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@xxxxxxxxxxxx>
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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