The design and implementation of a log-structured file system The design and implementation of a log-structured file system M. Rosenblum and J.K. Ousterhout Proceedings of the 13th ACM Symposium on Operating Systems Principles, December 1991

CS533 - Concepts of Operating Systems 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 the log structure: the log

CS533 - Concepts of Operating Systems 7 Challenges for Log-structured FS 1)How to retrieve information from the log? 2)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: inode  one inode per file: indirect blocks  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 inode map 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 Free space management Old log end Read these segments Writing memory buffer Cleaner thread: copy segments to memory buffer Free segment

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

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

CS533 - Concepts of Operating Systems 16 Free space management Old log endNew 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 1)Which segments to clean? 2)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 the Write 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 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) Low u = low write cost

CS533 - Concepts of Operating Systems 21 Achieving a bimodal distribution? First attempt: cleaner always chooses lowest u segments and sorts FAILURE! 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 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 Single 100MB file

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 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 A. Tanenbaum “Modern operating systems” 2 nd ed. (Chpt.4 “File systems”), Prentice Hall