Operating Systems

1. Operating Systems Operating Systems Session 9: Disk Performance Optimization File Systems

2. Introduction • Secondary storage is common bottleneck • Improvements in secondary storage performance significantly boost overall system performance • Physical aspects • moving head disk storage • access optimization • Logical aspects • file system organization COP 5614 - Operating Systems

3. Characteristics of Moving-Head Disk Storage • Physical layout of disk drives • Set of magnetic platters • Rotate on spindle • Made up of tracks, which in turn contain sectors • Vertical sets of tracks form cylinders COP 5614 - Operating Systems

4. Moving-Head Disk Storage COP 5614 - Operating Systems

5. Disk Surface: traditional COP 5614 - Operating Systems

6. Characteristics of Moving-Head Disk Storage • Performance measurements • Rotational latency • Time for data to rotate from current position to read-write head • Seek time • Time for read-write head to move to new cylinder • Transmission time • Time for all desired data to spin by read-write head COP 5614 - Operating Systems

7. Components of disk access COP 5614 - Operating Systems

8. Disk Speed Examples COP 5614 - Operating Systems

9. Disk Scheduling Evaluation Criteria • Throughput • Number of requests serviced per unit of time • Mean response time • Average time waiting for request to be serviced • Variance of response times • Measure of the predictability of response times • also: starvation COP 5614 - Operating Systems

10. Scenario: Disk request pattern COP 5614 - Operating Systems

11. Seek Optimization Strategies COP 5614 - Operating Systems

12. First-Come-First-Served (FCFS) • Pro: fair, no starvation, low overhead • Cons: Potential for extremely low throughput COP 5614 - Operating Systems

13. Shortest-Seek-Time-First (SSTF) • Pro: high throughput, lower response times • Cons: high variance, possible starvation COP 5614 - Operating Systems

14. SCAN Disk Scheduling • Pro: improved variance in response times • Cons: possible starvation, middle tracks preferred COP 5614 - Operating Systems

15. C-SCAN Disk Scheduling • Pro: improved variance in response times • Cons: possible starvation COP 5614 - Operating Systems

16. FSCAN and N-Step SCAN • Groups requests into batches • FSCAN: “freeze” the disk request queue periodically, service only those requests in the queue at that time • N-Step SCAN: Service only the first n requests in the queue at a time • Pros: prevent indefinite postponement, reduce variance of response times COP 5614 - Operating Systems

17. LOOK and C-LOOK • LOOK: Improvement on SCAN scheduling • Does not move the disk arm to the outer edges of the disk unless requests for those regions are pending • Pro: more efficient (less head movement) • C-LOOK improves C-SCAN scheduling • Combination of LOOK and C-SCAN • Pro: Lower variance of response times at the expense of throughput COP 5614 - Operating Systems

18. Rotational Optimization Strategies • SLTF • Shortest-latency-time-first • On a given cylinder, service request with shortest rotational latency first • SPTF • Shortest-positioning-time-first • positioning time: seek time plus rotational latency • SATF • Shortest-access-time-first • Access time: positioning time plus transmission time COP 5614 - Operating Systems

19. SLTF Scheduling requests are serviced in the indicated order regardless of the order in which they arrived COP 5614 - Operating Systems

20. SPTF Scheduling Order: B then A • Pro: good performance • Cons: possible starvation COP 5614 - Operating Systems

21. SATF Scheduling Order: B then A • Pro: good performance • Cons: possible starvation COP 5614 - Operating Systems

22. Another example: SPTF Scheduling Order: B then A COP 5614 - Operating Systems

23. SATF Scheduling: different order Order: A then B COP 5614 - Operating Systems

24. Optimization Strategies Weakness • require knowledge of • disk performance characteristics • physical disk geometry • which might not be readily available due to error-correcting data and transparent reassignment of bad sectors COP 5614 - Operating Systems

25. Other Disk Performance Techniques • Multiple copies of frequently-accessed data • Access the copy that is closest to read-write head • Can incur significant storage overhead • Record blocking • Read/write multiple records as single block of data • Disk arm anticipation • move disk arm to location of next access • If the disk arm incorrectly predicts future disk accesses, performance can significantly degrade COP 5614 - Operating Systems

26. Caching and Buffering • Store copy of disk data in faster memory • Location: • main memory • disk controller • onboard disk • Read access: • Vastly faster access times than access to disk • Depends on cache-hit ratio COP 5614 - Operating Systems

27. Caching and Buffering • Write access: • cache acts as buffer to delay writing of data until disk is under light load • caching model: • Write-back caching • Data not written to disk immediately • Flushed periodically • Write-through caching • Writes to disk and cache simultaneously • Reduces performance compared to write-back, but guarantees consistency COP 5614 - Operating Systems

