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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-7597) Clients seeking over webhdfs may crash the NN
Date Wed, 14 Jan 2015 16:20:37 GMT

    [ https://issues.apache.org/jira/browse/HDFS-7597?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14277150#comment-14277150
] 

Daryn Sharp commented on HDFS-7597:
-----------------------------------

Good discussion:
* The commons LRUMap is no-frills, lightweight, and specifically tailored to this use case.
* Guava in my experience is developer friendly at the expense of performance and excessive
garbage.  A quick glance at the cache builder code definitively confirms this.
* A CHM is neither useful nor performant unless you intend to cache many multiples of the
number of accessing threads.  Probably on the order of thousands which is overkill.

If we happen to rarely make 2 UGIs instead of 1, that's remarkably better than an unbounded
N-many.

Let me take a look at the test failure...

> Clients seeking over webhdfs may crash the NN
> ---------------------------------------------
>
>                 Key: HDFS-7597
>                 URL: https://issues.apache.org/jira/browse/HDFS-7597
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: webhdfs
>    Affects Versions: 2.0.0-alpha
>            Reporter: Daryn Sharp
>            Assignee: Daryn Sharp
>            Priority: Critical
>         Attachments: HDFS-7597.patch
>
>
> Webhdfs seeks involve closing the current connection, and reissuing a new open request
with the new offset.  The RPC layer caches connections so the DN keeps a lingering connection
open to the NN.  Connection caching is in part based on UGI.  Although the client used the
same token for the new offset request, the UGI is different which forces the DN to open another
unnecessary connection to the NN.
> A job that performs many seeks will easily crash the NN due to fd exhaustion.



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