Consider the bitmap representation of the free$-$space map, where for each block $$in the file, two bits are maintained in the bitmap. If the block is between $0$ and $30$ percent full the bits are $00,$ between $30$ and $60$ percent the bits are $01,$between $60$ and $90$ percent the bits are $10,$ and above $90$ percent the bits are $11.$Such bitmaps can be kept in memory even for quite large files.