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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-2373) Create a Codec to work with streaming and append-only filesystems
Date Fri, 09 Jul 2010 10:22:50 GMT

    [ https://issues.apache.org/jira/browse/LUCENE-2373?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12886671#action_12886671
] 

Michael McCandless commented on LUCENE-2373:
--------------------------------------------

Looks great Andrzej!  +1 to commit.

> Create a Codec to work with streaming and append-only filesystems
> -----------------------------------------------------------------
>
>                 Key: LUCENE-2373
>                 URL: https://issues.apache.org/jira/browse/LUCENE-2373
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>            Reporter: Andrzej Bialecki 
>             Fix For: 4.0
>
>         Attachments: appending.patch, appending.patch, LUCENE-2372-2.patch, LUCENE-2373.patch
>
>
> Since early 2.x times Lucene used a skip/seek/write trick to patch the length of the
terms dict into a place near the start of the output data file. This however made it impossible
to use Lucene with append-only filesystems such as HDFS.
> In the post-flex trunk the following code in StandardTermsDictWriter initiates this:
> {code}
>     // Count indexed fields up front
>     CodecUtil.writeHeader(out, CODEC_NAME, VERSION_CURRENT); 
>     out.writeLong(0);                             // leave space for end index pointer
> {code}
> and completes this in close():
> {code}
>       out.seek(CodecUtil.headerLength(CODEC_NAME));
>       out.writeLong(dirStart);
> {code}
> I propose to change this layout so that this pointer is stored simply at the end of the
file. It's always 8 bytes long, and we known the final length of the file from Directory,
so it's a single additional seek(length - 8) to read it, which is not much considering the
benefits.

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