Hyper + V and dynamic memory on failover cluster

[Rozzer]

Hi All,

 

Seem to be having a odd issue with my cluster. I have all my VMS on a cluster shared volume which is always owned by one of my cluster host servers. The server which hosts the cluster shared volume hosts the VMs perfectly with no memory errors. But the other server which connects through to the cluster shared volume all the VMS end up with warnings on dynamic memory. Should I have two separate cluster shared volumes for each server ?

 

Any ideas ?

 

Thanks,

 

Ross

[ChrisH]

The point in CSVs is so it can be exposed to 2 or more servers at once. What you are talking about is more like using a traditional lun, with that if you have more than one vm on that lun they will all fail over together.

[Rozzer]

So your suggesting having a partition/lun for each VM?

[ChrisH]

No that's the old way of doing it your set up currently should be okay. I was talking about your proposed solutions being the old way of doing it.

[Rozzer]

Ahh ok any possible suggestions or ideas to my situation?

[jamesfed]

Dynamic Memory is relating to RAM usage - when your VMs are being hosted on the 2nd server does it run low on RAM?

[Rozzer]

Nope the memory only becomes an issue on the server which is not hosting the cluster shared volume. Plenty of memory available! I have dual link MPIO to my san.

[jamesfed]

Are you running SCVMM with any host resurves set on the 2nd server?

[Rozzer]

I am running scvmm not sure what reserve sets are...

[jamesfed]

You will want to go to the hosts tab then select the host you are having problems with - just click into its properties and see what its all set to as reserves.

[Rozzer]

I'll give that a shot and get back to you ta for the advice

 

Ross

[Rozzer]

Its down as Over-committed. That doesn't sound good!

[jamesfed]

Is there a differance in RAM between the two servers at all?

 

Also this is where you should look to check the resurve settings - would be surprised if it is set at anything else.

[Rozzer]

Yes one server has 24GB and other has 32GB but both servers are barely going near the 50% mark as I have evenly shared out the VMs across both.

 

I have mimicked your settings as I noticed it was asking for a lot of hard drive space free.

 

This was what mine was set too.

 

[Rozzer]

Also here is whats happening.

 

 

The VM host on the left has the cluster shared volume allocated to it where the right server doesn't. If i transfer the cluster shared volume to the right server the issue will then start on the left server.

Edited April 12, 2012 by Rozzer

[jamesfed]

Phew well u've got me there then! Never seen anything like that

[Rozzer]

Yeah tell me about it very confusing!! Maybe worthy of a call to Microsoft.... I have checked iSCSI and MPIO seems to be setup and working as it should.

[Arthur]

Would it be possible to post a screenshot of the memory settings for one (or more) of the VMs with the warnings? e.g.

 

http://i.imgur.com/60LuG.png

 

The warning messages seem to indicate your hosts do not have enough physical RAM to supply the buffers...

 

http://i.imgur.com/lPh7r.png

 

Startup RAM

This is the amount of memory that is allocated to the virtual machine when it starts up. If you set this to 512 MB then 512 MB is instantly assigned from the shared pool to the virtual machine when it starts. If you set this to 1 GB then 1 GB is instantly assigned from the shared pool to the virtual machine when it starts up.

 

Think about this for a few seconds. Dynamic Memory is made possible via the DMSCV integration component. The integration components are a set of drivers or services. They only start up when the virtual machine’s guest operating system starts up. The guest operating system has a requirement for a certain amount of memory for it starts up. You can’t configure a virtual machine to start up with 128 MB of RAM and expect it to work with a Windows Server 2008 R2 guest operating system. Windows Server 2008 R2 will look for 512 MB to get up and running. Only when the OS is running will the DMVSC start. And that is when Dynamic Memory can start up and start adding more memory to the virtual machine to meet the demands of any other services or applications that are installed in the virtual machine.

 

That means that you must configure the Startup RAM setting to be at least the minimum amount of RAM required for the operating system to start up. That’s the official line. My recommendation is that you configure Startup RAM to be the amount of memory that is the minimum required for the operating system and applications in the VM to run at basic or normal levels. This will ensure that the virtual machine always has enough memory to meet normal demands, and can grow beyond this when unusual demands occur.

 

Maximum RAM

This is simple enough. How much memory do you want to limit this virtual machine to? By default this is 64 GB RAM. 64 GB is the maximum possible amount of memory that a HyperV virtual machine can have.

 

Note that Hyper-V can never allocate more memory than is available in the shared pool on the host machine. This means that in times of low general demand, a virtual machine that has a high demand could reach its maximum RAM setting. But in times of high general demand, there might not be any memory left in the pool to allocate to the virtual machine.

 

Memory Buffer

Once you try out Dynamic Memory you will see that it is able to add both small and large amounts of memory to a virtual machine very quickly. However, there is a process, and there will be some amount of latency that is not perceivable to us. There is always a chance that some service or application will demand memory more rapidly than Dynamic Memory can respond to. This is why Dynamic Memory allocates a spare amount of memory to each virtual machine. The Memory Buffer defines that extra assignment as a percentage of the committed RAM in the virtual machine. For example, if the virtual machine has demand for 4 GB of committed RAM, and the Memory Buffer is 50% (the default value is 20%), then the virtual machine will be allocated 6 GB of RAM (4 GB + 50% of 4 GB). (Source)

 

Here there are three new columns that show you what’s going on with the memory of a virtual machine:

 

• Assigned Memory: shows how much physical memory is being used by the virtual machine.
  
• Memory Demand: shows how much memory the virtual machine really needs. This is equal to the amount of committed memory in the guest operating system.
  
• Memory Status: this indicates how much of the buffer is free.
  

Memory Status can have one of three values:

 

• OK: there is enough free space to provide the full buffer
  
• Low: Hyper-V believes that the virtual machine should have a larger buffer than is currently assigned
  
• Warning: There isn’t enough physical memory on the host to supply the buffer
  

Edited April 12, 2012 by Arthur

[Rozzer]

Trust me the server has over 24GB free of ram. I have startup at 512MB which I am thinking maybe best to up to 1024MB and most servers have max of 4096GB. Set middle priority and 20% buffer although 5% on SQL servers.

 

Ross

[Rozzer]

After a bit more investigating I noticed that there were a lot of iSCSI error on one of my virtual hosts. So I have updated the nic drivers and updated the server and so far no errors.

 

Fingers crossed!!

 

Thanks for everyones help.

 

Ross