[Kimchi-devel] [RFC] Memory HotPlug changes to fix issues

[Aline Manera]

On 02/25/2016 03:15 PM, Rodrigo Trujillo wrote:
> Hi all,
>
> with latest changes in Kimchi backend/frontend (enable Memory 
> hotplug[UI] and support to update MaxMemory) I am facing
> a new issue related to maxMemory:
>
> How is done today:
> - whenever a user adds memory when guest is running, Kimchi is going 
> to add enough 1GiB Memory Device to guest XML;
> - when user changes the Memory in a offline guest, Kimchi is going to 
> change the value in  "NUMA -> Memory"  tag, which becomes 
> "currentMemory" tag
> - the value in "NUMA -> Memory" plus the sum of 1GiB Memory Devices 
> (DIMM) added , become the "memory" tag
>
> Problem:
> - the memory devices added are persistent, once added user will never 
> be able to remove them, except editing the guest XML;
> - libvirt does not provide memory hot-unplug;
> - so, when user turns off the guest that has memory devices and try to 
> increase or decrease the amount of memory,
>   the new value is going to be be sum with memory devices amount, 
> causing most of the time to errors with value of "maxMemory" configured.
>
> Example1:
> - Guest has configured 4GB maxMemory,  2GB currentMemory|Numa->memory 
> , 0 Memory Devices, 2GB memory
> - Run guest and add 2GB -> 4GB maxMemory,  2GB 
> currentMemory|Numa->memory,  2x1GB Memory Devices, 4GB memory
> - Stop guest and edit it. The memory field is going to show 4GB
> - Change memory fileld to 3GB (decrease) -> 4GB maxMemory,  3GB 
> currentMemory|Numa->memory,  2x1GB Memory Devices,  5GB memory
> Notice the error: memory > maxMemory
>
> Example2:
> - Guest has configured 4GB maxMemory,  1GB currentMemory|Numa->memory 
> , 0 Memory Devices, 1GB memory
> - Run guest and add 2GB -> 4GB maxMemory,  1GB 
> currentMemory|Numa->memory,  2x1GB Memory Devices, 3GB memory
> - Stop guest and edit it. The memory field is going to show 3GB
> - Change memory field to 4GB (increase) -> 4GB maxMemory, 4GB 
> currentMemory|Numa->memory,  2x1GB Memory Devices,  6GB memory
> Notice the error: memory > maxMemory
>
> ------
>
> Possible Solution 1:
> - Make memory devices NOT persistent. Then when guest is turn off, 
> memory back to lower values configured before the memory hotplug action;
>
> Possible Solution 2:
> - Change memory update function to check if there are memory devices 
> configured and adjust the number of devices according to the new value 
> passed.
> Ex.:
> - Stopped guest has:   6GB maxMemory,  2GB 
> currentMemory|Numa->memory,  2x1GB Memory Devices, 4GB memory
> - Change memory field to 3GB (decrease) -> 4GB maxMemory, 2GB 
> currentMemory|Numa->memory,  1x1GB Memory Devices,  3GB memory
> Notice: memory device was removed instead of change the currentMemory
>
>
> Solution 1 is simpler and quicker to implement.
> Solution 2 seems to be the best, but will demand more time to code and 
> specially test.
>

If I can choose I'd rather to go on solution 2 =)
But we are one week to code freeze - not sure it will be enough to get 
it in.


> What you think ?
>
> Rodrigo Trujillo
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