Slideshow:
Logical database addresses
Logical database addresses are
hash keys
How to access a disk block
using a logical database address
Method:
You can use a logical database address
anywhere for a
physical database address
A problem caused by Physical database
addresses
A problem caused by Physical database
addresses
Problem solved by using Logical database
addresses
Problem solved by using Logical database
addresses
- Logical (block/record) address:
- Logical address =
an arbitrary string of
fixed length
bits
(Logical addresses can be generated using a counter --- we add 1 to the counter when we need a new value)
- Each block/record is
assigned a unique
logical address
Each (valid) logical address is mapped to a physical address (of a block/record)
Example: map table
Logical Address Physical Address -------------------+------------------------------------ 101001.......10 | (HostID1, DiskID1, BlockID1) 010101.......01 | (HostID2, DiskID2, BlockID2) ... | ...
- Logical address =
an arbitrary string of
fixed length
bits
- Data structure for
the map table:
- Sample
map table:
Logical Address Physical Address -------------------+------------------------------------ 101001.......10 | (HostID, DiskID, ..., BlockID) 010101.......01 | (HostID, DiskID, ..., BlockID) ...
- To speed up the
look up operation
(using a logical address to
find a physical address),
the map table is
organized
as:
- a hash table.
- The map table is stored on disk (in a file)
- Sample
map table:
- How to use the map table:
- When given a
logical database address,
we can find the
corresponding
physical database address
(where the block/record is
found physically) as
follows:
Explanation:
- Hash the logical database address
- Locate the entry in map table
- If entry is
valid:
return the (corresponding)
physical database address
If entry is not valid: return t invalid (not found)
- When given a
logical database address,
we can find the
corresponding
physical database address
(where the block/record is
found physically) as
follows:
- How to use
logical addresses:
- When a record x need to
refer to
another record y,
we now store:
- the logical database address of y inside x
Example:
- When a record x need to
refer to
another record y,
we now store:
- Problem with
referencing another record
using a physical
(database) address:
- Example:
2 records X1 and X2
are referencing the
record Y
using
physical database addresses:
- Problem:
If record Y is
moved (stored) to a
different block on
the disk:
- We must update
multiple (=all)
(physical) database addresses:
- We must update
multiple (=all)
(physical) database addresses:
- Example:
2 records X1 and X2
are referencing the
record Y
using
physical database addresses:
- Better way:
store
logical (database) addresses as
reference inside
records:
- Example: 2 records X1 and X2
references the
record Y
using
logical database addresses:
- If we move the
record Y, we
only need to:
- Update the
(one) physical address in the
map table:
- Update the
(one) physical address in the
map table:
- Example: 2 records X1 and X2
references the
record Y
using
logical database addresses: