1. January 18 & 20, 20001 Csci 2111: Data and File Structures Week2, Lecture 1 & 2 (Cont’d) Secondary Storage and System Software: Magnetic Disks &Tapes

2. January 18 & 20, 20002 Part II: Tape Outline Description of Tape Systems Organization of Data on Nine-Track Tapes Estimating Tape Length Requirements Estimating Data Transmission Times Disk versus Tape

3. January 18 & 20, 20003 Description of Tape Systems No direct accessing facility, but very rapid sequential access. Compactness, resistance to rough environmental conditions, easy to store and transport, cheaper than disk Used to be used for application data Currently, tapes are primarily used as archival storage.

4. January 18 & 20, 20004 Organization of Data on Nine- Track Tapes I On a tape, the logical position of a byte within a file corresponds directly to its physical position relative to the start of the file. The surface of a typical tape can be seen as a set of parallel tracks each of which is a sequence of bits. These bits correspond to 1 byte + a parity bit. One Byte = a one-bit-wide slice of tape called a frame.

5. January 18 & 20, 20005 Organization of Data on Nine- Track Tapes II In odd parity, the bit is set to make the number of bits in the frame odd. This is done to check the validity of the data. Frames are organized into data blocks of variable size separated by interblock gaps (long enough to permit stopping and starting)

6. January 18 & 20, 20006 Estimating Tape Length Requirements I Let b= the physical length of a data block Let g= the length of an interblock gap, and Let n= the number of data blocks. The space requirement, s, for storing the file is s = n * (b+g) b= blocksize (i.e., bytes per block)/ tape density (i.e., bytes per inch)

7. January 18 & 20, 20007 Estimating Tape Length Requirements II The number of records stored in a physical block is called the blocking factor. Effective Record Density: a general measure of the effect of choosing different block sizes: (number of bytes per block)/ (number of inches required to store a block) ==> Space utilization is sensitive to the relative sizes of data blocks and interblock gaps.

8. January 18 & 20, 20008 Estimating Data Transmission Times Normal Data Transmission Rate= (Tape Density (bpi)) * (Tape Speed (ips)) Interblock gaps, however, must be taken into consideration ==> Effective Transmission Rate/ ((Effective Recording Density)* (Tape Speed))

9. January 18 & 20, 20009 Disk versus Tape In the past: Both Disks and Tapes were used for secondary storage. Disks were preferred for random access and tape was better for sequential access. Now (1): Disks have taken over much of secondary storage ==> Because of the decreased cost of disk + memory storage Now (2): Tapes are used as Tertiary storage (Cheap, fast & easy to stream large files or sets of files between tape and disk)