IndexDeletionPolicy
Expert: policy for deletion of stale index commits.
Implement this interface, and set it on to customize when older [ ] are deleted from the index directory.
The default deletion policy is KeepOnlyLastCommitDeletionPolicy, always removes old commits as soon as a new commit is done (this matches the behavior before 2.2).
One expected use case for this (and the reason why it was first created) is to work around problems with an index directory accessed via filesystems like NFS because NFS does not provide the "delete on last close" semantics that Lucene's "point in time" search normally relies on. By implementing a custom deletion policy, such as "a commit is only removed once it has been stale for more than X minutes", you can give your readers time to refresh to the new commit before IndexWriter removes the old commits. Note that doing so will increase the storage requirements of the index. See LUCENE-710 for details.