This is a quick blog to discuss where vSphere is at with memory management today. vSphere has many mechanisms to reclaim memory before resorting to paging to disk. Let’s briefly look at these methods.

Memory Reclamation

• Transparent Page Sharing (TPS)
• Think of this as deduplication for memory. Identical pages of memory are shared with many VM’s instead of provisioning a copy of that same page to all VM’s. This can have a tremendous impact on the amount of RAM used on a given host if there are many identical pages.
• Balooning
• This method increases the memory pressure inside the guest so that memory that is not being used can be reclaimed. If the hypervisor were to just start taking memory pages from guests, the guest Operating Systems would not react positively to that. So, balooning is a way to place artificial pressure on the guest VM so that the VM pages unused memory to disk. Then, the hypervisor can reclaim that memory without disrupting the guest OS.
• Memory compression
• This method attempts to compress memory pages that would normally be swapped out via hypervisor swapping. This is preferable to swapping as there can be a performance impact when memory is swapped to disk.
• Hypervisor swapping
• This is the last resort for memory management. The memory pages are swapped to disk. New in vSphere 5 is the support for swapping these memory pages to SSD’s. This increases the performance when swapping is needed.

As you can see there are many memory management techniques in vSphere that allow greater consolidation ratios. The hypervisor in the virtual infrastructure does much more than just host guest VM images. There is a lot going on under the hood to consider before choosing a specific hypervisor to serve as the foundation for your infrastructure. Feel free to contact me if you would like to discuss any of the “under the hood” features of vSphere.