[Database] Physical Storage Media and File Organisation
Physical Storage Media
$$ Access\ time = Seek\ time + Rotational\ latency + (Transfer\ time) $$
- A contiguous sequence of sectors from a single track
- Data is transferred between disk and main memory in blocks
- The operating system stipulates that a disk block can only contain one file, so the space occupied by the file can only be an integer multiple of the disk block
- Smaller blocks: more transfers from disk
- Larger blocks: more space wasted due to partially filled blocks
Therefore, Lareger block sizes may mean space wasted, smaller may lead to more IO times.
Typical block sizes today range from 4 to 16 kilobytes
The elevator algorithm:
Optimise block access time by organising the blocks to correspond to how data will be accessed
- Fixed-length records
- Variable-length records
- Allocate only as many records to a block as would fit entirely in the block
- Undesirable to delete records (requires additional block accesses)
When delete a record, the record's pointer is inserted in the free list. It will not be deleted immediately.
We change the header pointer to point to the next available record in the free list.
If no space is available, we add the new record to the end of the file.
Variable-length attributes are represented in the initial part of the record by a pair (offset, length)
- The actual records are allocated contiguously in the block, starting from the end of the block.
- The free space in the block is contiguous, between the final entry in the header array, and the first record
The header stores the information of each records.
If a record is inserted, space is allocated for it at the end of free space.
Free the space that it occupies and the records in the block before the deleted record are moved.
- Multitable clustering file organisation
Sequential File Organisation
It is applied in many databases, such as Rocksdb
locate the position where the record is to be inserted
- If there is free space insert there
- If no free space, insert the record in an overflow block
- In either case, pointer chain must be updated
use pointer chains
Multitable Clustering File Organisation
- good for queries involving join
- bad for queries involving only one table
- results in variable size records
- can add pointer chains to link records of a particular relation
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