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From "Todd Lipcon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-9150) Unnecessary DNS resolution attempts for logical URIs
Date Wed, 23 Jan 2013 07:02:13 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-9150?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13560436#comment-13560436
] 

Todd Lipcon commented on HADOOP-9150:
-------------------------------------

Hey Suresh. Thanks for reviewing the patch.

The reason that we need to create {{canonicalizeUri}} and allow implementations to override
it is that we have canonicalize the URI parameter in {{checkPath}}. Since we don't have a
FileSystem instance corresponding to the URI parameter, we have to add this method which takes
a URI for this to work out.

Certainly most systems can now use the default implementation of {{getCanonicalUri}} (which
just calls {{canonicalizeUri(getUri())}}, but making it final seemed a little bit dictatorial.
I'm generally not a fan of forcing users to not override things - seems better to just suggest
that implementors use the default implementation unless they have good reason not to.
                
> Unnecessary DNS resolution attempts for logical URIs
> ----------------------------------------------------
>
>                 Key: HADOOP-9150
>                 URL: https://issues.apache.org/jira/browse/HADOOP-9150
>             Project: Hadoop Common
>          Issue Type: Bug
>          Components: fs/s3, ha, performance, viewfs
>    Affects Versions: 3.0.0, 2.0.2-alpha
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>            Priority: Critical
>         Attachments: hadoop-9150.txt, hadoop-9150.txt, hadoop-9150.txt, hadoop-9150.txt,
hadoop-9150.txt, hadoop-9150.txt, hadoop-9150.txt, log.txt, tracing-resolver.tgz
>
>
> In the FileSystem code, we accidentally try to DNS-resolve the logical name before it
is converted to an actual domain name. In some DNS setups, this can cause a big slowdown -
eg in one misconfigured cluster we saw a 2-3x drop in terasort throughput, since every task
wasted a lot of time waiting for slow "not found" responses from DNS.

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