"File Systems" — Practice

Question 1

A block is 8 KB and a pointer is 8 bytes. How many pointers fit in one block?

Accepted Answer

8192 divided by 8, which is 1024 pointers.

Question 2

Which allocation method allows a file to be read at any offset with no extra disk access, and what does it cost?

Accepted Answer

Contiguous allocation, since the block address is computed arithmetically from the start block. It costs external fragmentation and the inability to grow a file beyond its neighbours.

Question 3

After deleting the only hard link to a file that also has a symbolic link, what happens to the data and to the symbolic link?

Accepted Answer

The link count drops to zero, so the inode and data blocks are freed. The symbolic link survives as a file but now dangles, and opening it fails.

Question 4

An inode has 12 direct, one single indirect and one double indirect pointer, with 4 KB blocks and 4-byte pointers. Compute the maximum file size.

Accepted Answer

Pointers per block is 1024. Direct gives 48 KB, single indirect gives 1024 times 4 KB, which is 4 MB, and double indirect gives 1024 squared times 4 KB, which is 4 GB. The total is 4 GB plus 4 MB plus 48 KB.

Question 5

With 4 KB blocks, 4-byte pointers and 12 direct pointers, how many disk accesses fetch the data at byte offset 5,000,000, given the inode is in memory?

Accepted Answer

The block index is 5,000,000 divided by 4096, which is 1220. Direct covers 0 to 11 and single indirect covers 12 to 1035, so 1220 lies in the double indirect range. That needs three accesses: the double indirect block, the second-level block, and the data block.

Question 6

Compute the free-space bit vector size for a 512 GB disk with 8 KB blocks.

Accepted Answer

The block count is 512 times 2 to the 30 divided by 8192, which is 2 to the 26, about 67 million blocks. One bit each gives 2 to the 26 bits, which is 2 to the 23 bytes, or 8 MB.

Question 7

How many disk accesses resolve /a/b/c/d from a cold cache, with each directory in one block?

Accepted Answer

Three directory components a, b and c cost two accesses each, an inode and a data block, giving six, plus one access for d's own inode, for a total of seven.

Question 8

A 200-block file must have one block inserted in the middle. Compare the cost under contiguous, linked and indexed allocation.

Accepted Answer

Contiguous must shift about 100 blocks, costing roughly 100 reads and 101 writes. Linked needs about 100 reads to reach the position, then two writes to splice, with no other block moving. Indexed needs one data block write plus one index block write after shifting entries in memory.

Question 9

Explain why fork lets two processes share a file offset but two separate opens of the same file do not.

Accepted Answer

Fork duplicates the per-process descriptor table, and both descriptors point at the same system-wide open-file table entry, which is where the offset lives. A separate open creates a new system-wide entry with its own offset, even though both refer to the same inode.

Question 10

Why does a crash between writing an inode and writing its directory entry corrupt the file system, and how does journaling fix it?

Accepted Answer

The inode is allocated but unreachable, so its blocks leak, or conversely a directory entry points at an unallocated inode. Journaling writes both intentions to a log first, and the write-ahead rule guarantees the log reaches disk before the changes, so recovery replays or discards the whole operation atomically.

Practice — "File Systems"

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