Things have changed …

1. The design and implementation ofa log-structured file systemM. Rosenblum and J.K. OusterhoutProceedings of the 13th ACM Symposium onOperating Systems Principles, December 1991

2. Things have changed … • CPU speeds have increased • Memories have become larger (cheaper) • Disk capacity has increased, but … • - disk performance has not kept pace • - dominated by seek & rotational latency CS533 - Concepts of Operating Systems

3. Consequences … • Applications are disk-bound • File systems have large memory caches - most read requests hit in cache - so they never get to the disk - but all writes must eventually go to disk! • Disk traffic is mostly writes! - but data placement is optimized for reads! CS533 - Concepts of Operating Systems

4. Why the poor performance? • Data is updated in place - can’t just write where the disk head is • Meta-data is updated synchronously - Even if data and meta-data blocks are clustered, there is still some seeking CS533 - Concepts of Operating Systems

5. Seek overhead in FFS Creating a new file in FFS requires 5 disk I/Os: • 2 for file i-node • 1 for file data • 2 for directory i-node and data With small files, most of the time is spent in seeking CS533 - Concepts of Operating Systems

6. Log-structured file systems • Buffer a series of writes in memory • and write them asynchronously to disk • Entire buffer copied to disk • in a single write to a contiguous segment • Includes data and meta data • Allocate a new version instead of • updating the old one in place: - All info on disk is in a single sequential structure: the log CS533 - Concepts of Operating Systems

7. Challenges for Log-structured FS How to retrieve information from the log? How to make sure there are large extents of free space available for writing contiguous log segments? CS533 - Concepts of Operating Systems

8. File location and reading Basic data structures analogous to Unix FFS: • one inode per file: • contains attributes, address of first 10 blocks or indirect blocks But inodes are in the log, i.e. not at fixed locations on disk… So how do we find the right version? CS533 - Concepts of Operating Systems

9. File location and reading New data structure: inode map • Located in the log • Fixed checkpoint region on disk holds addresses of all map blocks • Indexed by file id gives location of file’s inode CS533 - Concepts of Operating Systems

10. Checkpoint regions • Contains - addresses of all blocks in inode map - segment usage table - current time - pointer to last segment written • Two of them, for safety • Located at fixed positions on disk • Used for crash recovery CS533 - Concepts of Operating Systems

11. Free space management - 1 GOAL: keep large extents of free space to write new data • Divide disk into fixed-length segments (512kB or 1MB) • Write segments sequentially until end of disk space • - older segments get fragmented meanwhile …and then? CS533 - Concepts of Operating Systems

12. Free space management - 2 Need to clean segments periodically Segment cleaning: • Read a number of segments into memory • Identify live data • Write live data only back to smaller number of clean segments CS533 - Concepts of Operating Systems

13. Read these segments Old log end Free segment Writing memory buffer Cleaner thread: copy segments to memory buffer Free space management CS533 - Concepts of Operating Systems

14. Old log end New log end Writing memory buffer Cleaner thread: identify live blocks Free space management CS533 - Concepts of Operating Systems

15. Old log end New log end Writing memory buffer Cleaner thread: queue compacted data for writing Free space management CS533 - Concepts of Operating Systems

16. Free space management Old log end New log end Writing memory buffer Writer thread: write compacted and new data to segments, then mark old segments as free CS533 - Concepts of Operating Systems

17. Implementation Segment summary block – identifies each piece of information in segment • E.g.: for a file, each data block identified by version number+inode number (=unique identifier, UID) and block number • Version number incremented in inode map when file deleted • If UID of block not equal to that in inode map when scanned, block is discarded CS533 - Concepts of Operating Systems

18. Cleaning policies Which segments to clean? How should live blocks be grouped when they are written out? CS533 - Concepts of Operating Systems

19. Free space management – cleaning policies Cleaning policies can be compared in terms of theWrite cost: N = number of segments read U = fraction of live data in read segments (0 u <1) • Average amount of time disk is busy per byte if new data written (seek and rot. latency negligible in LFS) • Note: includes cleaning overhead • Note dependence on u CS533 - Concepts of Operating Systems

20. Cleaning policies Low u = low write cost • Note: underutilized disk gives low write cost, but high storage cost! • …But u defined only for read segment (not overall) • Achieve bimodal distribution: keep most segments nearly full, but a few nearly empty (have cleaner work on these) CS533 - Concepts of Operating Systems

21. Achieving a bimodal distribution? • First attempt: cleaner always chooses lowest u segments and sorts by age before writing – FAILURE! • Free space in “cold” (i.e. more stable) segments is more “valuable” (will last longer) • Assumption: stability of segment proportional to age of youngest block (i.e. older = colder) • Replace greedy policy with Cost-benefit criterion • Clean segments with higher ratio • Still group by age before rewriting CS533 - Concepts of Operating Systems

22. Cost-benefit - Results • Left: bimodal distribution achieved - Cold cleaned at u=75%, hot at u=15% • Right: cost-benefit better, especially at utilization > 60% CS533 - Concepts of Operating Systems

23. Performance – small files • SunOS based on Unix FFS • NB: best case for SpriteLFS: no cleaning overhead • Sprite keeps disk 17% busy (85% for SunOS) and CPU saturated: will improve with CPU speed (right) CS533 - Concepts of Operating Systems

24. Performance – large files Single 100MB file • Traditional FS: logical locality – pay additional cost for organizing disk layout, assuming read patterns • LFS: temporal locality – group information created at the same time – not optimal for reading randomly written files CS533 - Concepts of Operating Systems

25. Performance – cleaning overhead • Statistics over several months of real usage • Previous results did not include cleaning • Write cost ranges 1.2-1.6 - more than half of cleaned segments empty • Cleaning overhead limits write performance: to ~70% of bandwidth • Improvement: cleaning could be performed at night or in idle periods CS533 - Concepts of Operating Systems

26. Conclusions • Prototype log-structured FS implemented/tested • Due to cleaning overhead, segment cleaning policies are crucial - tested in simulations before implementation • Results in tests (without cleaning overhead) • Higher performance than FFS in writes for both small and large files • Comparable read performance (except one case) • Results in real usage (with cleaning) • Simulation results confirmed • 70% of bandwidth can be used for writing CS533 - Concepts of Operating Systems

27. References • M. Rosenblum and J.Ousterhout, “The design and implementation of a log-structured file system”, Proceedings of the 13th ACM Symposium on Operating Systems Principles, December 1991 • Marshall K. McKusick, William N. Joy, Samuel J. Leffler, and Robert S. Fabry, “A Fast File System for Unix”, ACM Transactions on Computer Systems, 2(3), August 1984, pp. 181-197 • A. Tanenbaum “Modern operating systems” 2nd ed. (Chpt.4 “File systems”), Prentice Hall CS533 - Concepts of Operating Systems