Last week I received the question how exactly Storage DRS picks a datastore.

On a SDRS the initial placement of a vm is done on the weight calculated based on the storage free and IO. My question is: when I have a similar weight between all the datastore in the cluster, which datastore is choose for the initial placement?

Storage DRS takes the virtual machine configuration into account, the platform & user-defined constraints and the resource utilization of the datastores within the cluster. Let’s take a closer look at the Storage DRS initial placement workflow.
User-defined constraint
When selecting the datastore cluster as a storage destination, the default datastore cluster affinity rule is applied to the virtual machine configuration. The datastore cluster can be configured with a VMDK affinity rule (Keep files together) or a VMDK anti-affinity rule (Keep files separated). Storage DRS obeys the affinity rule and is forced to find a datastore that is big enough to store the entire virtual machine or the individual VMDK files. The affinity rule is considered to be a user-defined constraint.
Platform constraint
The next step in the process is to present a list of valid datastores to the Storage DRS initial placement algorithm. The Storage DRS placement engine checks for platform constraints.
The first platform constraint is the check of the connectivity state of the datastores. Fully connected datastores (datastores connected to all host in the compute cluster) are preferred over partially connected datastores (datastores that are not connected to all host in the cluster) due to the impact of mobility of the virtual machine in the compute cluster.
The second platform constraint is applicable to thin-provisioned LUNs. If the datastore exceeds the thin-provisioning threshold of 75 percent, the VASA provider (if installed) triggers the thin-provisioning alarm. In response to this alarm Storage DRS removes the datastores from the list of valid destination datastores, in order to prevent virtual machine placement on low-capacity datastores.
Resource utilization
After the constraint handling, Storage DRS sorts the valid datastores in order of combined resource utilization rate. The combined resource utilization rate consists of the space utilization and the I/O utilization of a datastore. The best-combined resource utilization rate is a datastore that has a high level of free capacity and Low I/O utilization. Storage DRS selects the datastore that has the best-combined utilization rate and attempts to place the virtual machine. If the virtual machine is configured with a VMDK anti-affinity rule, Storage DRS starts with placing the biggest VMDK first.