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File Manager

The "magnetic disk storage manager"
  • manages its own pool of open file descriptors
  • each one represents an open relation file (Vfd)
  • may use several Vfd's to access data, if file > 2GB
  • manages mapping from PageId to file+offset.
PostgreSQL PageId values are structured: 

typedef struct
{
    RelFileNode rnode;    // which relation/file
    ForkNumber  forkNum;  // which fork (of reln)
    BlockNumber blockNum; // which page/block 
} BufferTag;