1. 1 CENG334 Introduction to Operating Systems Erol Sahin Dept of Computer Eng. Middle East Technical University Ankara, TURKEY URL: http://kovan.ceng.metu.edu.tr/ceng334 Disks and Filesystems Topics: Disks

2. 2 Today: Disks and Filesystems Physical operation of modern disk drives Operating system access to raw disk and disk I/O scheduling Overview of filesystem design Next lecture: Detailed look at filesystem implementation Adapted from Matt Welsh’s (Harvard University) slides.

3. 3 A Disk Primer Disks consist of one or more platters divided into tracks Each platter may have one or two heads that perform read/write operations Each track consists of multiple sectors The set of sectors across all platters is a cylinder Aperture Platter Heads Track Sector Adapted from Matt Welsh’s (Harvard University) slides.

4. 4 Hard Disk Evolution IBM 305 RAMAC (1956) First commercially produced hard drive 5 Mbyte capacity, 50 platters each 24” in diameter! Adapted from Matt Welsh’s (Harvard University) slides.

5. 5 Hard Disk Evolution Adapted from Matt Welsh’s (Harvard University) slides.

6. 6 Disk access time-1 Command overhead: Time to issue I/O, get the HDD to start responding, select appropriate head Seek time: Time to move disk arm to the appropriate track Depends on how fast you can physically move the disk arm These times are not improving rapidly Settle time: Time for head position to stabilize on the selected track Adapted from Matt Welsh’s (Harvard University) slides.

7. 7 Disk access time-2 Rotational latency: Time for the appropriate sector to move under the disk arm Depends on the rotation speed of the disk (e.g., 7200 RPM) Transfer time Time to transfer a sector to/from the disk controller Depends on density of bits on disk and RPM of disk rotation Faster for tracks near the outer edge of the disk – why? Modern drives have more sectors on the outer tracks! Adapted from Matt Welsh’s (Harvard University) slides.

8. 8 Example disk characteristics Seagate Barracuda 7200.10 320GB Hard Drive Capacity (GB): 320 Interface: Serial ATA-300 Spindle Speed (RPM): 7200 Buffer Memory: 16MB Average Latency (msec): 4.16 Maximum External Transfer Rate (Mbits/sec): 300 Data Transfer Rate on Serial ATA: Up to 3000 Mb/sec Logical Cylinders/Heads/Sectors per Track: 16,383/16/63 Bytes Per Sector: 512 form factor: 3.5” Disk interface speeds SCSI: From 5 MB/sec to 320 MB/sec ATA: from 33 MB/sec to 100 MB/sec Serial ATA (single wire): Starting at 150 MB/sec Firewire: 50 MB/sec Adapted from Matt Welsh’s (Harvard University) slides.

9. 9 Disk I/O Scheduling Given multiple outstanding I/O requests, what order to issue them? Why does it matter? Major goals of disk scheduling: Adapted from Matt Welsh’s (Harvard University) slides.

10. 10 Disk I/O Scheduling Given multiple outstanding I/O requests, what order to issue them? Why does it matter? Major goals of disk scheduling: 1) Minimize latency for small transfers Primarily: Avoid long seeks by ordering accesses according to disk head locality 2) Maximize throughput for large transfers Large databases and scientific workloads often involve enormous files and datasets Note that disk block layout also has a large impact on performance Where we place file blocks, directories, file system metadata, etc. Adapted from Matt Welsh’s (Harvard University) slides.

11. 11 Disk I/O Scheduling Given multiple outstanding I/O requests, what order to issue them? FIFO: Just schedule each I/O in the order it arrives What's wrong with this? Adapted from Matt Welsh’s (Harvard University) slides.

12. 12 Disk I/O Scheduling Given multiple outstanding I/O requests, what order to issue them? FIFO: Just schedule each I/O in the order it arrives What's wrong with this? Potentially lots of seek time! SSTF: Shortest seek time first Issue I/O with the nearest cylinder to the current one Why might this not work so well??? Adapted from Matt Welsh’s (Harvard University) slides.

13. 13 Disk I/O Scheduling Given multiple outstanding I/O requests, what order to issue them? FIFO: Just schedule each I/O in the order it arrives What's wrong with this? Potentially lots of seek time! SSTF: Shortest seek time first Issue I/O with the nearest cylinder to the current one Favors middle tracks: Head rarely moves to edges of disk SCAN (or Elevator) Algorithm: Head has a current direction and current cylinder Sort I/Os according to the track # in the current direction of the head If no more I/Os in the current direction, reverse direction CSCAN Algorithm: Always move in one direction, “wrap around” to beginning of disk when moving off the end Idea: Reduce variance in seek times, avoid discriminating against the highest and lowest tracks Adapted from Matt Welsh’s (Harvard University) slides.

14. 14 SCAN example Current track Direction Adapted from Matt Welsh’s (Harvard University) slides.

