IndexCacheSize defines the amount of RAM indexer --index uses to for the search index cache. When the search index cache size gets equal or greater than size, indexer flushes the collected index cache into the database and continues to index the next portion of documents.
The bigger cache size gives better performance at search time, because of smaller search index fragmentation and therefore fewer disk operation.
The size value understands the K, M and G multipliers meaning kilobytes, megabytes and gigabytes respectively.
The default value is 128M.
IndexCacheSize takes global effect for the entire configuration file and should be used only one time.