DB_BLOCK_SIZE Initialization Parameter (92)

The most commonly used block size should be picked as the standard block size. In
many cases, this is the only block size that you need to specify. Typically,
DB_BLOCK_SIZE is set to either 4K or 8K. If you do not set a value for this parameter,
the default data block size is operating system specific, which is generally adequate.

You cannot change the block size after database creation except by re-creating the
database. If the database block size is different from the operating system block size,
ensure that the database block size is a multiple of the operating system block size. For
example, if your operating system block size is 2K (2048 bytes), the following setting
for the DB_BLOCK_SIZE initialization parameter is valid:

DB_BLOCK_SIZE=4096

A larger data block size provides greater efficiency in disk and memory I/O (access
and storage of data). Therefore, consider specifying a block size larger than your
operating system block size if the following conditions exist:
■ Oracle Database is on a large computer system with a large amount of memory
and fast disk drives. For example, databases controlled by mainframe computers
with vast hardware resources typically use a data block size of 4K or greater.
■ The operating system that runs Oracle Database uses a small operating system
block size. For example, if the operating system block size is 1K and the default
data block size matches this, the database may be performing an excessive amount
of disk I/O during normal operation. For best performance in this case, a database
block should consist of multiple operating system blocks.

DB_BLOCK_SIZE参数
1. 只能为4K或8K , 与操作系统相关
2. 如果该值与操作系统块大小不一致 ,需保证该参数值为后者的整倍数
3. 大的快值提供了磁盘I/O与内存的效率
4. 如果以下条件存在 ,可考虑使用较大的块值:
a. 快的磁盘驱动器 , 大的内存机器上
b. 运行数据库的服务器操作系统使用较小的块值 ,如1K

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