Don’t use memory limits – Proofpudding - Eric Sloof

During my weekly VMware training sessions, I’m always getting a bit excited when we reach the module where the ESX and virtual machine memory is covered. I’m trying to explain how important it is; not to use the virtual machine memory limit setting and very often students believe me after checking out their own production environment for ballooning activity and 9 out of 10 times they find a few virtual machine which have a memory limit configured. The memory limit is the upper limit on memory the ESX host makes available to virtual machine. Host memory reclamation happens regardless of the current host free memory state. For example, even if host free memory is in the high state, memory reclamation is still mandatory when a virtual machine’s memory usage exceeds its specified memory limit. If this happens, the ESX server will employ ballooning and, if necessary, swapping to reclaim memory from the virtual machine until the virtual machines host memory usage falls back to its specified limit.

I’ve recorded a video which shows you the Proofpudding and collected some useful links about this topic.

Scott Herold: Memory Behavior when VM Limits are Set
VMware KB: Virtual machine boots very slowly when Memory Limit
Arnim van Lieshout: PowerCLI: Reset CPU and Memory Limits

Comments

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One reason to set a limit (at the same level as the VMs RAM) is that a VMSwap is not created if the Limit is the same as the VMs RAM. This can save a significant amount of expensive SAN storage space if you dont expect to overcomit your hosts from a memory persepective.

#1 Russ Hayward on 2010-06-29 22:02 (Reply)

I think you’re confusion the limit with the reservation, when you configure the reservation the same as configured memory there’s no swap file. Limit has nothing to do with it.

#1.1 Eric Sloof on 2010-06-29 22:15 (Reply)

I've employed both reservations and memory limits to prevent creation of swoap files. I've found that a swap file is created if I don't specify both. I specify memory limit=reservation=allocated ram. E.g. for a VM with 2 GB, I specify both the memory limit and reservation at 2048 MB. I'm not seeing balooning activity on these VMs, but the hosts are not memory constrained.

#1.1.1 Dave Finch on 2010-06-30 03:32 (Reply)

Yes, Russ speaks of memory reservations.

#2 Jason Boche (Homepage) on 2010-06-30 03:48 (Reply)

Limits have absolutely nothing to do with vswap. vswap will not be created when your reservation equals the amount of configured memory for your VM. A limit does not need to be set. Also, if you specify a reservation that equals your configured memory you will never, and let me repeat that, NEVER see ballooning or swapping as you just reserved that whole chunk of memory.

#3 Duncan (Homepage) on 2010-06-30 20:32 (Reply)

I'll confess to being fairly ignorant about the memory utilization but I have implemented limits on our secondary exchange mailstore to keep it from eating all the hosts memory and thus triggering alarms. I would have changed the actual memory allocated to the machine, but that would have caused a reboot. So instead I figured out how much the machine needed and used memory limits to give most of the extra back and stop hogging it. Maybe there's a better way. I'm open to suggestions and corrections

#4 Grant (Homepage) on 2010-09-23 16:59 (Reply)

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Don’t use memory limits – Proofpudding

Eric Sloof - NTPRO.NL

Monday, 28 June 2010

Don’t use memory limits – Proofpudding

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