How to successfully Virtualize MS Exchange – Part 3 – Memory

In Part 1 and Part 2, we discussed how to size and configure Exchange VMs to meet CPU requirements. In Part 3 we will focus on virtual memory (vRAM).

As Exchange 2013 is quite RAM intensive and is not unusual to have memory requirements of >128GB RAM in larger deployments. As such, one of the first things we should consider is Virtual Machine maximums.

Luckily in recent years the maximum VM size in vSphere has increased and is no longer a constraint for virtualizing even the largest of Exchange environments.

The current maximum vRAM configuration per VM is shown below:

vSphere Virtual Machine RAM Maximums

Maximum vRAM per VM1TB (vSphere 5.0 or later)
Maximum vRAM per VM: 255GB (vSphere 4.1)

The key point here is memory is in no way a constraining factor when virtualizing Exchange even in older vSphere 4.1 deployments.

Memory Sizing

For maximum Memory performance, sizing the Exchange VM within a NUMA node gives the maximum benefit from NUMA locality, meaning the latency between the CPU and RAM is minimized.

In the event the memory requirements exceed the NUMA node, consider scaling out until you have at least 4 Exchange VMs (across 4 ESXi hosts) before scaling Exchange VMs up. This ensures higher resiliency and aligns with a Virtualization friendly scale out approach. Once the environment has 4 or more Exchange VMs, scaling up beyond the size of a NUMA node can be a good option to reduce the number of Exchange instances to manage and license without significantly impacting resiliency.

Memory Overcommitment

ESXi has excellent memory overcommitment capabilities which can work very well depending on the Operating system and application running within the guest. However Exchange is generally considered a Business Critical Application and as such, overcommitting memory for Exchange is generally not a good idea and should be avoided where possible.

Memory Reservations

For Exchange VMs, I recommended configuring the VM with “All Memory Locked” or in other words, a 100% memory reservation.

This has two advantages, the first being consistent memory performance for MS Exchange which is critical to ensure a great end user experience.

The second is the potentially large storage saving as the vSwap file is eliminated. For example, if an Exchange VM has 128Gb RAM and no memory reservation, a 128Gb vSwap file will be created by default in the same Datastore as the VMs .vmx file which could impact storage sizing and performance.

ESXi Host / Cluster Sizing Considerations

Exchange VMs are typically larger than the average VM, as a result they can consume a significant percentage of an ESXi hosts memory resources. As a result it is important to size your ESXi hosts to have sufficient RAM for the Exchange VMs.

As such in cases where the Exchange VM is sized to exceed the NUMA node, I recommend sizing ESXi hosts to have at least 25% more physical RAM than the vRAM assigned to your Exchange VMs.

Example: If your Exchange VM is assigned 96Gb, the ESXi hosts in the cluster should have at least 128Gb. This ensures memory for the hypervisor and other smaller VMs such as Domain Controllers to service things like the global catalog requirements for Exchange without contention.


1. Set “All Memory Locked” (100% Memory Reservation) for Exchange VMs.
2. Where possible, size the Exchange VMs RAM within a NUMA node.
3. Where Exchange RAM requirements exceed that of the NUMA node ensure the size ESXi hosts to have at least 25% more RAM than the Exchange VM (or the largest vRAM VM in the cluster)
4. Ensure VMs vRAM is right sized after deployment to minimize waste (especially considering the recommendation to use memory reservations)

Back to the Index of How to successfully Virtualize MS Exchange.

How to successfully Virtualize MS Exchange – Part 1 – CPU Sizing

Part 1 will focus on CPU sizing for Exchange Mailbox (MBX) or Multi Server Role (MSR) deployments.

The Exchange 2013 Server Role Requirements Calculator v6.6 should be used to size the VM to ensure sufficient performance for the specific Exchange deployment.

One key input for the calculator is the “SpecInt2006 Rate Value” which can be found on the “Input” tab of the calculator  (shown below).


To find the SpecInt2006 Rate Value for your specific CPU, I recommend using the  Exchange Processor Query Tool which allows you to enter the Processor Model number of your servers and query the database for the rating of your CPU.

Note: This tool is applicable to Exchange 2010 and 2013 deployments despite the tool being titled “Exchange 2010 Processor Query Tool”.

