Recently VMware announced via the VMware Security Blog, that Transparent Page Sharing (TPS) will be disabled by default in an upcoming update of ESXi.
Since this announcement I have been asked how will this impact sizing vSphere solutions and as a result I’ve been involved in discussions about the impact of this on Business Critical Application, Server and VDI solutions.
Firstly what benefits does TPS provide? In my experience, in recent times with large memory pages essentially not being compatible with TPS, even for VDI environments where all VMs are running the same OS, the benefits have been minimal, in general <20% if that.
Memory overcommitment in general is not something that can achieve significant savings from because memory is much harder to overcommit than CPU. Overcommitment can be achieved but only where memory is not all being used by the VM/OS & Applications, in which case, simply right sizing VMs will give similar memory saving and likely result in better overall VM and cluster performance.
So to begin, in my opinion TPS is in most cases overrated.
Next Business Critical Applications (vBCA):
In my experience, Business Critical Applications such as MS Exchange, MS SQL , Oracle would generally have memory reservations, and in most cases the memory reservation would be 100% (All Memory Locked).
As a result, in most environments running vBCA’s, TPS has no benefits already, so TPS being disabled has no significant impact for these workloads.
Next End User Computing (EUC) Solutions:
There are a number of EUC solutions, such as Horizon View , Citrix XenDesktop and Citrix PVS which all run very well on vSphere.
One common issue with EUC solutions is architects fail to consider the vSwap storage requirements for Virtual Servers (for Citrix PVS) or VDI such as Horizon View.
As a result, a huge amount of Tier 1 storage can be wasted with vswap file storage. This can be up to the amount vRAM allocated to VMs less memory reservations!
The last part is a bit of a hint, how can we reduce or eliminate the need for Tier 1 storage of vSwap? By using Memory Reservations!
While TPS can provide some memory savings, I would invite you to consider the cost saving of eliminating the need for vSwap storage space on your storage solution, and the guarantee of consistent performance (at least from a memory perspective) outweigh the benefits of TPS.
Next Virtual Server Solutions:
Lets say we’re talking about general production servers excluding vBCAs (discussed earlier). These servers are providing applications and functions to your end users so consistent performance is something the business is likely to demand.
When sizing your cluster/s, architects should size for at least N+1 redundancy and to have memory utilization around the 1:1 mark in a host failure scenario. (i.e.: Size your cluster assuming a host failure or maintenance of one host is being performed).
As a result, any reasonable architectural assumption around TPS savings would be minimal.
As with EUC solutions, I would again invite you to consider the cost saving of eliminating the vSwap storage requirement and the guarantee of consistent performance outweigh the benefits of TPS.
Next Test/Dev Environments:
This is probably the area where TPS will provide the most benefit, where memory overcommitment ratios can be much higher as the impact to the applications(VMs) of memory saving techniques such as swapping/ballooning should not have as high an impact on the business as with vBCA, EUC or Server workloads.
However, what is Test/Dev for? In my opinion, Test/Dev should where possible simulate production conditions so the operational verification of an application can be accurately conducted before putting the workloads into production. As such, the Test/Dev VMs should be configured the same way as they are intended to be put into production, including Memory Reservations and CPU overcommitment.
So, can more compute overcommitment be achieved in Test/Dev, sure, but again is the impact of vSwap space, potentially inconsistent performance and the increased risk of operational verification not being performed to properly simulate product worth the minimal benefits of TPS?
If VMware believe TPS is a significant enough security issue to make it disabled by default, this is something architects should consider, however I would argue there are many other areas where security is a much larger issue, but that’s a different topic.
TPS being disabled by default is likely to only impact a small percentage of virtual workloads and with RAM being one of the most inexpensive components in the datacenter, ensuring consistent performance by using Memory Reservations and eliminating the architectural considerations and potentially high storage costs for VMs vSwap make leaving TPS disabled an attractive option regardless of if its truly a security advantage or not.