Sunday, 10 May 2009

Hot Add CPU - Vsphere

Seems a few peeps are playing...I mean evaluating the new hot add CPU/RAM feature within VSphere.

To summarize the following OS's only support hot add CPU/RAM regardless of whether a physical or virtual machine;
  • Windows 2003 Enterprise (RAM Only) and Datacenter (CPU & RAM)
  • Windows 2008 Enterprise (RAM Only) and Datacenter (CPU & RAM)
  • Linux running kernel 2.6.14 or above
When looking at how the hot add cpu feature works within the Windows OS it does appear that the running application and processes which are not system or SVCHOST based do not benefit on first plug of the CPU basically meaning that a reboot is required to ensure that the OS Can use the CPU Effectively and applications running as processes other than the supported can actually use the CPU's, I presume this would mean apps like MS Exchange with Store.exe or SQL servers process which are not currently going to benefit immediately.

This is something you will need to take into consideration as to whether your hot add of a CPU actually is going to benefit. Microsoft SQL 2008 Supports hot addition manually after first plug, so I presume most of Microsoft's future backoffice applications will too, not so sure how current MS applications likely to be virtualised will cope though.

Below are some links on more detailed information and resources i've picked this from for you to look in more detail. b4085a80e34e/SVR-T325_WH07.pptx

POST UPDATED 23.07.09 with more clarity on features supported on Enterprise OS's

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Very interesting post Daniel, do you know if the RAM hotplug has any case like this one ?

Not sure I understand the question, RAM hotplug seems to work perfectly ok on supported OS's.

If this dosnt answer the question then do comment again or email me.


Windows Storage Server 2008 x64 Enterprise also supports hot-add of Memory without a reboot but requires an add-plus-reboot to make a Hot-Plug of CPU effective. This is, of course, after the VM has previously had HotAdd/HotPlug engaged at a VM powered off state.


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