To do this, enter the model number of your CPU (example E5-2697 v2 shown below) and press query.


The calculator will then return the list of tested server in the right hand side of the spreadsheet an example of this is shown below.


The SpecInt2006 result for your CPU is highlighted in Orange in the “Result” column.

At this stage the drop box in Step 4 allows you to choose the number of physical cores planned to be used and it will then return the average result of all tested servers.


The above result for example assumes a Dual Socket physical server with 12 core Intel E5-2697 v2 processors.

As we are discussing Virtualizing Exchange, Step 7 is applicable.

Here the tool allows you to enter the overcommitment (vCPU to Physical Core) and the number of vCPUs (called virtual processors in the spreadsheet) which then results in what the spreadsheet calls

“Virtual mailbox server SPECint2006 Rate Value” shown below in Orange.


The calculator makes the assumption that CPU overcommitment of 2:1 degrades performance by 50% which is not strictly true, but can be used as general guidance that high levels of CPU overcommitment which may lead to CPU contention are not recommended for MS Exchange deployments. It is important to note, CPU overcommitment ≠ CPU contention, although the higher the overcommitment, the higher the possibility of contention (CPU Ready).

Now that we have the SPECint2006 Rate Value, this can be entered into the Primary and Secondary (if applicable) field of the Exchange 2013 Server Role Requirements Calculator (shown below).


The SPECInt2006 value is for the physical processor, which if it supports Hyper-threading (HT), means the rating includes the performance benefit of HT. The key point here is using just physical cores for sizing means your VM will not get the full performance of the SPECint2006 rating, it will be slightly less. This will be discussed in more detail in Part 2 – vCPU configurations.

The “Processor Cores / Server” field should be populated by the physical cores intended to be used.

While the “Processor Cores / Server” value does not impact the CPU utilization calculations, entering the “Processor Cores / Server” allows the calculator to report the number Processor Cores Utilized as shown below from the “Role Requirements” tab.


The number of cores utilized helps calculate the number of vCPUs required for the Exchange VM. If the “Server CPU utilization” is much lower than 80% (recommended maximum), the “SPECInt2006 rate value” and “Processor Cores / Server” can be reduced.

Example: If the calculator reports Server CPU Utilization at 40% and the CPU Type is Intel E5-2697 v2 with 12 physical cores with a SPECint2006 rating of 479. The Virtual Machine should be sized with 6 vCPU. To confirm this the SPECint2006 rating for Primary and Secondary (if applicable) field of the Exchange 2013 Server Role Requirements Calculator can be reduced by 50% (from 479) too 239.5 which will result in the calculator reporting Server CPU Utilization at 80%.

Another option is to review the number of Mailbox Servers are configured, and where the utilization is low as in the previous example 40%, you could choose to “scale up” each of the Exchange VMs. To do this, change the highlighted field on the “Input” tab of the calculator to 4, and you will see the utilization under “Server configuration” increase (on the role requirements tab) to 80%.


Scaling up reduces the number of Windows/Exchange instances licenses and ongoing maintenance (such as patching) required, but also increases the failure domain and impact of a failure so this decision needs to not only be a architectural/technical one, but a business decision.

As a general rule, I recommend customers scale out until they have 4 or more Exchange VMs (across 4 or more ESXi hosts), then scale up (and out) as required. This ensures the impact of a server failure is 25%, compared to 50% if it was a scaled up Exchange server deployment with only 2 VMs.

An important consideration for any business critical application deployment is the scalability of the solution. In this case, when discussing virtualizing one or more Exchange servers, the Virtual Machine maximums are critical.

The below shows the maximum vCPUs supported for a VMware based virtual machine.

vSphere Virtual Machine CPU Maximums

Maximum vCPUs: 64 (vSphere 5.1 or later)
Maximum vCPUs: 32 (vSphere 5.0)
Maximum vCPUs: 8 (vSphere 4.1)

The above numbers are dependant on the physical hardware chosen.

Recommendations for CPU sizing:

1. CPU overcommitment be less than 2:1, and ideally 1:1 for hosts servicing Exchange workloads. This will be discussed further in this series.

