The Bitmap Inode tracks the used/free space in the real-time device using an old-style bitmap. One bit is allocated per real-time extent. The size of an extent is specified by the superblock's sb_rextsize value.

The number of blocks used by the bitmap inode is equal to the number of real-time extents (sb_rextents) divided by the block size (sb_blocksize) and bits per byte. This value is stored in sb_rbmblocks. The nblocks and extent array for the inode should match this.

xfs_ino_t        sb_rbmino;