2.5 BAT

BAT is a region consisting of a single array of 64-bit values, with an entry for each block that determines the state and file offset of that block. The entries for the payload block and sector bitmap block are interleaved in a way that the sector bitmap block entry associated with a chunk follows the entries for the payload blocks in that chunk. For example, if the chunk ratio is 4, the table's interleaving would look like the following figure.

Figure 6: BAT layout example

The BAT layout is the same for all types of VHDX, whether fixed, dynamic, or differencing. However, in a fixed or dynamic VHDX, the sector bitmap blocks will not be allocated. In a dynamic VHDX all sectors are contained within the file. Because the sector bimap blocks are used to determine whether a parent VHDX file contains payload data, they are unnecessary in a dynamic VHDX. BAT entries for sector bitmap blocks exist for dynamic VHDX, because, in addition, the presence of the entries avoids the need to insert or remove the interleaving sector bitmap entries during conversion between dynamic and differencing virtual hard disk types.

The BAT region MUST be at least large enough to contain as many entries as required to describe the possible blocks for a given virtual disk size (see section 2.6.2.2 for more information on VirtualDiskSize).

The number of payload blocks can be calculated as

The number of sector bitmap blocks can be calculated as

For dynamic and fixed VHDX, the last BAT entry MUST locate the last payload block of the virtual disk. The total number of BAT entries for either dynamic or fixed VHDX can be calculated as

For a differencing VHDX, the last BAT entry must be able to locate the last sector bitmap block that contains the last payload sector. The total number of BAT entries can be calculated as