Database Systems: The Complete Book (2nd Edition) (Hardcover) – By : Hector Garcia-Molina (Author), Jeffrey D. Ullman (Author), Jennifer Widom (Author) Publisher : Prenctice Hall. Jonathan Ben-David

Agenda: USING MULTIPLE DISKS MIRRORING DISKS

USING MULTIPLE DISKS Option 1: A system with one disk and many heads locked together [1]. Option 2: A system with several disks with their independent heads [1]. Claim 1: Using several disks can increase the ability of a database system to handle heavy loads of disk – access requests [1]. Claim 2: As long as the system is not overloaded, there is no change in how long it takes to perform any single block access [1].

USING MULTIPLE DISKS If we have several disks, then the technique known as STRIPING will speed up access to large database objects [1]. Example: Striping a relation across four disks The first disk can receive blocks 1, 5, 9,.. The second disk holds blocks 2, 6, 10,.. The third disk holds blocks 3, 7, 11,.. The last disk holds blocks 4, 8, 12,..

USING MULTIPLE DISKS To retrieve the 256 blocks of R on one of the disks requires an average seek time, 6.46 milliseconds, plus four rotation of the disk, one rotation for each track [1] *8.33 = 39.8 milliseconds. This should speedup in the time to access R by about a factor of three on the average.

MIRRORING DISKS It makes sense to have two or more disks hold identical copies of data [1]. (mirrors of each other). First motivation: data will survive a head crash by either disk. Second motivation: if we have n disks, each holding the same data, then the rate at which we can read blocks goes up by a factor of n, since the disk controller can assign a read request to any of the n disks.

REFERENCES 1. Hector Garcia-Molina (Author), Jeffrey D. Ullman (Author), Jennifer Widom (Author) Publisher : Prenctice Hall. Database Systems: The Complete Book (2nd Edition) (Hardcover)

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