Slideshow:
Deploying the pointer swizzling method will cause
2 problems
Helpful fact to understand
problem 1
Block/record address
after performing
pointer swizzling
We cannot write the data block to disk
Problem 1:
"unswizzle"
all references in block when block is written back to disk
We must unswizzle all (virtual) memory address before write back
Helpful definition (referenced/
pinned block) to understand
problem 2
A referenced block in memory is also called a pinned block
The
referenced block
causes problem 2
We cannot free the memory space used to store a referenced block !!
Some record references will reference to non-existing data after the release !!!
Problem 2:
"unswizzle"
all references
to a block
when removing that block from memory
We must first unswizzle all (virtual) memory addresses that point to the freed block
This will unpin a pinned block....
- Problems (caveats)
when we use
pointer swizzling:
- Writing a
block back to
disk:
- We must unswizzle all virtual memory address back to a database address before the block can be written back to disk,
- Releasing (freeing)
the memory used
by a disk block:
- Can result in
invalid record references
in other records !!!
(Virtual memory addresses in other records can be pointing to the block that you want to release (free) !!!)
- Can result in
invalid record references
in other records !!!
- Writing a
block back to
disk:
- Facts:
- The permanent location to
store a
disk block/record is
on disk
- Disk blocks/records is read into computer memory so programs to operate (= read/write) on the data in the block/record
-
Record pointers stored
on disks that
points to
other records
must be stored as:
- Database addresses !!!
- The permanent location to
store a
disk block/record is
on disk
-
Caveat:
we
must
unswizzle all
virtual memory addresses when
we write a
block
back to disk:
- When the block in
memory is
written back to
disk:
We must:
- First perform an inverse swizzling operation on all swizzled pointers in the block before writing the block to disk !!!
Graphically:
We call this operation:
- Unswizzling a (virtual) memory address
- When the block in
memory is
written back to
disk:
- Referenced block:
- Referenced block = a block in memory with records that are referenced by memory addresses
Example:
- Errors
caused by
releasing (= freeing)
a block/record in
memory that is
being referenced
through a virtual memory address:
Error:
- The data that is referenced by the virtual memory addresses are now illegal !!!
- Pre-requisite to
releasing a
block stored
in memory:
- We must
unswizzle
all virtual addresses
that
references to the
released block !!!
Graphically:
- We must
unswizzle
all virtual addresses
that
references to the
released block !!!
- Pinned block:
- A block in memory that is
begin referenced
by a virtual memory address is
said to be:
- pinned
because the block cannot be safely released from memory
Example: Block 2 in this figure is pinned:
- pinned
- A block in memory that is
begin referenced
by a virtual memory address is
said to be: