File Structures & Data Processing

Unit-I Introduction : File structure design, File processing operations : open, close, read, write, seek. Unix directory structure. Secondary storage devices: disks, tapes, CD- ROM.Buffer management. I/O in Unix.

A Program to display the contents of a file on Screen. 1. Open the file for Input(reading). 2. While there are characters to read from the input files: Read a character from the file. Write the charater to the screen. 3. Close the files.

. //C program # include Void main(){ Char ch; File * infile; infile=fopen(“A.txt”,”r”); While(fread(&ch,1,1,infile)!=0){ fwrite(&ch,1,1,stdout); } fclose(infile); }

. //Cpp Program #include Void main(){ Char ch; fstream Infile; Infile.open(“A.txt”,ios::in); Infile.unsetf(ios::Skipws);//set flag so it doesn’t skip white spaces Infile>>ch; While(!Infile.fail()){ Cout<<ch; infile>>ch; } Infile.close(); }

Secondary Storage Devices Since secondary storage is different from main memory we have to understand how it works in order to do good file structure design. Two major types of storage devices are: 1.Direct Access storage devices. -Magnetic Disks.Hard drives(high capacity,low cost per bit).Floppy Disks(low capacity,slow,cheap) -Optical Disks.CD-ROM-Compact Disc,Read only memory(read only/write once,holds a lot of data,cheap production) 2. Serial Devices: -Magnetic Tapes(Very fast sequential access).

The organization of Disks

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. Cylinder: the set of tracks that are directly above or below each other.

. Each platter (disc-shaped) is coated with magnetic material on both surfaces. Each platter surface has arm extended from fixed position. Tip of the arm contains read/write head for reading or writing data. The arm moves the heads from the spindle edge to the edge of the disk.

. Internal Structure Track Width is 1-2 microns (micrometer). A sector contains fixed number of bytes. E.g. 215 bytes or 4096 bytes Divided in header (stores sector number), data and ECC (Error Correction Code). Width of 1 bit in a sector is 0.1 to 0.2 microns.

. Cylinder: is a set of tracks on all the surfaces at a fixed arm position. The internal structure of a disk is shown below:

. How data is read/ written: Each Block is identified (i.e. block address contains) Cylinder Number (i.e. Track Number) Surface Number Sector Number Based on the block address the disk controller (digital circuit)Moves the arm to designated track. Platter is rotated (spins angularly) until the desired sector is located. Once the head is aligned with header section, the reading or writing mechanics is performed.

. Read/ Write Mechanism: Disks record data by magnetizing a magnetic material in a pattern that represents the data. -Magnetic material is ferromagnetic substance such as iron oxide. Write: Head contains an induction coil through which the current passes. -This magnetizes the iron oxide. -Depending on the current direction, the magnetic particles align either in left or right direction.. Read: Head passes over the bit region to detect the magnetization of the material. -a current is generated in the coil, which is measured and value is measured determines with bit is 0 or 1.

. Since a cylinder consists of a group of tracks, a track consists of a group of sectors and a sector consists of a group of bytes, it is easy to compute track, cylinder, and drive capabilities.

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Organizing tracks by Sector

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Cluster, Extent and Fragmentation

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Fragmentation

Organizing Tracks by block

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Nondata Overhead

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The cost of a Disk Access

The Cost of a Disk Access : A disk access can be divided into three distinct physical operations, each with its own cost : 1. Seek Time 2. Rotational Delay 3. Transfer Time Seek Time : Is the time required to move the access arm to the correct cylinder. Average seek time of Hard disk is less than 10 msec and high performance disks have average seek times as low as 7.5 msec.

Rotational Delay : Is time it takes for the disk to rotate so the sector we want is under the read/write head. On average rotational delay is half a revolution. Transfer Time : The transfer time is given by the formula : Transfer time is depends on no. of sectors on track. If there are 63 sectors on track then transfer time is 1/63.

Disk as a Bottleneck

4. RAM Disk: The RAM disk refers to large amount of primary memory in the form of RAM which behaves like a mechanical disk. It can be used to achieve faster access speeds. Unlike disk, RAM disk is a semiconductor memory which needs no rotations. Also, the seeking time is negligible as compared to the seeking time of disks. Because of less seeking time and no rotation delay, the required data can be located with faster access speed. The only disadvantage of RAM disk is its volatile nature. 5. Disk Cache: It is same like that of normal cache memory. The disk cache is present along with secondary storage device i.e. magnetic disk which maintains frequently used pages of data from the disk. Whenever there is a request of a particular data, it first looks into the disk cache for it. If the data is not found, then it accesses the disk. The size of the disk cache is negligible as compared to that of the disk. Both RAM disk and disk cache are the examples of buffers.

Characteristics of Magnetic Tapes

Organization of Data on Nine-Track Tapes Surface of tape is a set of parallel tracks, each of which is a sequence of bits. Since tapes are accessed sequentially, there is no need for addresses to identify the locations of data on a tape. Track Frame Gap Data Block Gap

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Disk Versus Tape

Physical organization of CD-ROM

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The Compact Disc is a spin-off of Laserdisc technology.Laserdisc Diagram of CD layers. A. A polycarbonate disc layer has the data encoded by using bumps. B. A shiny layer reflects the laser. C. A layer of lacquer protects the shiny layer. D. Artwork is screen printed on the top of the disc. E. A laser beam reads the CD and is reflected back to a sensor, which converts it into electronic datascreen printed

. CLV Vs CAV : The space on a computer disc is arranged into individually addressable areas called sectors. There are two basic methods for arranging these sectors on a disc: 1) placed in concentric rings (called tracks) of equal angle per sector 2) the other is to have them in an Archimedean spiral with the physical length of sectors along the disc kept constant instead of the angle. All sectors themselves have identical capacity regardless of their physical size (area or length), although the density of the sector (the size of the individual bits) can vary.

. Discs arranged into discrete tracks (including floppy discs, DVDs, and hard drives) are constant angular velocity (CAV) discs: the disc spins at a fixed rate. This means that sectors at the outside of the disc pass under the head much faster than those at the centre, and thus the data is more spread out.

. This wastes physical space on the disc. CD-ROMs have a single, spiral track and are constant linear velocity (CLV) discs. CD-ROM drives change the speed at which the disc spins such that the amount of disc surface passing under the laser unit is constant; sectors at the outside and inside of the disc’s surface are the same size (same length). This results in increased capacity at the expense of a more complex format. (Vinyl records also have a spiral track but are nevertheless CAV.)

CD-ROM Strengths and Weaknesses : 1) Seek Performance 2) Data Transfer Rate 3) Storage Capacity 4) Read-Only Access 5) Asymmetric Writing and Reading