055:

bq. So, if no receiver set up, traces go nowhere?


bq.  To see anything, you'd have to load a receiver with the just-added trace command (i.e.HDFS-6956)?

There's two ways to turn on tracing:
* set {{hadoop.htrace.spanreceiver.classes}} (usually to {{LocalFileSpanReceiver}} or {{ZipkinSpanReceiver}}
* use "hadoop trace \-add" to add tracing to an already-running daemon.

If you're testing, it's probably easier to just set up your Hadoop XML configuration so that
tracing will be running as soon as the daemon starts up.  This is also the only option for
tracing a short-running program like the FSShell.  The dynamic changing is really more for
production deployments, where people might not want this on all the time, but don't want to
restart daemons to turn it on.

bq. If so, that seems cleaner. The alternative where files show up in /tmp if no receiver
is set is less so. Drop this facility from this patch?

Well, the files don't show up in /tmp unless {{LocalFileSpanReceiver}} is configured.  By
default, there are no trace span receivers configured, and nothing shows up in {{/tmp}}.

bq. On the patch, are these defined anywhere "NeverSampler"? Should we make it so they are?
Or should they be toString of the enums?

This is another thing I'd like to add to HTrace.  I think HTrace should have some method that
creates a sampler based on a string which is the class name.  But right now that doesn't exist,
so I had to add a hacked up version of it to Hadoop (for now).  We can easily take that out
once HTrace has it, though.

I'd like to get the basic stuff in and then start shifting more things that we think could
be common code into HTrace.

> 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, screenshot-get-1mb.png
> Add tracing to DFSInputStream.

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