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There are two types of Blocks: payload and sector bitmap. Payload blocks contain virtual disk payload data, while sector bitmap blocks contain parts of the sector bitmap.
Payload blocks are the size of the BlockSize field defined by the File Parameters metadata item (section 2.6.2.1) and can be virtually indexed: payload block 0 contains the first BlockSize bytes of the virtual disk; payload block 1 contains the second BlockSize bytes of the virtual disk; and so on.
Sector bitmap blocks are always 1 MB in size and can be virtually indexed similarly: sector bitmap block 0 contains the first 1 MB of the sector bitmap; sector bitmap block 1 contains the second 1 MB of the sector bitmap; and so on.
Each sector bitmap block contains a bit for each logical sector in the file, representing whether the corresponding virtual disk sector is present in this file. Bit 0 (that is, bit 0 of byte 0) is the entry for the first virtual sector, bit 1 is the entry for the second, and so on. For each bit, a value of 1 indicates that the payload data for the corresponding virtual sector should be retrieved from this file, while a value of zero indicates that the data should be retrieved from the parent VHDX file.
The number of sectors that can be described in each sector bitmap block is 223, so the number of bytes described by a single sector bitmap block is 223 times the logical sector size (see section 2.6.2.4 for more information on LogicalSectorSize). This value is known as the chunk size. A virtually contiguous, chunk-size aligned and chunk-sized portion of the virtual disk is known as a chunk. The chunk ratio is the number of payload blocks in a chunk, or equivalently, the number of payload blocks per sector bitmap block.
