Source: Microsoft Support

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The entire contents of a file that is crawled by Microsoft SharePoint may not be searchable.

The chunk buffer size used by the Search service may be too small.

As the actual textual data in files may vary, it is impossible to predict what buffer size will work for all files. For example, a 10MB PPT file may not require a larger chunk buffer whereas a 4MB TXT file containing a high percentage of unique terms will. This is because unique words for a PPT could include words that are displayed in images, which does not get indexed. Hence the amount of chunk buffer space needed is directly related to the number of unique words that can be indexed in a file.

The alternative is to increase the buffer size gradually till the entire contents of a file representative of what is in your content source is searchable.

Note increasing these values will cause the search service to consume more memory on the indexer machine and should not be done randomly.

To increase the chunk buffer size, change the following registry values on the Indexer:

Important This section, method, or task contains steps that tell you how to modify the registry. However, serious problems might occur if you modify the registry incorrectly. Therefore, make sure that you follow these steps carefully. For added protection, back up the registry before you modify it. Then, you can restore the registry if a problem occurs. For more information about how to back up and restore the registry, click the following article number to view the article in the Microsoft Knowledge Base:

How to back up and restore the registry in Windows 
http://kbalertz.com/Feedback.aspx?kbNumber=322756/


NOTE: For SharePoint 2010 12.0 will show as 14.0 in the registry keys below

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\12.0\Search\Global\Gathering Manager\CB_ChunkBufferSizeInMegaBytes

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office Server\12.0\Search\Global\Gathering Manager\CB_MinBytesReservedForDoc

CB_ChunkBufferSizeInMegaBytes is expressed in MB while CB_MinBytesReservedForDoc is expressed in bytes.
 

Because there are other structures aside from raw data that need to be maintained within a chunk buffer, CB_MinBytesReservedForDoc should be approximately 2MB less than CB_ChunkBufferSizeInMegaBytes.

For instance, if CB_ChunkBufferSizeInMegaBytes is increased to 10 (decimal), CB_MinBytesReservedForDoc should be: 8388608 (decimal).

The search service must also be restarted for the changes to take effect.

Q: What are chunk buffers ?
A: A Chunk Buffer is a shared block of memory used by Filter Daemons (mssdmn.exe) to provide filtered, word broken data and raw data to the Indexer (mssearch.exe). The Indexer populates the indexes with the data in chunk buffers.
  
Q: What are the Default values?
A: By default, SharePoint Server ships with the following values: 

32bit defaults:
CB_ChunkBufferSizeInMegaBytes Default value: 0x2
CB_MinBytesReservedForDoc Default value: 0xb2000 or 729,088 bytes 

64bit defaults:
CB_ChunkBufferSizeInMegaBytes Default value: 0x8
CB_MinBytesReservedForDoc Default value: 0x300000 or 3,145,728 bytes 

Q: What are the number of chunk buffers created by default?
A: Four (4) 

Q: What parameters needs to be monitored to observe impact of changes?
A: Before modifying these values, using perfmon, locate and note the maximum values for the Process:private bytes and Process:Virtual bytes counters for the MSSearch.exe process after a full crawl.  At a minimum, these values will increase by 4 * CB_ChunkBufferSizeInMegaBytes.  The increased buffer size may negatively impact search service performance/reliability if these values will exhaust available physical memory and/or virtual address space.

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