1. Chap3. Secondary Storage and System Software

2. Chapter Objectives  Describe the organization of typical disk drives, including basic units of organization and their relationships  Identity and describes the factors affecting disk access time, and describe methods for estimating access times and space requirements  Describe magnetic tapes, identify some tape applications, and investigate the implications of block size on space requirements and transmission speeds  Identify fundamental differences between media and criteria that can be used to match the right medium to an application  Describe in general terms the events that occur when data is transmitted between a program and a secondary storage device  Introduce concepts and techniques of buffer management  Illustrate many of the concepts introduced in the chapter, especially system software concepts, in the context of UNIX

3. Contents 3.1 Disks 3.2 Magnetic Tape 3.3 Disk versus Tape 3.4 Introduction to CD-ROM 3.5 Physical Organization of CD-ROM 3.6 CD-ROM Strengths and Weaknesses 3.7 Storage as a Hierarchy 3.8 A Journey of a Byte 3.9 Buffer Management 3.10 I/O in UNIX

4. Disks  Random Access Device  Disk device - DASD(Direct Access Storage Devices)  Magnetic disk hard disk - most common floppy disk - inexpensive, little capacity magnetic tape - serial access Iomega Zip (100 M byte), Jaz (1 G byte)  Nonmagnetic disk optical discs - increasingly important for secondary storage e.g. CD, DVD

5. Organization of Disks(1)  Components of Disk Tracks Sectors Ü the smallest addressable portion of a disk Disk pack Üa collection of a lot of platters Cylinder (vertical collection of tracks) –Arms are moving together  Seeking r/w arm movement the slowest part of reading data from disk

6. Organization of Disks(2) TrackSector Gaps Surface of disk showing tracks and sectors

7. Boom Read/write heads Spindle Platters Schematic illustration of disk drive Organization of Disks(3)

8. Estimating Capacities(1)  Cylinder --> Track --> Sector --> Byte  Track capacity = # of sectors per track * bytes per sector  Cylinder capacity = # of tracks per cylinder * track capacity  Drive capacity = # of cylinders * cylinder capacity ** Inner track, Outer track? Same capacity?

9. Estimating Capacities(2)  Ex) file with 50,000 fixed-length data records on 2.1G disk # of bytes per sector = 512 # of sectors per track = 63 # of tracks per cylinder = 16 # of cylinders = 4092  How many cylinders does the file require if each data record requires 256 bytes? 50,000/2 = 25,000 sectors 63x16 = 1008 sectors in a cylinder 25,000/1008 = 24.8 cylinders

10. Organizing Tracks by Sector(1)  Two ways to organize data on a disk by sector & by user-defined block  Physical placement of sectors logical view and physical view adequate way to view a file logically, but always not good way to store sectors physically interleaving — problems occur due to the gap between disk revolution speed and disk controller speed — high performance disks (with high speed disk controller) now offer 1:1 interleaving

11. 1 2 3 4 21 22 23 32 33 34 If interleaving factor is 3..... Disk Interleaving Organizing Tracks by Sector(2)

12. Organizing Tracks by Sector(3)  Clusters (for improving file access speed) One file consists of one or more clusters One cluster is a fixed number of contiguous sectors Cluster is the smallest unit of space for a file Contiguous sectors per cluster and Mapping table No additional seek (arm movement) for one cluster Very useful when access granularity is medium large cluster vs. small cluster FAT (File Allocation Table) –contains a list of all the clusters in file –each cluster entry --> physical location of the cluster

13. FAT(File Allocation Table) - A table containing mappings to the physical locations of all the clusters in all files on disk storage Cluster number Cluster location 123...123... File allocation table(FAT) The part of the FAT pertaining to our file...... 2 1 Organizing Tracks by Sector(3)

14. Organizing Tracks by Sector(4)  Extents (further attempt for file access speed) Contiguous clusters per extent and Mapping table File may have several extents Very useful when accessing the whole file sequentially Many extents of a file increase amount of seeking file extents extent

