The Memory Hierarchy. Desired data carried to read/write port, access times in seconds. Most common: racks of tapes; newer devices: CDROM “juke boxes,” tape “silo's.” Capacities in terabytes. Typically magnetic disks, magnetooptical (erasable), CDROM.
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Desired data carried to read/write port,
access times in seconds.
Most common: racks of tapes; newer
devices: CDROM “juke boxes,” tape
Capacities in terabytes.
under a microsecond, random access, perhaps 512Mb
fastest, perhaps 1Mb
To motivate many of the ideas used in DBMS’es, we must examine the operation of disks in detail.
1. Buffer data in and out of disk.
2. Schedule the disk heads.
3. Manage the ``bad blocks'' so they are not used.
On average, the desired sector will be about half
way around the circle when the heads arrive at
That is, the block might be on a track over which the head is positioned already, and the first sector of the block might be about to pass under the head.
That is, 10.97/360 is the fraction of a rotation need to read the entire block, and .00833 seconds is the amount of time for a 360-degree rotation.
The heads are positioned at the innermost cylinder, and the block we want to read is on the outermost cylinder (or vice versa).
Average number of cyls to travel, if the heads are currently positioned at cyl i.
Avg number of cyls to travel if the block is on the left.
Probability the block is on the left
Probability the block is on the right
Avg number of cyls to travel if the block is on the right.