Storing Databases

Storing Databases

1. Two Main Categories of Storage

   • Primary Storage
     • - Includes storage media that can be operated directly by the CPU. - Computer's main memory and other cache memories. - Provides fast access to data. - Limited storage capacity and more expensive than lower access.
       • Cache Memory: Static Ram. Used by the CPU to speed up execution of program instructions. MOST EXPENSIVE
       • Dynamic Ram (DRAM) aka MAIN MEMORY provides the main work area for the CPU and keeps the program instructios and data. Low cost but slow speed. Possible memory loss.
   • Secondary and Tertiary
     • Magnetic disks, optical disks, tapes. Hard-disk drives are 2ndary. Removable media are tertiary storage. Larger capacity and less cost but cannot be accessed directly by CPU.
       • Flash Memory: High density and high performance. Fast access speed but entire block must be erased and writted over simultaneously.
       • Magnetic disks: basic unit of data is the bit. Magnetizing one area on the disks can be made to represent a bit value of either 0 or 1. Number of bytes it can store = capacity of disk. Disks are gouped in a disk pack to increase storage.

2. Data must be stored physically on a storage medium.

3. Magnetic Disks

   • 100-1000 tracks per disk
   • divided into hard-coded sectors
   • Division of a track into equal-sized disk blocks is set by the operating system during formatting.
     • Block size is fixed and can't be changed.
     • Separated by interblock gaps

4. Random Access

   • Hard disks are random access addressable devices.
   • Transfer between disk and main meory is measured in disk blocks.
   • Hardware addres of a black is a combination of: cylinder #, track # and block #

5. Disk Reading/Writing

   • Head is the mechanism for reading and writing blocks to/from the disk
   • Disks rotate continuously at a constant speed
   • Once it's positions on the right track, disk spins to correct block and data is read/written.
   • Reads/Writes take time.
     • Seek Time: time for disk controller to position R/W head to correct track..
     • Rotational Delay: Waiting for the beginning of the desired block to rotate into position under the R/W head.
     • Block Transfer Delay: Time needed to transfer the data from the disk to the system.