Summary

LFS introduces a new approach to updating the disk. Instead of overwriting files in places, LFS always writes to an unused portion of the disk, and then later reclaims that old space through cleaning. This approach, which in database systems is called shadow paging“Physical Integrity in a Large Segmented Database” by R. Lorie. ACM Transactions on Databases, Volume 2:1, 1977. The original idea of shadow paging is presented here. and in file-system-speak is sometimes called copy-on-write, enables highly efficient writing, as LFS can gather all updates into an in-memory segment and then write them out together sequentially.

The large writes that LFS generates are excellent for performance on many different devices. On hard drives, large writes ensure that positioning time is minimized. On parity-based RAIDs, such as RAID-4 and RAID-5, they avoid the ...