difference between FSDirectory and MMap Directory?
caching, java, lucene
Solution
`MMapDirectory` is one of the concrete subclasses of the abstract `FSDirectory` class. It uses memory-mapped files to access the information in the index.
The other options are `SimpleFSDirectory` and `NIOFSDirectory`, which use different methods. You should have a look at the documentation for `FSDirectory` for a brief explanation of all three. As you will see there, `FSDirectory.open(File)` tries to pick the best implementation for your environment.
In my own experience, I haven't noted any significant difference in performance between `NIOFSDirectory` and `MMapFSDirectory`, but you should do some performance testing using your own data and hardware setup.
In case you end up using `MMapFSDirectory`, virtual memory and index size could only be a problem on a 32-bit machine (unless your indexes are larger than 2^48 bit = 32TB).
Problem
Can any one explain me what is the difference between FSDirectory and MMapDirectory? I want to warm up my cache. I read that this could be useful but couldn't find how this will be helpful in warming up the cache. Explain me if you have any idea. Even pointers are welcome. Lucene documentation says that MMap uses virtual memory to speed up the lookup of the indices. How the speedp up is achieved and what happens if my indices are large so that they won't fit in my virtual memory>