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From "Michael McCandless (JIRA)" <j...@apache.org>
Subject [jira] Commented: (LUCENE-532) [PATCH] Indexing on Hadoop distributed file system
Date Sat, 11 Nov 2006 16:03:40 GMT
    [ http://issues.apache.org/jira/browse/LUCENE-532?page=comments#action_12448989 ] 
            
Michael McCandless commented on LUCENE-532:
-------------------------------------------

Alas, in trying to change the CFS format so that file offsets are stored at the end of the
file, when implementing the corresponding changes to CompoundFileReader, I discovered that
this approach isn't viable.  I had been thinking the reader would look at the file length,
subtract numEntry*sizeof(long), seek to there, and then read the offsets (longs).  The problem
is: we can't know sizeof(long) since this is dependent on the actual storage implementation,
ie, for the same reasoning above.  Ie we can't assume a byte = 1 file position, always.

So, then, the only solution I can think of (to avoid seek during write) would be to write
to a separate file, for each *.cfs file, that contains the file offsets corresponding to the
cfs file.  Eg, if we have _1.cfs we would also have _1.cfsx which holds the file offsets.
  This is sort of costly if we care about # files (it doubles the number of files in the simple
case of a bunch of segments w/ no deletes/separate norms).

Yonik had actually mentioned in LUCENE-704 that fixing CFS writing to not use seek was not
very important, ie, it would be OK to not use compound files with HDFS as the store.

Does anyone see a better approach?

> [PATCH] Indexing on Hadoop distributed file system
> --------------------------------------------------
>
>                 Key: LUCENE-532
>                 URL: http://issues.apache.org/jira/browse/LUCENE-532
>             Project: Lucene - Java
>          Issue Type: Improvement
>          Components: Index
>    Affects Versions: 1.9
>            Reporter: Igor Bolotin
>            Priority: Minor
>         Attachments: indexOnDFS.patch, SegmentTermEnum.patch, TermInfosWriter.patch
>
>
> In my current project we needed a way to create very large Lucene indexes on Hadoop distributed
file system. When we tried to do it directly on DFS using Nutch FsDirectory class - we immediately
found that indexing fails because DfsIndexOutput.seek() method throws UnsupportedOperationException.
The reason for this behavior is clear - DFS does not support random updates and so seek()
method can't be supported (at least not easily).
>  
> Well, if we can't support random updates - the question is: do we really need them? Search
in the Lucene code revealed 2 places which call IndexOutput.seek() method: one is in TermInfosWriter
and another one in CompoundFileWriter. As we weren't planning to use CompoundFileWriter -
the only place that concerned us was in TermInfosWriter.
>  
> TermInfosWriter uses IndexOutput.seek() in its close() method to write total number of
terms in the file back into the beginning of the file. It was very simple to change file format
a little bit and write number of terms into last 8 bytes of the file instead of writing them
into beginning of file. The only other place that should be fixed in order for this to work
is in SegmentTermEnum constructor - to read this piece of information at position = file length
- 8.
>  
> With this format hack - we were able to use FsDirectory to write index directly to DFS
without any problems. Well - we still don't index directly to DFS for performance reasons,
but at least we can build small local indexes and merge them into the main index on DFS without
copying big main index back and forth. 

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