FFS, LFS, and RAID

1. FFS, LFS, and RAID Andy Wang COP 5611 Advanced Operating Systems

2. UNIX Fast File System • Designed to improve performance of UNIX file I/O • Two major areas of performance improvement • Bigger block sizes • Better on-disk layout for files

3. Block Size Improvement • Quadrupling of block size quadrupled amount of data gotten per disk fetch • But could lead to fragmentation problems • So fragments introduced • Small files stored in fragments • Fragments addressable (but not independently fetchable)

4. Disk Layout Improvements • Aimed toward avoiding disk seeks • Bad if finding related files takes many seeks • Very bad if find all the blocks of a single file requires seeks • Spatial locality: keep related things close together on disk

5. Cylinder Groups • A cylinder group: a set of consecutive disk cylinders in the FFS • Files in the same directory stored in the same cylinder group • Within a cylinder group, tries to keep things contiguous • But must not let a cylinder group fill up

6. Locations for New Directories • Put new directory in relatively empty cylinder group • What is “empty”? • Many free i_nodes • Few directories already there

7. The Importance of Free Space • FFS must not run too close to capacity • No room for new files • Layout policies ineffective when too few free blocks • Typically, FFS needs 10% of the total blocks free to perform well

8. Performance of FFS • 4 to 15 times the bandwidth of old UNIX file system • Depending on size of disk blocks • Performance on original file system • Limited by CPU speed • Due to memory-to-memory buffer copies

9. FFS Not the Ultimate Solution • Based on technology of the early 80s • And file usage patterns of those times • In modern systems, FFS achieves only ~5% of raw disk bandwidth

10. The Log-Structured File System • Good, large buffer caches can catch almost all reads • But most writes have to go to disk • So file system performance can be limited by writes • So, produce a FS that writes quickly • Like an append-only log

11. Basic LFS Architecture • Buffer writes, send them sequentially to disk • Data blocks • Attributes • Directories • And almost everything else • Converts small sync writes to large async writes

12. A Simple Log Disk Structure File A Block 7 File Z Block 1 File M Block 202 File A Block 3 File F Block 1 File A Block 7 File L Block 26 File L Block 25 Head of Log

13. Key Issues in Log-Based Architecture 1. Retrieving information from the log No matter how well you cache, sooner or later you have to read 2. Managing free space on the disk You need contiguous space to write - in the long run, how do you get more?

14. Finding Data in the Log File A Block 7 File Z Block 1 File M Block 202 File A Block 3 File F Block 1 File A Block 7 File L Block 26 File L Block 25 Give me block 25 of file L Or, Give me block 1 of file F

15. Retrieving Information From the Log • Must avoid sequential scans of disk to read files • Solution - store index structures in log • Index is essentially the most recent version of the i_node

16. Finding Data in the Log Foo Block 1 Foo Block2 Foo Block3 Foo Block1 (old) How do you find all blocks of file Foo?

17. Finding Data in the Log with an I_node Foo Block 1 Foo Block2 Foo Block3 Foo Block1 (old)

18. How Do You Find a File’s I_node? • You could search sequentially • LFS optimizes by writing i_node maps to the log • The i_node map points to the most recent version of each i_node • A file system’s i_nodes cover multiple blocks of i_node map

19. How Do You Find the Inode? The Inode Map

20. How Do You Find Inode Maps? • Use a fixed region on the disk that always points to the most recent i_node map blocks • But cache i_node maps in main memory • Small enough that few disk accesses required to find i_node maps

21. Finding I_node Maps New i_node maps An old i_node map

22. Reclaiming Space in the Log • Eventually, the log reaches the end of the disk partition • So LFS must reuse disk space like superseded data blocks • Space can be reclaimed in background or when needed • Goal is to maintain large free extents on disk

23. Example of Need for Reuse Head of log New data to be logged

24. Major Alternatives for Reusing Log • Threading + Fast - Fragmentation - Slower reads Head of log New data to be logged

25. Major Alternatives for Reusing Log • Copying +Simple +Avoids fragmentation -Expensive New data to be logged

26. LFS Space Reclamation Strategy • Combination of copying and threading • Copy to free large fixed-size segments • Thread free segments together • Try to collect long-lived data permanently into segments

27. A Threaded, Segmented Log Head of log

28. Cleaning a Segment 1. Read several segments into memory 2. Identify the live blocks 3. Write live data back (hopefully) into a smaller number of segments

29. Identifying Live Blocks • Clearly not feasible to track down live blocks of all files • Instead, each segment maintains a segment summary block • Identifying what is in each block • Crosscheck blocks with owning i_node’s block pointers • Written at end of log write, for low overhead

30. Segment Cleaning Policies • What are some important questions? • When do you clean segments? • How many segments to clean? • Which segments to clean? • How to group blocks in their new segments?

