1. 13.6 Representing Block and Record Addresses
Deepika Verma
CS257 Section 1
ID: 106

2. Outline 13.6.1 Addresses in Client-Server Systems
Logical and Structured Addresses
Pointer Swizzling
Returning Blocks to Disk
Pinned Records and Blocks
Exercises for Section 13.6

3. Introduction Address of a block and Record In Main Memory
Address of the block is the virtual memory address of the first byte
Address of the record within the block is the virtual memory address of the first byte of the record
In Secondary Memory
Sequence of Bytes describe the location of the block : the device Id for the disk, Cylinder number, etc.

4. 13.6.1 Addresses in Client-Server Systems
Physical Addresses:
Byte strings referring to the place within the secondary storage system where the record can be found.
Physical Address bits are used to indicate:
Host to which the storage is attached
Identifier for the disk
Number of the cylinder
Number of the track
Offset of the beginning of the record
Logical Addresses:
arbitrary string of bytes of some fixed length that maps to physical address

5. Map Table
Stored on disk in a known location, relates logical addresses to physical addresses.
Logical
Physical
Logical Address
Physical Address

6. 13.6.2 Logical and Structured Addresses
Purpose of logical address:
Gives more flexibility, when we
Move the record around within the block
Move the record to another block
Easy updating of records
Structured address
Gives us an option of deciding what to do when a record is deleted
Unused
Record 4
Record 3
Record 2
Record 1
Offset table
Header

7. Pointer Swizzling
Having pointers is common in an object-relational database systems
Every data item (block, record, etc.) has two addresses:
database address: address on the disk
memory address, if the item is in virtual memory
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)

8. Pointer Swizzling (Contd…)
Translation Table: Maps database address to memory address
All addressable items in the database have entries in the map table, while only those items currently in memory are mentioned in the translation table
Dbaddr
Mem-addr
Database address
Memory Address

9. Pointer Swizzling (Contd…)
Pointer consists of the following two fields
Bit indicating the type of address
Database or memory address
Example 13.17
Disk
Memory
Swizzled
Block 1
Block 1
Unswizzled
Block 2

10. Example 13.7
Block 1 has a record with pointers to a second record on the same block and to a record on another block
If Block 1 is copied to the memory
The first pointer which points within Block 1 can be swizzled so it points directly to the memory address of the target record
Since Block 2 is not in memory, we cannot swizzle the second pointer

11. Types of Swizzling Automatic Swizzling Swizzling on Demand
As soon as block is brought into memory, swizzle all relevant pointers.
Swizzling on Demand
Only swizzle a pointer if and when it is actually followed.
No Swizzling
Pointers are not swizzled they are accesses using the database address.

12. Programmer Control of Swizzling
Unswizzling
When a block is moved from memory back to disk, all pointers must go back to database (disk) addresses
Use translation table again
Important to have an efficient data structure for the translation table

13. 13.6.4 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. 13.6.5 Pinned records and Blocks
A block in memory is said to be pinned if it cannot be written back to disk safely
If block B1 has swizzled pointer to an item in block B2, then B2 is pinned
Unpin a block, we must unswizzle any pointers to it
Keep in the translation table the places in memory holding swizzled pointers to that item
Unswizzle those pointers (use translation table to replace the memory addresses with database (disk) addresses

15. Thank You