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Home -> Community -> Mailing Lists -> Oracle-L -> RE: SHMMAX setting with multiple instances? Solaris 2.8,Oracle 8.1.7.4
Carmen,
SHMMAX determines the largest size a single shared memory segment can = be. It does NOT govern the total amount of shared memory that can be = allocated, just the max allowable size of a single chunk.
The value you show, 4,294,967,295 (shown w/ commas to demonstrate it's =
4GB-1, not 400MB) is fine. In a 32 bit
environment, 4GB is the largest amount of memory that a program can =
address, since 2^32=3D4GB.
So, if your Oracle installation is 32-bit code, that value is more than = enough. Even if it was 64-bit code, and you specified an SGA larger = than 4GB, having SHMMAX set to 4,294,967,295 would not harm you. You = would just see that your SGA was made up of multiple shared memory = segments. (Do 'ipcs -m' on Solaris to see all the shared memory = segments.) Note that there has been a vicious rumor that Solaris must = have your entire SGA in a single shared memory segment, if ISM (Intimate = Shared Memory) would be disabled. This is NOT TRUE. There is no harm = in having your SGA in multiple shared memory segments.
Bottom line, you should go ahead with:
set shmsys:shminfo_shmmax=3D4294967295
and forget about it. You'll be fine, and it won't cause you any = problems.
-Mark
-----Original Message-----
From: Carmen Rusu [mailto:carmen.rusu_at_rrc.state.tx.us]
Sent: Friday, February 13, 2004 9:57 AM
To: mgogala_at_adelphia.net; oracle-l_at_freelists.org
Subject: Re: SHMMAX setting with multiple instances? Solaris 2.8,Oracle
8.1.7.4
Here's from Sun Tech support. Does it make any sense to you? To me, it doesnt :-(
> customer. I wouldset=20
> shmmax=3DTHE_HIGHEST_NUMBER_OF_BYTES_TO_BE_USED_BY_THE_SGA+ some extra
> head room.
>=20
Carmen Rusu
>>> mgogala_at_adelphia.net 2/13/2004 7:31:05 AM >>>
On 02/13/2004 08:17:14 AM, Carmen Rusu wrote:
> Oracle install guide recommends the minimum value
>=20
> set shmsys:shminfo_shmmax=3D4294967295
>=20
> How does this number depend on the the available physical RAM on the
> box?
>=20
> Does this number depend on the number of instances on the box ?
>=20
> Did you experiment with this setting? Horror stories?
>=20
> Thanks,
>=20
> Carmen Rusu
>
Carmen, the number in question is 400MB and it is the maximum size of a shared memory segment on Slowaris. It also represents the largest SGA that oracle can construct in a single piece. It doesn't have any direct impact on your RAM unless you really =20 allocate the SGA.
--=20
Mladen Gogala
Oracle DBA
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