15. 15 SCAN example Current track Direction Adapted from Matt Welsh’s (Harvard University) slides.

16. 16 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

17. 17 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

18. 18 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

19. 19 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

20. 20 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

21. 21 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

22. 22 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

23. 23 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

24. 24 SCAN example Direction Current track Adapted from Matt Welsh’s (Harvard University) slides.

25. 25 SCAN example Direction Current track What is the overhead of the SCAN algorithm? Count the total amount of seek time to service all I/O requests In this case, 12 tracks in --> direction 15 tracks for long seek back 5 tracks in <-- direction Total: 12+15+5 = 32 tracks Adapted from Matt Welsh’s (Harvard University) slides.

26. 26 ATA and IDE Interfaces IDE stands for Integrated (or “Intelligent”) Drive Electronics Same as “ATA” (Advanced Technology Attachment) Standard interface to hard drives that integrate a drive controller in the drive itself 1 or 2 drives on a chain Enhanced IDE (EIDE) and ATA-2 Faster version of ATA/IDE that supports Direct Memory Access (DMA) transfers Ultra ATA: Speed enhancements to ATA standard Versions running at 33, 66, and 100 Mbytes/sec Serial ATA: Emerging standard using a serial (not parallel) interface Speeds starting at 150 Mbyte/sec Can drive longer cables at much higher clock speeds than parallel cable Serial ATA Rounded parallel ATA Parallel ATA

27. 27 SCSI Interface Standard hardware interface to wide range of I/O devices Disks, CDs, DVDs, tapes, etc. Bus-based design: single shared set of I/O lines that all devices connect to Access model using logical blocks on disk On-disk controller maps logical block # to sector/track/head combination SCSI-1: 8-bit bus, 5 Mhz = 5 Mbytes/sec max speed Supported up to 8 devices on a single bus Lots of problems with termination: required physical connector on end of cable to avoid signal refraction! SCSI-2: The next generation Fast SCSI: 10 Mhz clock speed Wide SCSI: 16 bit bus width Fast wide SCSI: 10 Mhz + 16 bit bus = 20 MB/sec throughput SCSI-3: Ramping up on speed and bus width Highest speed now is “Ultra320 SCSI”: 160 Mhz x 16 bits = 320 MB/sec max speed

28. 28 Relative Interconnect Speeds (from macspeedzone.com) USB 3.0 standard established in 2007 supports speeds upto 5GBps

29. 29 CENG334 Introduction to Operating Systems Erol Sahin Dept of Computer Eng. Middle East Technical University Ankara, TURKEY Filesystems and their interface Topics:

30. 30 Filesystems A filesystem provides a high-level application access to disk As well as CD, DVD, tape, floppy, etc... Masks the details of low-level sector-based I/O operations Provides structured access to data (files and directories) Caches recently-accessed data in memory Adapted from Matt Welsh’s (Harvard University) slides.

31. 31 Filesystems Essential requirements for long term storage: It must be possible to store a very large amount of information. The information must survive the termination of the process using it. Multiple processes must be able to access the information concurrently. Think of a disk as a linear sequence of fixed-size blocks and supporting reading and writing of blocks. Questions that quickly arise: How do you find information? How do you keep one user from reading another’s data? How do you know which blocks are free? Slide adapted from: Tanenbaum, Modern Operating Systems 3 e, (c) 2008 Prentice-Hall, Inc. All rights reserved. 0-13-6006639

32. 32 Filesystem Operations Filesystems provide a standard interface to files and directories: Create a file or directory Delete a file or directory Open a file or directory – allows subsequent access Read, write, append to file contents Add or remove directory entries Close a file or directory – terminates access What other features do filesystems provide? Accounting and quotas – prevent your classmates from hogging the disks Backup – some filesystems have a “$HOME/.backup” containing automatic snapshots Indexing and search capabilities File versioning Encryption Automatic compression of infrequently-used files Adapted from Matt Welsh’s (Harvard University) slides.

33. 33 File Concept File is a logical storage unit abstraction provided by the operating system. Files are mapped by the operating system onto physical devices (disks, tapes, CDs, etc..) From the user point of view, file is the only unit through which data can be written onto storage devices. The information in a file as well as the attributes of the file is determined by its creator. Data Numeric/character/binary Program When a file is created, it becomes independent of the process, the user and even the system that created it.

