LC

Implement Copy-on-Write Filesystem for Snapshots

hard9/10

Implement Block Cache for Filesystem Performance Understand fsync, I/O Barriers, and Disk Write Ordering

Language

Compiler

2.0s Limit 256MB

Implementation

Implement simple CoW filesystem.

Requirements:

Track blocks with reference counts
On write: allocate new block, copy data, update metadata
On delete: decrement ref count, free if zero
Implement snapshot: copy metadata, share blocks
Track space savings
Show block allocation and sharing
Demonstrate CoW benefits

Test:

Create file with data
Create snapshot (no data copy)
Modify original (only changed blocks copied)
Show reference counts
Show space savings

Why This Matters

Btrfs, ZFS, and Apple's APFS all use exactly this copy-on-write-plus-reference-counting design to make snapshots instantaneous and clones nearly free, which is why `zfs snapshot` and Time Machine local snapshots return immediately even on multi-terabyte volumes. Forgetting to check the reference count before an in-place write is exactly the bug class that would corrupt a snapshot that's supposed to be immutable.

Hints

0 / 3 revealed

Validation Suite

main.c

Workspace 1.0