Cache Format
Contents
Cache Format
General Information
The old engine cache is made up two types of files.
Data file
The data file holds all of the files in the cache and is named main_file_cache.dat. It is therefore very big, typically ~10-20 megabytes..
Index file
There are several index files, named main_file_cache.idx and then postfixed with a number. Each index file holds 'pointers' to where a file is located in the main cache. Each index file represents a type of file.
Format
Index file format
The index file is made up of 6 byte blocks which hold information about where a file can be located in the data file. The format of a single block is as follows:
tribyte fileSize tribyte initialDataBlockId
Data file format
The data file is made up of 520 byte blocks. The format of each of these blocks is as follows:
short nextFileId short currentFilePartId tribyte nextDataBlockId byte nextFileTypeId byte[512] blockData
Explanation
An example will be used here as it is easier to follow.
Let us say, the client wishes to fetch file type 2, file id 17.
First off, it will open the main_file_cache.idx2 file and seek to the index 17 * 6 (102). It will then read two tribytes.
fileSize = 1200 intialDataBlockId = 4
The client will now open the main_file_cache.dat file and seek to the index 4 * 520 (2080). The values it reads will be:
nextFileId = 17 currentFilePartId = 0 nextDataBlockId = 5 nextFileTypeId = 2 blockData = ...
It will read the first 512 bytes of the file and then knows that there is 688 bytes left. Therefore, it has to read the next block.
nextFileId = 17 currentFilePartId = 1 nextDataBlockId = 6 nextFileTypeId = 2 blockData ...
It reads these next 512 bytes of the file and now knows that there are 176 bytes left. So for a final time, it will read the next block.
nextFileId = 18 currentFilePartId = 2 nextDataBlockId = 7 nextFileTypeId = 2 blockData = ...
It can ignore most of these values (the next ones are meaningless at this stage) and read the final 176 bytes. The whole 1200 byte file has now been read.
