Virtual Machine Resources Settings: Memory Limit

[nsmallcomb]

What is the point of the limit control on the VM Settings > Resources tab > Memory.

If you specify a limit that is GREATER than the amount of \*physical* memory you allocated to the VM, the swap file gets bigger, but the system still has the same amount of \*physical* memory. Why would you want to do this?

If you specify a limit that is LESS than the amount of \*physical* memory you allocated to the machine, you are now limited by that number. Why would you do this?

Any input?

[MR-T]

Using these settings allows multiple machines to play nicely.

By capping the amount of memory or CPU a VM can use, it prevents a rogue application running away with all the resources on a server.

[nsmallcomb]

Consider reading a bit deeper into my question. I understand the point of Resource allocation/Resource pools. I am specifically asking about the Limit slider on the Memory screen of the Resources tab of a VM's settings.

[esiebert7625 · Accepted solution]

Limit is misleading, a VM cannot use more memory then it actually has assigned to it. All that setting is good for is to limit the maximum amount of host memory a VM can use, once it reaches the limit it digs into it's vswp file. The limit slider should actually be 0 to the amount of RAM a VM has assigned to it not 0 to the amount of total RAM the host has.

So if you had a server with 1024MB of RAM and set the limit to 512MB the VM could only use 512MB of physical host memory and 512MB of vswp memory. The VM stills see 1024MB of memory, ESX just controls how much physical host memory it gets through limits.

See this post...

http://www.vmware.com/community/thread.jspa?messageID=632903

Message was edited by:

esiebert7625

[nsmallcomb]

great answer. thank you.

[nsmallcomb]

Why would you want to set the limit HIGHER than amount of RAM assigned to the VM?

[esiebert7625]

You wouldn't, you could set it higher but the VM will not get that extra memory. The default is the total amount of host RAM which I think they should change because it is misleading. Regardless of what you set the limit to if it is higher then the VM has assigned to it, the VM is only going to get the amount of RAM it has assigned. So if you have a VM with 1GB of memory and you set the limit to 2GB all that means is the VM is able to use up to 1GB of host memory. Reservations on the other hand guarantee a certain amount of physical host memory.

From the docs...

http://www.vmware.com/pdf/esx3_memory.pdf

"An ESX Server host allocates the memory specified by the limit parameter to each virtual machine

unless memory is overcommitted. An ESX Server host never allocates more memory to a virtual machine than its specified physical memory size. For example, a 1GB virtual machine might have the default limit (unlimited) or a user-specified limit (for example 2GB). In both cases, the ESX Server host never allocates more than 1GB, the physical memory size that was specified for it."

Message was edited by:

esiebert7625