15. Organizing Tracks by Sector(5)  Fragmentation the space that goes unused within a cluster, block, track, or other unit of physical storage internal fragmentation: loss of space within a sector or cluster trade-offs in use of large cluster sizes ex) size of sector is 512 bytes and size of all records in file is 300 bytes –we may enforce one record / sector –we may allow span sectors

16. Organizing Tracks by Block(1)  User-defined size of blocks can be fixed or variable  No sector spanning ==> No internal fragmentation  Blocking factor the number of records that are to be stored in each block  In block-addressing, each block of data is accompanied by one or more subblocks containing extra information about the data block  Note: the “block” has a different meaning in the context of the UNIX I/O system

17. Sector organization vs. Block organization 1 1 1 1 1 1 2 2 22 2 3 3 3 44 4 4 5 5 5 Sector 1Sector 2Sector 3Sector 4Sector 5 (a) sector organization (b) block organization 1 1 1 1 1 1 2 2 22 2 3 3 3 44 4 4 5 5 5

18. Organizing Tracks by Block(2)  Subblocks count subblocks –contain the num of bytes in the accompanying data block key subblocks –contain key for last record in data block Count subblock Count subblock Key subblock Data subblock Count subblock Data subblock Count subblock Key subblock Data subblock Count subblock Key subblock Data subblock

19. Nondata Overhead(1)  Stored during preformatting  Sector-addressable disks (physical) sector/track address, condition, gaps, and synchronization marks of no concern to programmer  Block-organized disks (user-defined) some of nondata for programmer more nondata provided with blocks than with sectors

20. Nondata Overhead(2)  Ex) block-addressable disks 20,000 bytes/track subblock, interblock gap equivalent to 300 bytes/block If we want to store a file containing 100-byte records, how many records can be stored per track if blocking factor is 10 or 60 ? èif blocking factor is 10, then 20000 / ( (10*100)+300) = 15 blocks ==> 150 records èif blocking factor is 60, 180 records(=3 blocks)

21. Cost of Disk Access u Disk Access Time è Seek time + Rotational delay + Transfer time u Seek time u time to move arm to locate cylinder u more costly in multi-user environment u average seek time for a particular file operation u Rotational delay u time taken for disk to rotate correct sector under read/write arm u Transfer time (for actual data) number of bytes transferred number of bytes on a track rotation time

22. Timing Computation  Seagate Cheetah 9 Gbyte drive Minimum seek (track-to-track): 0.78msec Maximum seek: 19 msec Average seek: 8 msec Rotation delay: 3 msec Read one track: 6 msec Read one cluster: 0.28 msec  Read 8704K size file sequentially access the whole file: 1.7 sec randomly access the whole file: 9.25 sec

23. Effect of Block Size on Performance (In UNIX)  UC Berkeley CSRG team  Many experiments in BSD UNIX System  Trade-off between block size, fragmentation, access time  Doubling block size to 1,024 bytes improves performance by more than a factor of 2  In 512-byte block size: the slow throughput & low internal fragmentation (6.9%)  In 4,096-byte block: the fastest throughput & internal fragmentation (45.6%) ==> Invent the cluster concept!

24. Disk as Bottleneck  Disk performance is increasing, but still slow!  Even high-performance network is faster than disk (5 M/ sec HD is slower than 100M bps LAN)  Disk bound jobs CPU and network must wait  Solution techniques multiprogramming striping - parallel I/Os avoid accessing disk –RAM disk, disk caches - locality

25. Magnetic Tape  A set of parallel tracks  9 tracks - parity bit  Frame one-bit-wide slice of tape  Interblock gaps permit stopping and starting

26. Nine-track tape TrackFrame 011010010011010010 Gap Data block Gap

27. Estimating Tape Length(1)  There is an interblock gap for each data block  Space requirement s ès = n * ( b + g ) b is the physical length of a data block g is the length of an interblock gap n is the number of data blocks  Tape density  Tape speed  Size of interblock gap

