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.
There are two ways to limit the cache size: using element count and element size. When using element count, in disk store there will be at most MaxKeySize elements (not disk blocks). When using element size, there will be at most KeySize kB of elements stored in the data file. As the Block Disk Cache doesn't need optimization, the data file will be not longer than specified. The limit does not cover the key file size. The mode is chosen using DiskLimitType. Allowed values are: COUNT and SIZE.