1. Representing Block & Record Addresses

2. Outline Types of Addresses Pointer Swizzling Returning Blocks to Disk
Automatic swizzling
Swizzling on demand
No swizzling
Programmer control of swizzling
Returning Blocks to Disk
Pinned Records and Blocks

3. Types of Addresses What is address? continued..
Commonly, database system consists of
Client – Server architecture
Client application uses virtual address space
Server uses database address space, referred to as blocks
continued..

4. Types of Addresses Physical Address Logical Address
Byte strings referring to the place within secondary storage where data(record) is saved
Logical Address
Arbitrary string of bytes of some fixed length that maps to physical address
continued..

5. Types of Addresses Physical Address Host name: My PC
Disk name:Megatron747
Cylinder # 10 (total #216 )
Track (Surface) # 3 (total # 24) (within cylinder)
(A sector has total # of 212 bytes.
Block # 2 (total # 4 sectors) (total # 28 sectors per track (average) ) )
Offset of the beginning record at10th byte

6. Types of Addresses Physical Properties Rotation 11 miliseconds
Acceleration of disk arms is about 5Gravational force
Start and stop need 1 milisecond

7. Types of Addresses Disk name:Megatron747
Compute the bytes in the following devices
Cylinder
Track
Block

8. Map Table

9. More on Logical Address
Purpose
Easy updating of records
Structured address

10. Pointer Swizzling
As discussed earlier, every record in memory has two forms of addresses
We can use either to access record
When block is moved from secondary storage to main memory pointers within block are “swizzled”,i.e translated from database address space (Server) to virtual address space (Client)
Consists of a bit indicating if pointer is currently a database address or memory address
The database or memory pointer itself

11. 1 Pointer Swizzling

12. Types of Swizzling Automatic Swizzling Swizzling on Demand
Whenever a block is moved to main memory swizzle all its pointers
Swizzling on Demand
Leave all pointers unswizzled when block is first loaded into memory, swizzle the pointer only if we need to follow it
Programmer Control of Swizzling
No Swizzling
Never swizzle the pointers. Advantage is there is no need to load record in main memory

13. Returning Blocks to Disk
When a block is moved from memory back to disk, any pointers within that block must be unswizzled, i.e. memory address must be replaced by database address.

14. Pinned Records and Blocks
‘Pinned’ means if the block cannot be written back to disk safely
Indicated by a bit located in the header of the block
To write a pinned block back to disk we need to ‘unpin’ the block
Unswizzle any pointers associated to it