VMware vSphere Metro Storage Cluster (vMSC) is a feature that extends VMware HA with active-active stretched storage. VMware HA, as stated in the previous section, requires shared storage to be presented to all hosts in a cluster to enable restart of virtual machines in the event of a disaster. Currently, VMware only supports VMFS for vMSC.
In many scenarios, a cluster might have hosts in two entirely separate datacenters or geographical locations. For VMware HA to restart a VM on a host in the cluster that is in a different datacenter, that host must see that storage too. There are a few options to achieve this.
Scenario 1: Two-site host cluster, single-site storage
It is possible to cross-connect a storage array in one datacenter to hosts in its own datacenter and a second datacenter. Therefore, storage is only provided by one site. If the power goes out (for example) in site B, no hosts will have access to the storage. Consequently, a one-site storage configuration does not really provide much additional resiliency.
Figure 1. Scenario 1: Stretched host cluster with non-stretched/single-site storage
Scenario 2: Two-site host cluster, single-site storage in a third site
Another option is to put an array in a third site and provide access to the hosts in two other sites to that storage. While there are many obvious problems with this one, the main one is that this does not protect against is the array failures or a network partition of site C. Either of these failures will cause all hosts to lose access.
Figure 2. Scenario 2: Stretched host cluster with non-stretched/single-site storage in third site
Scenario 3: Two-site host cluster, dual-site storage
The third option is stretched storage. In this situation, a storage array is in site A and another array is in site B. Both sites have hosts that can see one or both arrays. One or more storage volumes are created on one array and an exact synchronous copy is created on the second array. All writes are synchronously mirrored between the arrays so the volume appears as the same in both sites and can be read from and written to simultaneously. If hosts in datacenter A fail (or the storage, or the entire datacenter), hosts in datacenter B can take over their workloads by using the array in datacenter B. This is the scenario that will be discussed in this paper.
Figure 3. Scenario 3: Stretched host cluster with stretched/dual-site storage
The combination of stretched storage AND stretched ESXi clusters provides an extremely high level of resiliency to an infrastructure. When the automatic failover and recovery features offered by VMware vSphere HA is enabled on top of these two topologies, RTO is reduced even further as VMware can quickly and intelligently respond and react to host, storage or site failures to bring virtual machines back online.