Buffer Hit Ratio

[fiz911]

The maximum value of the overall multiblock read factor is calculated as:

(db_block_size * db_file_multiblock_read_count)

The value of db_file_multiblock_read_count is limited by the multiblock read factor in the operating system level.

Regardless of the MAX_IO_SIZE value allowed by your operating system,
db_file_multiblock_read_count cannot exceed:

(db_block_buffers/4)


NOTE: Benchmarks of values higher than 32 have shown little, if any performance gain.

As quoted by note: 1037322.6
Metalink.

[yongchoi75]

How can I find out the size that unix reads when it performs I/O to disk?..so are you guys saying if my unix can read 256K I can leave as it is, but if it can only read 64k, I should reduce to 8 right? and this will improve my buffer hit ratio?I'm confuse now.

[yongchoi75]

Regardless of the MAX_IO_SIZE value allowed by your operating system,
db_file_multiblock_read_count cannot exceed:

(db_block_buffers/4)


If above statement is true, then

db_block_buffer=16384 / 4 = 4096k is that mean my db_file_multiblock_read_count should be 4?

[gandolf989]

16384*8192 / 4 = 32768k
buffer blocks * block size / 4 = max multiblock read count

It sounds like db_file_multiblock_read_count = 8192 would work fine.

[julian]

Originally posted by yongchoi75
How can I find out the size that unix reads when it performs I/O to disk?..so are you guys saying if my unix can read 256K I can leave as it is, but if it can only read 64k, I should reduce to 8 right? and this will improve my buffer hit ratio?I'm confuse now.

1. Ask your system administrator.
2. Yes.
3. No.