HP Requesting SWAP = RAM

[Michael Weber]

Good morning!

If memory serves, the kernel goes into emergency memory saving mode when it detects the available swap space is less than 50% of physical ram.  It does this to keep from crashing should memory usage go up.  There is a pretty steep performance hit when it does, as you've found out.  The kernel really does want swap => ram.  It likes its elbow room.  Even if you have enough memory that you should never need to swap out a memory page, you still have to have the swap space available.  

Swapping the drives is one way to get the space needed, or you could stay with the iSCSI space.  This assumes that you really will never need to use the swap, because iSCSI speed isn't near what DAS is.  If there is even a remote chance your memory usage will go up, grab the drives now.

IMHO, of course!




Michael Weber
Network Administrator
 
Allied National, Inc.
4551 W . 107th St.
Suite 100
Overland Park, KS 66207

913-945-4313 is my direct number
>>> "Hearn, Stan J." <stan.hearn at nscorp.com> 4/27/2010 6:36 AM >>>
We have a new HP ProLiant BL460c G6 with 64GB RAM and mirrored 146GB drives running HP Network Node Manager 8.13.

We are having issues with the embedded PostreSQL software.  

First HP said their software was not supported on RHEL 5.4, only RHEL 5.2.  But I refused to downgrade and only offered to downgrade a defined list of packages.  But that list was never provided.

Then they asked that we modify vm.overcommit_memory=2, but that didn't fix the problem.  

So HP told us that 18GB of swap was not large enough, we needed at least 64GB of swap to equal the physical RAM.

Our usage statistics show less than 32 GB RAM ever being used and 0% of swap being used.

However to humor them we added an iSCSI target of 46GB to swap and the problem was resolved.

HP support writes "... it is not a matter of the space showing up as used. When our software poll command runs, It looks for a certain amount of swap and our experiences show that the recommended OS setting usually works and fixes the issue."

HP also referred to an old statement found on the linux.com "The general rule of thumb for swap size is that your total available swap space should be around double your RAM size. ..."  I guess we should be thankful they are not asking for 128 GB of swap.  At this same site I found many other sane answers to how much swap for a 64GB system.

The system owner is now wanting to replace the 146GB drives with 300GB ones so that we can create this huge swap file.  This effects several systems.

Why does this seem wrong to me?   I don't feel that I have much power to push back.   I can't take out memory to reduce the swap space needed.   I don't want to continue to run iSCSI on swap.  I'm afraid if I relent all my users are going to want really large swap files for illegitimate reasons.

Help, give me some ammunition or options?   Anyone else having vendors require ridiculous amounts of swap?  Anyone else running HP NNM with swap issues?

Thanks,
Stan

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