The Design and Implementation of a Log-Structured File System

Notes by Thorsten von Eicken, Feb. 1998

Goals

• Improve file system performance over FFS. Also improve recovery time.
• In particular, improve the performance of small writes, including the updating of file system meta data.
• Maintain standard Unix filesystem semantics

Key insights

• write updates to the disk in the form of a sequential log
• divide the disk into large segments, write segments from start to end, use cleaner process to turn partially full segments into full and empty ones
• use the same on-disk data structures as for normal file systems, but add a level of indirection to allow inode blocks to be placed at the end of the log instead of at a fixed location on disk

Results

• LFS outperforms FFS in every respect, except for the case of random writes followed by sequential reads.
• LFS recovers basically instantaneously
• cleaning process is non-obvious and may introduce some unpredictability into the FS performance