An existing vSAN OSA cluster has this specification:Four ESXi hosts with all flash configurationEach with...

Adding one more disk group per host with the same configuration and adding an ESXi host with identical device configuration are the two methods that the vSAN administrator should use to meet the goal of increasing capacity and performance with the least amount of impact. Adding one more disk group per host will increase the raw storage capacity by 20% and also improve the performance by distributing the I/O load across more cache devices and disk groups. Adding an ESXi host with identical device configuration will increase the raw storage capacity by 25% and also improve the performance by adding more compute and network resources to the cluster. Both methods can be done without disrupting any ongoing operations or requiring any data evacuation or resynchronization.

The other options are incorrect for the following reasons:

Replacing all capacity devices with a similar larger device is incorrect because it will not increase the performance and will have a significant impact on the cluster. Replacing the capacity devices requires deleting the disk groups, which will erase all data on them and trigger a resynchronization of the affected objects. This can be disruptive and time-consuming, and also introduce additional network and disk traffic.

Replacing all cache devices with a larger device is incorrect because it will not increase the capacity and will have a significant impact on the cluster. Replacing the cache devices also requires deleting the disk groups, which will have the same drawbacks as replacing the capacity devices. Moreover, increasing the cache size may not improve the performance significantly, as vSAN OSA uses afixed cache ratio of 70% for write buffer and 30% for read cache, regardless of the cache device size.

Adding faster cache devices is incorrect because it will not increase the capacity and will have a significant impact on the cluster. Adding faster cache devices also requires deleting the disk groups, which will have the same drawbacks as replacing the cache devices. Furthermore, adding faster cache devices may not improve the performance significantly, as vSAN OSA uses a fixed cache ratio of 70% for write buffer and 30% for read cache, regardless of the cache device speed. References:

VMware vSAN Specialist v2 Exam Preparation Guide, page 10

Expanding a vSAN Cluster