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From "Uwe Schindler (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (SOLR-5150) HdfsIndexInput may not fully read requested bytes.
Date Fri, 16 Aug 2013 18:00:51 GMT

    [ https://issues.apache.org/jira/browse/SOLR-5150?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13742465#comment-13742465
] 

Uwe Schindler commented on SOLR-5150:
-------------------------------------

Hi Mark,
I think your version should be preferred in both cases. The Apache Blur upstream version looks
like SimpleFSIndexInput (which has synchronization on the RandomAccessFile). The difference
is here, that reading from a real file has no network involved (at least not for local filesystems)
so the time spent in the locked code block is shorter. Still SimpleFSDir is bad for queries.
When merging the whole stuff works single-threaded per file so you would see so difference
in both approaches. If the positional readFully approach would be slower, then this would
be clearly a bug in Hdfs.
Another alternative would be: When cloning a file also clone the underlying Hdfs connection.
With RandomAccessFile we cannot do this in the JDK (we have no dup() for file descriptors),
but if Hdfs supports some dup() like approach with delete on-last close semantics (the file
could already be deleted when you dup the file descriptor) you could create 2 different connection
for each thread.
The backside: Lucene never closes clones - one reason why I gave up on implementig a Windows-Optimized
directory that would clone underlying file descriptor: The clone would never close the dup
:(
                
> HdfsIndexInput may not fully read requested bytes.
> --------------------------------------------------
>
>                 Key: SOLR-5150
>                 URL: https://issues.apache.org/jira/browse/SOLR-5150
>             Project: Solr
>          Issue Type: Bug
>    Affects Versions: 4.4
>            Reporter: Mark Miller
>            Assignee: Mark Miller
>             Fix For: 4.5, 5.0
>
>         Attachments: SOLR-5150.patch
>
>
> Patrick Hunt noticed that our HdfsDirectory code was a bit behind Blur here - the read
call we are using may not read all of the requested bytes - it returns the number of bytes
actually written - which we ignore.
> Blur moved to using a seek and then readFully call - synchronizing across the two calls
to deal with clones.
> We have seen that really kills performance, and using the readFully call that lets you
pass the position rather than first doing a seek, performs much better and does not require
the synchronization.
> I also noticed that the seekInternal impl should not seek but be a no op since we are
seeking on the read.

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