34. 34 File Operations OS provides a number of minimal operations on a file. Create Allocate space and then make an entry in the directory Write Requires name of the file, and the information to be written Search the directory to find file’s location. Keep a write-pointer to the location in the file Update the pointer after each write Read Requires name of the file, and the information to be read Search the directory to find file’s location. Keep a read-pointer to the location in the file Update the pointer after each read Reposition within file Change the value of the file-position pointer Delete Deallocate the space and remove the entry Truncate Change the allocated space to zero, and deallocate its space File-position pointer

35. 35 File Attributes Name – only information kept in human-readable form Identifier – unique tag (number) identifies file within file system Type – needed for systems that support different types Location – pointer to file location on device Size – current file size Protection – controls who can do reading, writing, executing Time, date, and user identification – data for protection, security, and usage monitoring Information about files are kept in the directory structure, which is maintained on the disk

36. 36 File Types – Name, Extension The file type provides information on what can be done with that file to the OS. Typically implemented as the extension of the filename. In UNIX systems, a crude “magic number” is stored at the beginning of some files to indicate the type of the file (executable/shell script..) In Mac OS X, each file has a type TEXT/APPL. Each file also has a creator attribute that is set to the program that created it.

37. 37 File Structure None - sequence of words, bytes This is the structure supported by UNIX systems Simple record structure Lines Fixed length Variable length Complex Structures Formatted document Relocatable load file Can simulate last two with first method by inserting appropriate control characters Who decides: Operating system Program

38. 38 Access Methods Sequential Access read next write next reset no read after last write (rewrite) Direct Access (available when the file is made up of fixed-length logical records, useful in databases) read n write n position to n read next write next rewrite n n = relative block number

39. 39 Open Files Most file operations require searching the directory for the entry associated with the file. To avoid this constant search, most systems require that file be “open”ed, before its use. Open(F i ) – search the directory structure on disk for entry F i, and move the content of entry to memory Close (F i ) – move the content of entry F i in memory to directory structure on disk Several pieces of data are needed to manage open files: File pointer: pointer to last read/write location, per process that has the file open File-open count: counter of number of times a file is open – to allow removal of data from open-file table when last processes closes it Disk location of the file: cache of data access information Access rights: per-process access mode information

40. 40 Open File Locking Provided by some operating systems and file systems Mediates access to a file Mandatory or advisory: Mandatory – access is denied depending on locks held and requested Advisory – processes can find status of locks and decide what to do

41. 41 Example Program Using File System Calls (1) Figure 4-5. A simple program to copy a file.

42. 42 Example Program Using File System Calls (2) Figure 4-5. A simple program to copy a file.

43. 43 Directory Structure A collection of nodes containing information about all files F 1 F 2 F 3 F 4 F n Directory Files Both the directory structure and the files reside on disk Backups of these two structures are kept on tapes

44. 44 Operations Performed on Directory A directory is effectively a symbol table that translates file names into their directory entries. Search for a file Given a name or a pattern of names, we should be able to find all the files that use it. Create a file touch assignment3.c Delete a file rm assignment3.c List a directory ls Rename a file mv assignment3.c odev3.c Traverse the file system cd include

45. 45 Organize the Directory (Logically) to Obtain Efficiency – locating a file quickly Naming – convenient to users Two users can have same name for different files The same file can have several different names Grouping – logical grouping of files by properties, (e.g., all Java programs, all games, …)

46. 46 Single-Level Directory A single directory for all users Naming problem: Who will use the name assignment3.c? Each student has to use a different name: e123456- assignment3.c Grouping problem Listing would be very crowdy.

47. 47 Two-Level Directory Separate directory for each user Path name In MS-DOS, C:\userx\test.bat In VMS, volume:[userx.home]test.bat;1 Can have the same file name for different user Efficient searching No grouping capability

48. 48 Tree-Structured Directories A directory is simply another file that needs to be treated in a special way.

49. 49 Tree-Structured Directories (Cont) Efficient searching directory entry sizes would be manageable Grouping Capability Current directory (working directory) cd /spell/mail/prog type list

50. 50 Tree-Structured Directories (Cont) Absolute or relative path name Creating a new file is done in current directory Delete a file rm Creating a new subdirectory is done in current directory mkdir Example: if in current directory /mail mkdir count mail progcopyprtexpcount Deleting “mail”  deleting the entire subtree rooted by “mail”

51. 51 Acyclic-Graph Directories Have shared subdirectories and files

52. 52 Acyclic-Graph Directories (Cont.) Two different names (aliasing) If dict deletes list  dangling pointer Solutions: Backpointers, so we can delete all pointers Variable size records a problem Backpointers using a daisy chain organization Entry-hold-count solution New directory entry type Link – another name (pointer) to an existing file Resolve the link – follow pointer to locate the file

53. 53 General Graph Directory

54. 54 File System Mounting A file system must be mounted before it can be accessed A unmounted file system is mounted at a mount point

55. 55 Access Lists and Groups Mode of access: read, write, execute Three classes of users RWX a) owner access 7  1 1 1 RWX b) group access 6  1 1 0 RWX c) public access1  0 0 1 Ask manager to create a group (unique name), say G, and add some users to the group. For a particular file (say game) or subdirectory, define an appropriate access. ownergrouppublic chmod761game Attach a group to a file chgrp G game

56. 56 Quotas are kept track of on a per-user basis in a quota table. Disk Quotas