28. Estimating Tape Length(2)  Ex) one million 100-byte records 6,250 BPI tape 0.3 inches of interblock gap  How much tape is needed? when blocking factor is between 1 and 50  Nominal recording density  Effective recording density: num of byte per block / num of inches for block

29. Estimating Data Transmission Times  Factors of data transmission rate interblock gaps effective recording density nominal recording density speed of r/w head time to start/stop the tape

30. Disk VS. Tape u Disk u Random access u Immediate access u Expensive seek in sequential processing u Tape u Sequential access u Long-term storage u No seek in sequential processing uDecrease in cost of disk and RAM uMore RAM space is available in I/O buffers, so disk I/O decreases uTertiary storage for backup: CD-ROM, tape...

31. Introduction to CD-ROM  CD-ROM: Compact Disc Read-Only Memory Can hold over 600MB(200,000 pages) Easy to replicate Useful for publishing or distributing medium But, not storing and retrieving data  CD-ROM is a child of CD audio  CD audio provides High storage capacity Moderate data transfer rate But, against high seek performance  Poor seek performance

32. History of CD-ROM: Videodisc  Videodisc technology developed in late 1960's and early 1970's The goal was to store movie A number of methods for storing video signals 1. Use a needle to respond mechanically to grooves in a disc like a vinyl LP record 2. Use optical storage Many companies were fighting which approach should become a standard Videodisk  LaserVision  CD audio  CD-ROM

33. History of CD-ROM: LaserVision  LaserVision Emerged as the winner of standard battles about standard Support CLV(Constant Linear Velocity) and CAV(Constant Angular Velocity) format Fast seek performance by using CAV format Data are stored in analog form Why did they fail? –Earlier disputes over the physical format of the video disc, many incompatible encoding scheme and error correction techniques were used by many firms –No standard format  the market never grew

34. History of CD-ROM: CD-ROM  CD-ROM Philips and Sony developed CD-ROM in 1984 in order to store music on a disc Use a digital data format The development of CD-ROM as a licensing system results in widely acceptance in the industry Promised to provide a standard physical format Any CD-ROM drive can read any sector which they want Computer applications store data in a file not in terms of sector, thus, file system standard should be needed In late 1985, videodisc/digital data industry moved into CD-ROM industry In early summer of 1986, an official standard for organizing files was worked out

35. Physical Organization of Master Disk  Master Disc Formed by using the digital data, 0 or 1 Made of glass and coated that is changed by the laser beam  Two part of CD-ROM –Pit –The areas that is hit by the laser beam –Scatter the light –Land –Smooth, unchanged areas between pits –Reflect the light laser beam pit land light

36. Encoding Scheme of CD-ROM  Encoding scheme The alternating pattern of high- and low-density reflected light is the signal 1 : transition from pit to land and back again 0s : the amount of time between transitions  Constraint The limits of the resolution of the optical pickup (generic hardware limit), there must be at least two 0’s between any pair of 1’s (no two adjacent 1s) We cannot represent all bit patterns, thus, we need translation scheme We need at least 14 bits to represent 8 bits under this constraint  EFM(eight to fourteen modulation) coding is used 1 10 10000100000000 EFM 00000001 2

37. Format of CD-ROM  CD audio chose CLV format instead of CAV format because CD audio requires large storage space CD audio is played from the beginning to the end sequentially  Format of CD-ROM CLV(Constant Linear Velocity) A single spiral pattern Same amount of space for each sector Capability for writing all of sectors at the maximum density Rotational speed is slower in reading outer edge than in inner edge Finding the correct speed though trial and error Characteristics –Poor seek performance –No straightforward way to jump to a specified location

38. Constant Angular Velocity Disk  Magnetic disk usually uses CAV(Constant Angular Velocity) Concentric tracks and pie-shaped sectors Data density is higher in inner edge than in outer edge Storage waste: total storage is less than a half of CLV Spin the disc at the same speed for all positions Easy to find a specific location on a disk  good seek performance CAVCLV

