RE: Calculating SHMMAX

From: Bobak, Mark <>
Date: Tue, 16 Feb 2010 11:26:39 -0500
Message-ID: <>

Hi Peter,

I think the real meaning of shmmax is often misunderstood. shmmax controls the maximum size of a *single* shared memory segment. If you attempt to start an Oracle instance with more memory than shmmax, you will *not* get an error. You'll get an SGA made up of multiple shared memory segments.

Bottom line, you probably don't want it set too small, just for practical reasons, looking at ipcs output could be really confusing if you have dozens, or more, shared memory segments listed. If you like to have one shared memory segment per instance, then it probably makes sense to set shmmax to at least the size of the largest SGA you plan to create.

shmall actually governs the maximum allowable amount of shared memory, and is expressed in pages, whereas shmmax is in bytes. So, setting shmmax to shmall*page_size is an attempt to ensure that all available shared memory can be allocated to a single shared memory segment. It's probably not harmful, but also probably unnecessary. In terms of Oracle, it's not necessary, as Oracle will simply allocate n number of shared memory segments, in multiples of shmmax, till it's allocated enough to satisfy the instance's memory requirements. It's important to have shmall set to a large enough value though, as setting it too low could cause an out of memory error.

If all that is a bit more than you wanted to know, I recommend the following minimums:

shmall = (sum of sizes of all SGAs on server)/page_size shmmax = (largest single SGA on server)

Hope that helps,

-----Original Message-----

From: [] On Behalf Of Peter Nedeljkovich Sent: Tuesday, February 16, 2010 10:41 AM To: ''
Subject: Calculating SHMMAX

I've been using shmall*page_size to calculate the value of SHMMAX. We're now at 14 databases on a 4 node 11gr1 RAC on Linux 4.7 64bit with 64GB ram. Does anyone have any recommendation about changing this formula based on the number of databases?

-----Original Message-----

From: [] On Behalf Of Rich Jesse Sent: Tuesday, February 16, 2010 10:34 AM To:
Subject: HTML MOS favorites not the same as Flash MOS?


Does anyone use both the HTML MOS and Flash MOS? Since I use FF3.6 which doesn't like whatever the Flash MOS is doing, I need to use HTML MOS for now, but my favorites haven't transferred to the HTML version, save for one.

I tried searching on that, but it wound up in an pointless loop like in an MS Bing commercial. I was hoping to find an answer/workaround here before attempting the MOS Feedback mechanism.




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-- Received on Tue Feb 16 2010 - 10:26:39 CST

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