Thought I’d give some information around index files becoming corrupt on front end servers within a SharePoint farm.  This can sometimes happen due to a network or database failure while a crawl is in progress.  It will result in errors appearing in the Application Event Logs ressembling the following:

Event Type: Error
Event Source: Office Server Search
Event Category: Search service
Event ID: 10038
Date:                       8/8/2009
Time: 9:53:02 AM
User: N/A
Computer: WFE1
Description:
Query machine WFE1′ has been taken out of rotation due to this error: The
content index is corrupt.   0xc0041800.  It will be retried in 15 seconds.
Component: c9dba2c2-0077-46f7-b352-aeeddb6ac2fd

In the case where there are multiple front end servers in the farm, end users will remain unaffected since their search queries will be served by the other front end server, but this issue can cause crawls to hang and will log errors to the application logs every time a crawl is attempted.

This happened on one of our production environments recently, and the first thought was that we should reset the indexes and recrawl the content (which would have worked, but would have taken about 3 days to re-crawl everything and we wanted it working by Monday morning).  So in trying to avoid a recrawl, we did the following:

• Open the Operations tab of the Central Administration site
• Select Services on Servers and choose the offending WFE from the list
• Stop the Office Server Search Service on the WFE
• Start the service again, but give a new location for the index file
• The next time the crawl runs, the index will be propagated to the new location and bring the server back into rotation.

Hope this helps avoid many hours of re-crawling content.