The Block Disk Cache stores cached values on disk. Like the Indexed Disk Cache, the Block Disk Cache keeps the keys in memory. The Block Disk Cache stores the values in a group of fixed size blocks, whereas the Indexed Disk Cache writes items to disk in one chunk.
The Block Disk Cache has advantages over the normal indexed model for regions where the size of the items varies. Since all the blocks are the same size, the recycle bin is very simple. It is just a list of block numbers. Also, the Block Disk Cache will never need to be optimized. Once the maximum number of keys is reached, blocks will be reused.