39. Addressing of CD-ROM  Addressing Magnetic disk: cylinder/track/sector approach CD-ROM: a sector-addressing scheme  Track density varies thus, each second of playing time on a CD is divided into 75 sectors 75 sectors/sec, 2 Kbytes/sector At least one-hour of playing time Maximum capacity can be calculated: 600 Mbytes 60 min * 60 sec/min * 75 sectors/sec = 270,000 sectors  We address a given sector by referring minutes, second, and sector of play 16:22:34 means 34th sector in the 22nd second in the 16th minutes of play

40. Fundamental Design of the CD disc  Initially designed for delivering digital audio information  Store audio data in digital form  Wave patterns should be converted into digital form  Measure of the height of the sound: 65,536 different gradation(16 bits)  Sampling rate: 44.1 kHz, because of 2 times of 20,000 Hz upto which people can listen  16 bits sample, 44,100 times per second, and two channel for stereo sound, we should store 176,400 bytes per seconds  Storage capacity of CD is 75 sectors per seconds, we have 2,352 bytes per sector  CD-ROM divides this raw sector as shown in the following figure 12 bytes synch 4 bytes sector ID 2,048 bytes user data 4 bytes error detection 8 bytes null 276 bytes error correction

41. File Structure Problem of CD-ROM  The most important problem in using CD-ROM as a storage media  Strongness and weakness of CD-ROM Strong aspects of CD-ROM –Data transfer rate: 75 sectors/sec –Storage capacity : over 600 Mbytes –Inexpensive to duplicate and durable Weak aspects of CD-ROM –Poor seek performance (weak random access) –Magnetic disk: 30 msec, CD-ROM : 500 msec Comparison of access time of a large file from several media –RAM: 20 sec, Disk: 58 days, CD-ROM: 2.5 years  We should have a good file structure avoiding seeks to an even greater extent that on magnetic disk

42. Storage as a Hierarchy  Primary storage 1/1,000,000,000 sec semi-conductors: registers, RAM, RAM disk, and disk cache  Secondary storage 1/1,000 sec magnetic disks(DASD), tape and mass storage(Serial)  Off-line storage (archival & back-up) 10 sec removable magnetic disks, optical discs, tapes

43. A Journey of a Byte(1)  What happens when a program writes a byte to a file on a disk? WRITE(TEXT, c, 1); User Program ---> OS File manager --> I/O buffer ---> I/O processor ---> Disk controller ---> Disk

44. A Journey of a Byte(2) 1The program asks the OS to write the contents of the variable c to the next available position in TEXT. 2The OS passes the job on to the file manager 3The file manager looks up TEXT in a table containing information about it, such as whether the file is open and available for use, what types of access are allowed, if any, and what physical file the logical name TEXT corresponds to. 4The file manager searches a FAT for the physical location of the sector that is to contain the byte. 5The file manager makes sure that the last sector in the file has been stored in a system I/O buffer in RAM, then deposits the ‘P’ into its proper position in the buffer. Logical layer

45. A Journey of a Byte(3) 6The file manager gives instructions to the I/O processor about where the byte is stored in RAM and where it needs to be sent on the disk 7The I/O processor finds a time when the drive is available to receive the data and puts the data in proper format for the disk. It may also buffer the data to send it out in chunks of the proper size for the disk 8The I/O processor sends the data to the disk controller. 9The controller instructs the drive to move the r/w head to the proper track, waits for the desired sector to come under the r/w head, then sends the byte to the drive to be deposited, bit-by-bit, on the surface of the disk. Physical layer 3.8 A Journey of a Byte

46. What is buffer?  Definition - the part of main memory available for storage of copies of disk blocks  Program buffers vs. System I/O buffers  Buffer manager subsystem responsible for the allocation for blocks goal: –minimize the number of disk access –utilize the memory space effectively

47. Buffer Management  Buffer bottlenecks What if program performs both input and output on one character at a time, and only one I/O buffer is available? At least two buffer - for input and output  I/O bound jobs: wait for I/O completion  Buffering Strategies for Performance use more than one buffer I/O system processes next block when CPU is processing current one

