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Introduction Addresses in Client-Server Systems Logical and Structured Addresses Pointer Swizzling Programmer Control of Swizzling Pinned Records and Blocks
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 in the overall system
Sequence of Bytes describe the location of the block : the device Id for the disk, Cylinder number, etc.
The addresses in address space are represented in two ways◦ Physical Addresses: byte strings that determine the
place within the secondary storage system where the record can be found.
◦ Logical Addresses: arbitrary string of bytes of some fixed length
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
Map Table relates logical addresses to physical addresses.
Logical Physical
Logical Address
Physical Address
ADDRESSES IN CLIENT-SERVER SYSTEMS (CONTD..)
Purpose of logical address? Gives more flexibility, when we
◦ Move the record around within the block◦ Move the record to another block
Gives us an option of deciding what to do when a record is deleted.
Record 4
Record 3
Record 2
Record 1
HeaderOffset table
Unused
Having pointers is common in an object-relational database systems
Important to learn about the management of pointers
Every data item (block, record, etc.) has two addresses:◦ database address: address on the disk◦ memory address, if the item is in virtual
memory
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
Pointer consists of the following two fields◦ Bit indicating the type of address◦ Database or memory address◦ Example 13.17
Disk
Block 2
Block 1
Memory
Swizzled
Unswizzled
Block 1
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
Three types of swizzling◦ Automatic Swizzling
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.
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
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