Figure 13.9 shows an inode data structure. Suppose a file system that uses this structure has filled up the blocks stemming from the doubly indirect pointers.How many disk accesses will it take to write one more byte to the file? Assume that the inode and free block bitmap are both completely in memory, but there is no buffer cache.Also assume that blocks do not have to be initialized.

Question

What will be an ideal response?

Answer 1

This would take four writes and no reads for a total of three disk accesses. Allocating the four necessary blocks would be free because the free block bitmap is in the main memory. Connecting the triply indirect pointer of the inode to the block that contains doubly indirect pointers does not require any disk accesses, because the inode is already in main memory. Connecting the block that contains the doubly indirect pointer to the block that contains the indirect pointers takes one disk access. Connecting the block that contains the indirect pointers to the block that contains the data block pointers requires the second disk access. Connecting the data block pointers to the actual data block requires the third disk access. Finally, writing the one byte to the data block requires the fourth disk access.