Why must the bit map for file allocation be kept on mass sto

Why must the bit map for file allocation be kept on mass storage, rather than in main memory?

Solution

In case of system crashes or memory fails, the free-space list would not be lost as it would be if the bit map had been stored in main memory. If the system crashes, causing a memory failure, storing the free-space bit map on disk prevents it from getting lost. Also, for a very large disk, e.g. an 80 GB disk with 4 KB blocks, the 20 Mbit free block bit map might require too much memory, so that is the reasion why we are kepting bit map file allocation on mass storage