Take a snapshot of the most recent commit to the index. You must call release() to free this snapshot. Note that while the snapshot is held, the files it references will not be deleted, which will consume additional disk space in your index. If you take a snapshot at a particularly bad time (say just before you call optimize()) then in the worst case this could consume an extra 1X of your total index size, until you release the snapshot.
SnapshotDeletionPolicy Class | Lucene.Net.Index Namespace