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Posted to dev@lucene.apache.org by "Lance Norskog (JIRA)" <ji...@apache.org> on 2010/05/07 00:33:48 UTC
[jira] Commented: (LUCENE-2373) Change StandardTermsDictWriter to
work with streaming and append-only filesystems
[ https://issues.apache.org/jira/browse/LUCENE-2373?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12864954#action_12864954 ]
Lance Norskog commented on LUCENE-2373:
---------------------------------------
Another reason to create files in a fully sequential mode is that SSD drives do not like random writes - they can get very slow. SSDs function well with sequential writes, sequential reads, and random reads, so if this issues is fixed, they should work well with Lucene.
> Change StandardTermsDictWriter 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
>
>
> 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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