Re: minimal sga_target

A bit of hand-waving, but it's probably mostly down to CPU_COUNT, and might be fairly easy to kick around.
I have notes that
processes = cpu_count * 80 + 40
sessions = processes * 1.5 + 24
transactions = sessions * 1.1
Then public redo thread count = cpu_count/16 Private redo thread count = transactions / 10

Oracle likes 1MB of buffer cache per process (or maybe it's session - I always forget), so that probably gets assumed. The size of the private redo thread is 128KB - so that's a chunk of the SGA to calculate
Somewhere I've seen a note about the public redo thread default size.

So the main component left would be the shared_pool demanded per process and for some reason I think it's 4MB - but I don't know where that idea came from.

If no-one else has any better answers - just keep on bouncing.

Regards
Jonathan Lewis

On Wed, 14 Jul 2021 at 14:09, Noveljic Nenad <nenad.noveljic_at_vontobel.com> wrote:

> The following error is generated on startup if sga_target is too small:
>
>
>
> ORA-00821: Specified value of sga_target 1024M is too small, needs to be
> at least 2576M
>
>
>
> (I defined a very small sga_target just to reproduce this error.)
>
>
>
> The minimum 2576M seems to be host dependent, rather than hardcoded.
>
>
>
> The MOS note “DBCA Fails With ORA-00821 Error (sga_target Is Too Small)
> (Doc ID 2409633.1)” confirms my observation:
>
>
>
> “If DBCA cannot find the initialization parameter file, it calculates
> default values for processes and memory allocation. It uses the amount of
> RAM, the number of cores, and the number of threads to calculate these
> values and this can create a very large value for processes and increases
> the size of the SGA to accommodate that value.”
>
>
>
> How does Oracle calculate this minimum, i.e. 2576M?
>
>
>
> Best regards,
>
>
> Nenad
>
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