Thursday, June 19, 2008

Advantages and Disadvantages of using smaller and bigger data blocks

Whether I will use bigger or smaller data blocks in my database it can be specified by parameter DB_BLOCK_SIZE or DB_nK_CACHE_SIZE. With the settings I can I can differentiate three types of data blocks in oracle.

1)Small Block(2KB-4KB)
2)Medium Block(8KB)
3)Large Block(16KB-32KB)

Advantages of Bigger Blocks
•Using bigger blocks means more data transfer per I/O call. So faster data transfer from disk to memory.

•Using bigger blocks means more space for key storage in the branch nodes of B*-tree indexes, which reduces index height, which improves the performance of indexed queries.

•When using large block there are less probability of chained and migrated rows, which in turn reduced the number of reads required to get the information.

Disadvantages of bigger Blocks

•If the rows are predominated random then you are increasing the possibility of contention in the buffer cache. Because now with same same amount of memory in buffer cache as it was in small blocks, we need more memory in the buffer cache to keep the same amount of buffers in memory in the buffer cache.

•If you have high transactional concurrency to a segment, using bigger blocks is just going to make the concurrency even higher.


Advantages and disadvantages of these blocks
1)Small Block(2KB-4KB):
The advantage of small blocks are they reduce block contention and they are really good where there is small rows or the selectivity of rows are highly random.

The disadvantages of small blocks are they have relatively larger overhead.

2)Medium Block(8KB): The advantage of medium blocks are if the rows are of medium size then you can bring a number of rows in a single I/O.

The disadvantage of it is space in the buffer cache will be wasted if you are doing random access to small rows and have a large block size. For example, with an 8KB block size and 60 byte row size, you are wasting 8000-60=7940 bytes in the buffer cache when doing random access.

3)Large Block(16KB-32KB): If you use larger block then relatively less overhead. Per I/O you can fetch more data. This is very good for sequential access, or very large rows.

Large block size is not good for index blocks used in an OLTP(Online Transaction Processing) type environment, because they increase block contention on the index leaf blocks.

Related Documents:

General Idea of Database Block Size and BLOCKSIZE
Choose an optimal Data block size in Oracle

1 comment:

Frederick Tang said...

Would you have some test results to demonstrate the advantages?