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Introduction: locating blocks/records

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Slideshow:

Prelude problem 1: Priority of data blocks in memory over data blocks on disk
When a disk block is read into memory (for operation), the memory copy has precedence over the disk copy:
 
First problem created by "disk block in memory" situation
When a disk block is read into memory (for operation), the memory copy has precedence over the disk copy:
 
Problem 1: how can we tell if a disk block is in memory ?

Prelude problem 2: database address used to reference block/records
Database addresses are the only form of identification used in stored data (on disk):

What happen when the referenced block is stored in memory ??
 
Second problem created by "disk block in memory" situation
 
Summary

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• Previously: there are 2 places to find a (same) data block/record
  • From previous discussion:

    A disk block/record is permanently stored of disk A disk block/record on a disk (drive) is identified by its database address A disk block/record can also be stored in memory (when the data in the disk block/record is being accessed/updated) A disk block/record in computer memory is identified by a virtual memory address

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  • Important fact:

    When   a disk block/record has been read into memory: We must use the disk block/record in memory            because: The copy of the disk block/record in memory has a more upto date value !!! (Because we may have made updates to the copy in memory !!!)

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• Conclussions and summary before we move on
  • Very important conclusions:

    When a database object (block/record) is only stored on disk: We must use its database address to access the database object (block/record) on disk Graphically: When a database object (block/record) is read into main memory: We must use its virtual memory address to access the database object (block/record) Graphically:

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• Problems that we must solve
  • There are 2 problems posed in the above conclussions:

    How can we tell (= know) when: A block is only stored on disk ? A copy of a block is stored in memory ?             The native references used to identify blocks/records is: Using its database address (i.e.: hostID:disk#:block#) When a copy of a block is stored in memory: We must use a memory address to find (= locate) (and use) the block (in memory) Problem: How to find the memory address of a block (using its database address ???

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  • Solution for both problems:

    Use a translation table