In previous releases of vSphere, scaling up resources such as storage or memory for vCenter Server was a huge pain-point for our customers. Before the various vCenter Server services can consume the new resources, some additional manual steps were required. Though this type of an operation is usually infrequent, there is still an operational overhead which can potentially lead to increased downtime of your vCenter Server.

For example, increasing storage capacity for the VCSA was an offline operation that required adding an additional disk and then copying the existing content to the new disk which can be quite error prone and lead to a significant amount of downtime. In vSphere 6.0, the VCSA now uses LVM which provides the ability for online storage capacity increase without any downtime to vCenter Server. Increasing memory was also challenging because you had to manually adjust several configuration files that manages the JVM heap settings for various vCenter Server services as described in this VMware KB. Having complex workflows to perform basic resource expansion can increase risk of errors, especially when the process is foreign to those performing it for the very first time.

To help solve this problem, in vSphere 6.0 vCenter Server (Windows & VCSA) now includes a built-in dynamic memory reconfiguration process that automatically runs at boot up. This process includes a dynamic algorithm that inspects the current amount of CPU, Memory and Storage that is available to determine the appropriate size to configure the vCenter Server. This means that if you no longer have to tweak individual JVM settings for the various services within vCenter Server, this will happen automatically by analyzing the resources that are available and then calculating the configuration based on the supported maximums for vCenter Server.

Note: In vSphere 6.0, there are additional services going beyond just the core vCenter Server, vSphere Web Client, vCenter SSO and Inventory Services.

The dynamic memory algorithm is configured to understand the minimal amount of resources for running a vCenter Server and is bounded between a "Tiny" configuration which is 2vCPU and 8GB memory and a "Large" configuration which is 16vCPU and 32GB memory. This is important to note because if you try to configure the vCenter Server with less memory than the minimal supported, though the algorithim will dynamically distribute the available memory to the various resources, it could lead to performance degradation as the different services may not be receiving the amount of memory they require to run. YMMV if you decide to reduce the supported amount of memory but the algorithm will distribute what's available.

The process which does all the magic is a utility called cloudvm-ram-size and there are several useful options to be aware of. To view the current memory assignment for the various vCenter Server services including the OS, you can run the following command on the VCSA as an example:

cloudvm-ram-size -l

From the screenshot above, we can see a very simple break down of the current memory assignment for a "Tiny" deployment which has 8GB of memory.

To show that the dynamic memory algorithm is in fact running when more memory is added, the example below is of a VCSA that was initially configured with 8GB of memory. I then capture the running configuration and then shut down the vCenter Server and increased its memory to 10GB. I then power on the VCSA and capture the running state and you can see differences in the screenshot below.

Another useful command to be aware is being able to see the current memory usage for all services. You can do this by running the following command:

cloudvm-ram-size -S

As you can see the dynamic memory algorithm is a very much welcome feature for vCenter Server and will greatly simplify the operational tasks when needing to scale up or down resources such as CPU and Memory. I know this is definitely one of the enhancements I have been waiting for and I am glad to see it here in the new vSphere 6.0 release! As of right now, a system reboot is required but who knows maybe in the future we can increase memory while the VCSA is still running and simply reloading the services ...