Re: DB_FILE_MULTIBLOCK_READ_COUNT

Kelly Young wrote:

> I'm also somewhat confused by the "block" terminology used in the
> Oracle manuals. We are running DEC Unix. It is my understanding
> that the physical disk block is 512 bytes and the Unix (OSF/1)
> operating system does I/O in blocks of 8192 bytes. Therefore,
> when Oracle refers to "Operating System Blocks" I assume they are
> referring to 8192 (not 512), correct? I've never been able to
> confirm this.

No. Generally when Oracle is referring to O/S blocks it is for *sizing* purposes, so it normally means the 512 byte disk block size, not the max I/O request size of your OS.

If you were setting, say, your Oracle block size, you would always make sure it was a multiple of the disk block size - 512 bytes. This is why you see recommended Oracle block sizes of 2K, 4K, 8K and so on.

The I/O size may be a limiting factor to consider when performing multi-block reads, as any requests which are larger than the I/O size for your system will just be broken up into multiple smaller requests. Example: 2K block (Oracle), multiblock read count 8: 8x2k = 16K read. The OS, with an 8K I/O limit will actually break this request down into 2 I/O requests, as it can't do a single 16K read, so you will get no advantage by setting large multi-block read counts.

Always check your OS specific Oracle installation guide for more info.

Steve Phelan. Received on Mon Mar 03 1997 - 00:00:00 CST