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memory management for multiple SQL instances

I have a SQL 2012 VM that is running 40 GBs of RAM and 6 instances.  As I understand I need to set the max of each to where the sum of all of them is lower than 40 GBs of RAM.  My only confusion is the minimum.  The article below says the minimum should have a sum slightly lower than the total what my total physical memory is..  That doesn't make sense to me.  Can someone elaborate on this?

https://www.conetrix.com/Blog/post/Best-Practices-for-Memory-Management-of-Multiple-Instances-of-SQL-Server-2008-R2.aspx
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gopher_49
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gopher_49
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indianguru2Commented:
You can leave the min memory to the default which is 0. What it actually means is you have to decide based on the load of each instance, how much memory should it be allocated to start with so that it will have that much memory at any given time and is not taken away when not being used.

The actual article mentioned above is from here:
http://msdn.microsoft.com/en-US/library/ms178067.aspx
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gopher_49Author Commented:
Okay.  I have historical graphs capturing the memory used by each instance.  Ill check the stats after peak usage to see what each instance actually uses. I'll then set the max to 25% higher than what's actually being used.  I think thats an accurate way to approach this.
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indianguru2Commented:
And min as the peak usage what you see. Just make sure you don't assign more than you physically have after summing up all the instances :)
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Carl TawnSystems and Integration DeveloperCommented:
It is basically saying that if you use the Minimum Memory approach then you need to set them so the sum total is 1-2Gb less than the physical RAM available. You always need to leave 1 or 2Gb of RAM free for the OS to use.
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gopher_49Author Commented:
Okay.  I plan on allocating 12 GBs of RAM to the virtual machine.  Currently there is 40 GBs.  We plan to reduce it for we can't migrate it to other ESXi hosts due to its memory allocation being too high.  Plus..  We want the ability to utilize the memory on other VM's.  So..  For example.  If my 6 x instance never exceed 500MBs of RAM (determined via performance counters) I could make each instances min to 500 and max to 750.  That way I know its getting enough memory and some to grow.  I'll keep at least 2 GBs for the OS when everything allocated.

Does this seem like a sound config?
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Carl TawnSystems and Integration DeveloperCommented:
That sounds reasonable to me, if you're working on the assumption that each instance is holding roughly the same amount of data and has roughly the same workload. Otherwise you might need to weight that memory allocation towards the heavier use instances.
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Anthony PerkinsCommented:
I plan on allocating 12 GBs of RAM to the virtual machine.
It is a shame you did not mention the fact that it was a virtual machine in you original question as that changes everything.  The answer is not the same as on a physical box.

For a virtual server (assuming VMWare) I would urge you to read up on Brent Ozar's blog on the subject.  Let me know if you cannot track it down.
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gopher_49Author Commented:
In the title of my question I state it's a VM..  But you are correct.  I didn't mention it in the most recent post...  I'll read the blog.
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Anthony PerkinsCommented:
In the title of my question I state it's a VM.
Ah good point.  I overlooked it entirely.

This is the articles I was referring to:
How to Set SQL Server Max Memory for VMware

Although this one also applies:
How Much Memory Does SQL Server Need?
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gopher_49Author Commented:
I'm using performance counters to get a good understanding of how much each instance uses over a week..  I'll then set my min and max based on these stats.
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