These problems should be done on your own. They are not to be turned in. Getting help from AI (besides being ruled out by the course policies) will actually hurt you, since the point of these questions isn’t so you can deliver an answer somewhere and check a box; it’s for you to gain the practice and experience of working through the problems. You will need that general skill (of absorbing something by practicing it) in life, and in this semester you will need the specific problem-solving skills that are emphasized in these homeworks.
File systems: data structures
Is it fundamentally necessary to store on disk information about the unallocated disk sectors (the bitmap, etc.)? Explain why or why not.
File systems: fault-tolerance
Suppose you have a file system with: multi-level indexing (with 14 direct pointers, 1 indirect inode, 1 doubly indirect pointer, and 1 triple indirect pointer in the inode), directories, and inodes statically allocated in an array 0..MAX_INUM in a known location on disk. Also assume there is an on-disk bitmap of free inodes and free blocks. Assume that the file containing the root directory is stored in a well-known inode with inumber ROOT_INUM.
Assume each inode consumes exactly one block.
Consider creating a new file "/foo/bar" in an existing directory "foo" and writing one block of data to that file.
Assume no in-memory cache.
- List the reads and writes that must be executed to accomplish this task (and explain how each disk address is determined.)
- Write down the writes in the order they should occur.
- List the actions that the FSCK program must take to ensure the disk is brought to a correct state on recovery, and argue that these actions guarantee file system consistency.
- Suppose we want to ensure reliable updates using logging (as in a journaling file system). List the series of writes (to the disk and log) in the order they should occur. Also describe what actions (if any) must be taken on recovery after a crash.
Crash recovery: Copy-on-write
Consider a copy-on-write filesystem. Alice executes rm /home/alice/lab.c, where lab.c has 2 data blocks. Assume all directory entries are in direct blocks. List the operations performed by the filesystem when deleting lab.c, and specify any ordering constraints that must be obeyed between operations. Additionally, state any assumptions you make about how directory inodes are stored.
Crash recovery: Undo-redo logging
Consider the redo-undo logging protocol from class. Why can’t we eliminate the undo pass, and just have a redo pass in which we redo only sub-operations from committed transactions?
Crash recovery
Your friend wants to build a file system that tolerates crashes. Your friend proposes write-behind journaling. In this proposal, there is a journal, but the file system writes to the journal only after checkpointing (“checkpointing”, recall, means applying an operation to the on-disk data structures). Specifically, (1) the file system writes a TxnEnd record for a given transaction only after the TxnBegin record and all journal entries for the given transaction are written, and (2) the file system writes individual journal entries only after checkpointing the operation described by that entry. The recovery protocol looks for incomplete operations (those that are part of a transaction that lacks a TxnEnd record) and undoes those operations, similar to the way that recovery works for undo-only logging.
Assume that a crash can happen at any time. Does your friend’s proposal work? If so, argue that it is correct. If not, explain why not. Use no more than four sentences.