28. RAID: Redundant Arrays of Independent Disks • Data stored in strips • Spread across set of disks • Strips form stripes • Set of strips at same location on each disk • Fine-grained strip • Yields high transfer rates • (many disks service request at once) • Array can only process one request at once • Coarse-grained strips • Might fit an entire file on one disk • Allow multiple requests to be filled at once COP 5614 - Operating Systems

29. RAID strips and stripes Strips and stripe created from a single file in RAID systems COP 5614 - Operating Systems

30. RAID drawbacks • disks have MTTF (mean time to failure) • more disks decreases MTBF (between) • RAID allows data duplication feature • mirroring • strip-based ECC COP 5614 - Operating Systems

31. RAID levels COP 5614 - Operating Systems

32. File System Organizes files and manages access to data • Responsible for file management, auxiliary storage management, file integrity mechanisms and access methods • Primarily concerned with managing disk storage space COP 5614 - Operating Systems

33. File Systems characteristics • device independence • use file names rather than physical device ids • backup and recovery capabilities • to prevent either accidental loss or malicious destruction of information • encryption and decryption capabilities • to make information useful only to its intended audience COP 5614 - Operating Systems

34. Basic File System Concept: Directory • contains names and locations of other files: • to organize and quickly locate files • entry stores information such as: • File name • Location • Size • Type • Accessed • Modified and creation times COP 5614 - Operating Systems

35. Common: Hierarchical File System COP 5614 - Operating Systems

36. File System Concept: Link • Directory entry that references a data file or directory located in a different directory • Facilitates data sharing and can make it easier for users to access files located throughout a file system’s directory structure • Soft link: • entry containing the pathname for another file • Hard link • entry that specifies the location of the file (typically a block number) on the storage device • needs updating when file moves COP 5614 - Operating Systems

37. File System Concept: Metadata • Information that protects integrity of file system • Cannot be modified directly by users • example: file systems superblock • stores critical information that protects the integrity of the file system: • The file system identifier • The location of the storage device’s free blocks • storage device holds redundant copies of superblock COP 5614 - Operating Systems

38. File System Concept: File Descriptor • file open operation returns a file descriptor • index into open-file table • all file access through the file descriptor • open-file table contains file control block: • file attributes: • symbolic name • location in secondary storage • access control data • etc. COP 5614 - Operating Systems

39. File System Concept: Mounting • Combines multiple file systems into one • Mount point: • directory where the mounted file system is inserted • Mount table: • information about the location of mount points and the devices to which they point • When the native file system encounters a mount point, it uses the mount table to determine the device and type of the mounted file system COP 5614 - Operating Systems

40. Mounting Links: only soft links across mounted file systems COP 5614 - Operating Systems

41. File Allocation • Problem similar to memory organization • contiguous allocation schemes have generally been replaced by more dynamic noncontiguous allocation • Files tend to grow or shrink over time • Users rarely know in advance how large their files will be COP 5614 - Operating Systems

42. Contiguous File Allocation • Place file data at contiguous addresses on disk • Advantages • Successive logical records typically are physically adjacent to one another • Disadvantages • External fragmentation • Poor performance can result if files grow and shrink over time • If a file grows beyond the size originally specified and no contiguous free blocks are available, it must be transferred to a new area of adequate size, leading to additional I/O operations. COP 5614 - Operating Systems

43. Non-Contiguous File Allocation • linked-list • tabular • indexed COP 5614 - Operating Systems

44. Linked-List Noncontiguous File Allocation block based: • data • pointer to next block COP 5614 - Operating Systems

45. Linked-List Noncontiguous File Allocation • Locating a record • linked list must be searched from the beginning • if blocks are dispersed throughout the storage device (which is normal), the search process can be slow as block-to-block seeks occur • Insertion and deletion are done by modifying the pointer in the previous block COP 5614 - Operating Systems

46. Linked-List Noncontiguous File Allocation • Large block sizes • Can result in significant internal fragmentation • Small block sizes • May cause file data to be spread across multiple blocks dispersed throughout the storage device • Poor performance as the storage device performs many seeks to access all the records of a file COP 5614 - Operating Systems

47. Tabular Noncontiguous File Allocation • directory entry lists first block of file • table cell stores link to next block • empty table cell marks end of file ex.: FAT file system COP 5614 - Operating Systems

48. Tabular Noncontiguous File Allocation • Pros: • reduces the number of lengthy seeks required to access a particular record • table can be cached so that the chain of blocks that compose a file can be traversed quickly • Cons: • cell entry size/block size dictates file system size • table can grow too large to cache COP 5614 - Operating Systems

49. Indexed Noncontiguous File Allocation • directory entry lists index block(s) • index blocks lists data blocks • index block reserves last few entries for more index blocks COP 5614 - Operating Systems

50. Indexed Noncontiguous File Allocation • Pros: • Searching takes place in the index blocks • File systems typically place index blocks near the data blocks they reference, so the data blocks can be accessed quickly after their index block is loaded COP 5614 - Operating Systems