48. Buffering Strategies(1): Multiple buffering  Double buffering swapping roles of two buffers after I/O finished allows O/S operating on one buffer while the other buffer is being loaded or emptied  any number of buffers can be used  Buffer pooling takes from a pool of available buffers decides which buffer to take from a buffer pool étake buffer by LRU

49. Double Buffering Program data area To disk I/O buffer 1 I/O buffer 2 (a) (b)

50. Buffering Strategies(2): Move & Locate mode  Move mode (using both system buffer & program buffer) moving data from one place in RAM to another before they can be accessed sometimes, unnecessary data moves  Locate mode (using system buffer only or program buffer only) perform I/O directly between secondary storage and program buffer (program’s data area) system buffers handle all I/Os, but program uses locations through pointer variable

51. Move mode & Location mode system buffer program’s data area system buffer disk user’s program location (pointer) Move mode Locate mode

52. Buffering Strategies(3): Scatter/Gather IO  A file with many blocks and each block with header and data  Need to put the headers in one buffer and the data in a different buffer ==> may occur complication  Scatter-input mode a single READ can scatter data into a collection of buffers  Gather-output mode a single WRITE can gather several buffers and output

53. Scatter Input & Gather Output disk scatter input gather output read() write() buffer 1 buffer 2 buffer 1 buffer 3 buffer 2 3.9 Buffer Management

54. I/O in UNIX 3.10 I/O in UNIX I/O system PROCESS KERNEL user programs librariesshell commands system call interface block I/O system (normal files) character I/O system (terminals, printers, etc.) network I/O system (sockets) block device drivers diskdisk... character device drivers consolesprinters... network interface drivers...networks...

55. UNIX Kernel(1)  The central part of UNIX operating system  View all I/O as operating on a sequence of bytes  Make all operations below top layer independent of application’s logical view of file  Data structures related to unix files file-descriptor table: owned by user process open-file table: owned by kernel index-node: a kind of FAT (one inode for each file in use) index-node table: owned by kernel 3.10 I/O in UNIX

56. UNIX Kernel(2)  File descriptor table associates each file descriptor to open file table every process has its own file descriptor table  Open file table entries for every open file file structures: r/w mode, offset, reference count 3.10 I/O in UNIX

57. UNIX Kernel(3) file descriptor file table entry o(keyboard) 1(screen) 2(error) 3(normal) 4(normal) 5(normal). to open file table descriptor table 3.10 I/O in UNIX

58. UNIX Kernel(4) R/W mode # of processes using it Offset of next access ptr to write routine inode table entry...... write1100...... to inode table write() routine for this type of file open file table 3.10 I/O in UNIX

59. UNIX Kernel(5)  Inode(Index node) data structure used to describe a file when a file is opened, a copy of inode is loaded into RAM for rapid access has a list of disk blocks of the file this list is UNIX counterpart to FAT  Device driver I/O processor program invoked by kernel performing I/O for devices 3.10 I/O in UNIX

60. Inode Structure device permissions owner’s userid file size block count file allocation table............ An inode 3.10 I/O in UNIX

61. Hard/Soft Link & File types  Linking file names to  Linking file names to files hard link: direct reference –pointer from a directory to the inode of a file –ln src-file target-file soft(symbolic) link: pathnames –link a file name to another file name –ln -s src-file target-file  File types normal file (governed by block IO system) special file: stream, signal control of device (governed by character IO system) sockets: endpoints of IPC (governed by network IO system) 3.10 I/O in UNIX

62. Let’s Review !!! 3.1 Disks 3.2 Magnetic Tape 3.3 Disk versus Tape 3.4 Introduction to CD-ROM 3.5 Physical Organization of CD-ROM 3.6 CD-ROM Strengths and Weaknesses 3.7 Storage as a Hierarchy 3.8 A Journey of a Byte 3.9 Buffer Management 3.10 I/O in UNIX