Structure of the _id and document storage in mongodb

[Moditha Hewasinghage]

Hi,

Can anyone direct me to the correct path where I can find the exact structure of how the index for _id is maintained, blocks allocated and the data stored? I was trying to insert simple documents ({a:1,b:2}) with default _id creation. When I check the stats of the data it says document size is 44b and the storage size is16384 (using wired tiger without compression to get better idea). It says 3 blocks are allocated (block size 4KB) so I'm not sure what the other block is being used for (16384/4096=4). Moreover, when inserting the same sized documents at the 4th document (data is 176b), the storage expands to 7 blocks. If someone could help me with how exactly the data is stored it would be really helpful or where to find it in the source code.

Thank you,
Moditha