GT.M partitions GDS blocks into 512-block groups. The first block of each group contains a local bitmap. The first block described by the bitmap is itself and is, therefore, always marked busy.
The bitmap reports whether each of the 512 blocks is currently busy or free and whether it ever contained valid data that has since been KILLed.
The two bits for each block have the following meanings:
00 - Busy
01 - Free and never used before
10 - Not currently a legal combination
11 - Free but previously used
The consequences of corrupted bitmaps are:
Possible loss of data when GT.M overwrites a block that is incorrectly marked as free (malignant).
Reduction in the effective size of the database by the number of blocks incorrectly marked as busy (benign).