Use the vSphere Cluster Sizing Calculator to confirm overcommitment ratios for your cluster or to validate your design.

2. Size Exchange Server VMs to less than 80% CPU Utilization

This allows for burst activity such as increased load or DAG failovers.

3. Scale up a single Exchange VM per ESXi host as opposed to running multiple smaller Exchange VMs per host.

4. Do not oversize Exchange VMs Day 1, Size for Day 1 demand and scale vCPUs as required (which can be done quickly and easily thanks to the virtual layer).
5. CPU reservations do not solve CPU scheduling contention (a.k.a CPU Ready). CPU reservations should not be required in properly sized environments.
6. Size Exchange VMs using Physical Cores and assume no benefit from HT
7. Leave HT turned on at the ESXi layer

Back to the Index of How to successfully Virtualize MS Exchange.

Example Architectural Decision – Transparent Page Sharing (TPS) Configuration for QA / Pre-Production Servers

Problem Statement

In a VMware vSphere environment, with future releases of ESXi disabling Transparent Page Sharing by default, what is the most suitable TPS configuration for an environment running Quality Assurance or Pre-Production server workloads?


1. TPS is disabled by default
2. Storage is expensive
3. Two Socket ESXi Hosts have been chosen to align with a scale out methodology.
4. Average Server VM is between 2-4vCPU and 4-8GB Ram with some larger.
5. Memory is the first compute level constraint.
6. HA Admission Control policy used is “Percentage of Cluster Resources reserved for HA”
7. vSphere 5.5 or earlier


1. The environment must deliver consistent performance
2. Minimize the cost of shared storage


1. Reduce complexity where possible.
2. Maximize the efficiency of the infrastructure

Architectural Decision

Leave TPS disabled (default) and leave Large Memory pages enabled (default).


1. QA/Pre-Production environments should be as close as possible to the configuration of the actual production environment. This is to ensure consistency between QA/Pre-Production validation and production functionality and performance.
2. Setting 100% memory reservations ensures consistent performance by eliminating the possibility of swapping.
3. The 100% memory reservation also eliminates the capacity usage by the vswap file which saves space on the shared storage as well as reducing the impact on the storage in the event of swapping.
4. RAM is cheaper than Tier 1 storage (which is recommended for vSwap storage to ensure minimal performance impact during swapping) so the increased cost of memory in the hosts is easily offset by the saving in Tier 1 shared storage.
5. Simplicity. Leaving default settings is advantageous from both an architectural and operational perspective.  Example: ESXi Patching can cause settings to revert to default which could negate TPS savings and put a sudden high demand on storage where TPS savings are expected.
6. TPS savings for server workloads is typically much less than with desktop workloads and as a result less attractive.
7. The decision has been made to use 2 socket ESXi hosts and scale out so the TPS savings per host compared to a 4 socket server with double the RAM will be lower.
8. HA admission control will calculate fail-over requirements (when using Percentage of cluster resources reserved for HA) so that performance will be approximately the same in the event of a fail-over due to reserving the full RAM reserved for every VM leading to more consistent performance under a wider range of circumstances.
9. Lower core count (and lower cost) CPUs will likely be viable as RAM will likely be the first constraint for further consolidation.
10. Remove the real or perceived security risk of sensitive information being gathered from other VMs using TPS as described in VMware KB 2080735


1. Using 100% memory reservations requires ESXi hosts and the cluster be sized at a 1:1 ratio of vRAM to pRAM (Physical RAM) and should include N+1 so a host failure can be tolerated.
2. Increased RAM costs
3. No memory overcommitment can be achieved
4. Potential for lower CPU utilization / overcommitment as RAM may become the first constraint.


1. Use 50% reservation and enable TPS
2. Use no reservation, Enable TPS and disable large pages

Related Articles:

1. Transparent Page Sharing (TPS) Example Architectural Decisions Register

2. The Impact of Transparent Page Sharing (TPS) being disabled by default @josh_odgers (VCDX#90)

3. Future direction of disabling TPS by default and its impact on capacity planning –@FrankDenneman (VCDX #29)

4. Transparent Page Sharing Vulnerable, Yet Largely Irrelevant – @ChrisWahl (VCDX#104)