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Home -> Community -> Usenet -> c.d.o.server -> Re: Question: amount of data in 1 read constrained by os-limit?
On 28 Aug 1998 13:06:52 GMT, Jeroen van Sluisdam
<Jeroen.van.Sluisdam_at_vrijuit.nl> wrote:
>...[SNIP]...
>3) Experimenting with your suggestion gives the following result on our
>test db:
>WAIT #1: nam='db file sequential read' ela= 2 p1=13 p2=13432 p3=1
>WAIT #1: nam='db file scattered read' ela= 2 p1=13 p2=13433 p3=8
>- What does scattered read mean here ?
>- p3= 1 did lead me to lookup db_file_multi_block_read_count in
>v$parameter and to my suprise
> this said while the init file says 32. How this can be possible is
>a very large question. So this could
> mean we're not reading 256K at all but only 64K
>- Is it possbile that Oracle is undermining the initfile by adjusting
>it fitting to OS-limits ?
Yes, that is exactly what Oracle does. If you set DB_FILE_MULTIBLOCK_READ_COUNT higher as the OS specific maximum value for this parameter, Oracle will set it to a lower value. The maximum value for this parameter is OS specific an also depends on your DB_BLOCK_SIZE. Here is a qoute from the "Getting Started Release 7.3.4 for Windows NT" manual:
<BEGIN QUOTE>
MAX_IO_SIZE is not an initialization parameter. However, it is used to
calculate the maximum value of DB_FILE_MULTIBLOCK_READ_COUNT. On
Windows NT, MAX_IO_SIZE is set to 64 KB - 512, or 65012 bytes. The
maximum value of DB_FILE_MULTIBLOCK_READ_COUNT for a given block size
is 65012 divided by the value of DB_BLOCK_SIZE. This result is rounded
down to the next whole number. So, for DB_BLOCK_SIZE equal to 8192,
the maximum value of DB_FILE_MULTIBLOCK_READ_COUNT is 65012/8192,
which rounds to 7. For DB_BLOCK_SIZE of 2048, the maximum value is 31,
and so on.
<END QUOTE>
I have a DB_BLOCK_SIZE of 4096. If I set DB_FILE_MULTIBLOCK_READ_COUNT = 16 in my initfile, Oracle will actualy set it to 15.
>Thanks,
>
>Jeroen
HTH,
--
Jurij Modic <jmodic_at_src.si>
Certified Oracle7 DBA (OCP)
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