Maximum: Hot-Add memory for this power on -- limited to 2GB?

[CornellU]

I have noticed that with our RedHat x64 VMs created with 1GB of memory (hot-add enabled) - only support a hot-add of 2GB of memory.  It was my understanding that the default hot-add value should be 16x the powered on memory value.  I did confirm that this was the case with newly created Windows 2008 x64 VMs.  I created a new Windows VM (hot-add enabled) powered it on with 1 GB of memory and was able to hot-add up to 16GB.

Configuring a linux VM and powering on with 1GB only allows hot-add too a max of 3GB.  I can of course add more if I power the VM off.  In fact this is the case with any linux VM powered on with less than 3 GB of memory, hot-add then only supports an increase to 3GB.

If I configure a new linux VM with say 4GB of memory I am able to hot-add up to 64GB (validating the x16 hot-add factor).

Anyone else encounter this, or have any VMware docs to this effect?

Jason

[Darius Davis]

Hi Jason,

Memory hot-add is disabled for some scenarios and situations where there is a high likelihood of the operation causing instability in the guest.  There are various problems within various versions of Linux guest kernels when a memory hot-add operation would result in the addition of physical memory above the 4 GByte address mark where previously all of the memory fit below 4 GBytes.  Since 1 GByte of physical address space below 4 GBytes is reserved for the PCI hole, this means that a memory hot add from 3072 MBytes or below to 3076 MBytes or above will meet the failure conditions.

For one example, see this article in the VMware KB: Linux Guest OS may become unresponsive after hot-adding memory from under 3 GB to over 3 GB.

As described in that article, you will need to power-off the guest and "cold-add" the memory when you cross the boundary – otherwise the guest would most likely malfunction and/or crash.

Hope this helps!

--

Darius