Global allocation map (GAM) and Shares Global Allocation Map (SGAM) pages in sql server

SQL Server uses a structure called the Global Allocation Map (GAM) to keep track of all free extents in a database file. The GAM is consists of several GAM pages. GAM pages records what extents have been allocated for any use. GAM has bit for every extent. If the bit is 1, the corresponding extent is free, if the bit is 0, the corresponding extent is in use as uniform or mixed extent. A GAM page can hold information of around 64000 extents. That is, a GAM page can hold information of (64000X8X8)/1024 = 4000 MB approximately. In other ward we can say that, a data file of size 7 GB will have two GAM pages.

SGAM pages record what extents are currently being used as mixed extent and also have at least one unused page. SGAM has bit for every extent. If the bit is 1, the corresponding extent is used as a mixed extent and has at least one page free to allocate. If the bit is 0, the extent is either not used as a mixed extent or it is mixed extent and with all its pages being used. A SGAM page can hold information of 64000 extents. That is, a SGAM page can hold information of (64000X8X8)/1024 = 4000 MB. . In other ward we can say that, a data file of size 7 GB will have two GAM pages.