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From robert engels <reng...@ix.netcom.com>
Subject Re: [jira] Updated: (LUCENE-624) Segment size limit for compound files
Date Wed, 26 Jul 2006 22:23:47 GMT
Why does more segment files improve search performance? I can see  
that if you have many smaller files, the merge process for  
incremental adds might be faster, but more segments should actually  
make searching slower.

On Jul 26, 2006, at 5:18 PM, Michael Busch (JIRA) wrote:

>      [ http://issues.apache.org/jira/browse/LUCENE-624?page=all ]
>
> Michael Busch updated LUCENE-624:
> ---------------------------------
>
>     Attachment: cfs_seg_size_limit.patch
>
> I attach the patch file for this improvement.
>
> This patch adds two new methods to the API of IndexWriter and  
> IndexModifier:
>   /** Get the current value of the compound file segment size limit.
>    *  Note that this just returns the value you set with  
> setCompoundFileSegmentSizeLimit(int)
>    *  or the default. You cannot use this to query the status of an  
> existing index.
>    *  @see #setCompoundFileSegmentSizeLimit(int)
>    */
>   public int getCompoundFileSegmentSizeLimit();
>
>   /** Sets the limit of documents a segment can have, so that
>    *  compound format is being used for that segment. A high
>    *  limit will decrease the number of files per index, whereas
>    *  a lower limit will improve search performance but
>    *  increase the number of files.
>    */
>   public void setCompoundFileSegmentSizeLimit(int value);
>
> Furthermore I added a constant to IndexWriter:
> public final static int DEFAULT_COMPOUND_FILE_SEGMENT_SIZE_LIMIT =  
> Integer.MAX_VALUE;
>
> Since the default value is set to Integer.MAX_VALUE, the behavior  
> of IndexWriter/IndexModifier only changes if the user uses  
> setCompoundFileSegmentSizeLimit(int) to change the value explicitly.
>
>> Segment size limit for compound files
>> -------------------------------------
>>
>>                 Key: LUCENE-624
>>                 URL: http://issues.apache.org/jira/browse/LUCENE-624
>>             Project: Lucene - Java
>>          Issue Type: Improvement
>>          Components: Index
>>            Reporter: Michael Busch
>>            Priority: Minor
>>         Attachments: cfs_seg_size_limit.patch
>>
>>
>> Hello everyone,
>> I implemented an improvement targeting compound file usage.  
>> Compound files are used to decrease the number of index files,  
>> because operating systems can't handle too many open file  
>> descriptors. On the other hand, a disadvantage of compound file  
>> format is the worse performance compared to multi-file indexes:
>> http://www.gossamer-threads.com/lists/lucene/java-user/8950
>> In the book "Lucene in Action" it's said that compound file format  
>> is about 5-10% slower than multi-file format.
>> The patch I'm proposing here adds the ability to the IndexWriter  
>> to use compound format only for segments, that do not contain more  
>> documents than a specific limit "CompoundFileSegmentSizeLimit",  
>> which the user can set.
>> Due to the exponential merges, a lucene index usually contains  
>> only a few very big segments, but much more small segments. The  
>> best performance is actually just needed for the big segments,  
>> whereas a slighly worse performance for small segments shouldn't  
>> play a big role in the overall search performance.
>> Consider the following example:
>> Index Size:                            1,500,000
>> Merge factor:                        10
>> Max buffered docs:             100
>> Number of indexed fields: 10
>> Max. OS file descriptors:    1024
>> in the worst case a not-optimized index could contain the  
>> following amount of segments:
>> 1 x 1,000,000
>> 9 x   100,000
>> 9 x    10,000
>> 9 x     1,000
>> 9 x       100
>> That's 37 segments. A multi-file format index would have:
>> 37 segments * (7 files per segment + 10 files for indexed fields)  
>> = 629 files ==> only about 2 open indexes per machine could be  
>> handled by the operating system
>> A compound-file format index would have:
>> 37 segments * 1 cfs file = 37 files ==> about 27 open indexes  
>> could be handled by the operating system, but performance would be  
>> 5-10% worse.
>> A compound-file format index with CompoundFileSegmentSizeLimit =  
>> 1,000,000 would have:
>> 36 segments * 1 cfs file + 1 segment * (7 + 10 files) = 53 ==>  
>> about 20 open indexes could be handled by the OS
>> The OS can handle now 20 instead of just 2 open indexes, while  
>> maintaining the multi-file format performance.
>> I'm going to create diffs on the current HEAD and will attach the  
>> patch files soon. Please let me know what you think about this  
>> improvement.
>
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