31. When to Clean • Periodically • Continuously • During off-hours • When disk is nearly full • On-demand • LFS uses a threshold system

32. How Many Segments to Clean • The more cleaned at once, the better the reorganization of the disk • But the higher the cost of cleaning • LFS cleans a few tens at a time • Till disk drops below threshold value • Empirically, LFS not very sensitive to this factor

33. Which Segments to Clean? • Cleaning segments with lots of dead data gives great benefit • Some segments are hot, some segments are cold • But “cold” free space is more valuable than “hot” free space • Since cold blocks tend to stay cold

34. Cost-Benefit Analysis • u = utilization • A = age • Benefit to cost = u*A/(u + 1) • Clean cold segments with some space, hot segments with a lot of space

35. What to Put Where? • Given a set of live blocks and some cleaned segments, which goes where? • Order blocks by age • Write them to segments oldest first • Goal is very cold, highly utilized segments

36. number of segments number of segments empty 100% full empty 100% full Goal of LFS Cleaning

37. Performance of LFS • On modified Andrew benchmark, 20% faster than FFS • LFS can create and delete 8 times as many files per second as FFS • LFS can read 1 ½ times as many small files • LFS slower than FFS at sequential reads of randomly written files

38. Logical Locality vs. Temporal Locality • Logical locality (spatial locality): Normal file systems keep a file’s data blocks close together • Temporal locality: LFS keeps data written at the same time close together • When temporal locality = logical locality • Systems perform the same

39. Major Innovations of LFS • Abstraction: everything is a log • Temporal locality • Use of caching to shape disk access patterns • Cache most reads • Optimized writes • Separating full and empty segments

40. Where Did LFS Look For Performance Improvements? • Minimized disk access • Only write when segments filled up • Increased size of data transfers • Write whole segments at a time • Improving locality • Assuming temporal locality, a file’s blocks are all adjacent on disk • And temporally related files are nearby

41. Parallel Disk Access and RAID • One disk can only deliver data at its maximum rate • So to get more data faster, get it from multiple disks simultaneously • Saving on rotational latency and seek time

42. Utilizing Disk Access Parallelism • Some parallelism available just from having several disks • But not much • Instead of satisfying each access from one disk, use multiple disks for each access • Store part of each data block on several disks

43. Disk Parallelism Example open(foo) read(bar) write(zoo) File System

44. Data Striping • Transparently distributing data over multiple disks • Benefits – • Increases disk parallelism • Faster response for big requests • Major parameters are number of disks and size of data interleaf

45. Fine-Grained Vs. Coarse-Grained Data Interleaving • Fine grain data interleaving • High data rate for all requests • But only one request per disk array • Lots of time spent positioning • Coarse-grain data interleaving • Large requests access many disks • Many small requests handled at once • Small I/O requests access few disks

46. Reliability of Disk Arrays • Without disk arrays, failure of one disk among N loses 1/Nth of the data • With disk arrays (fine grained across all N disks), failure of one disk loses all data • N disks 1/Nth as reliable as one disk

47. Adding Reliability to Disk Arrays • Buy more reliable disks • Build redundancy into the disk array • Multiple levels of disk array redundancy possible • Most organizations can prevent any data loss from single disk failure

48. Basic Reliability Mechanisms • Duplicate data • Parity for error detection • Error Correcting Code for detection and correction

49. Parity Methods • Can use parity to detect multiple errors • But typically used to detect single error • If hardware errors are self-identifying, parity can also correct errors • When data is written, parity must be written, too

50. Error-Correcting Code • Based on Hamming codes, mostly • Not only detect error, but identify which bit is wrong