vSAN storage polices define storage requirements for your virtual machines. These policies determine how storage is allocated to management and workload VMs.

The first cluster created in your VMware Cloud on Public Cloud SDDC includes two vSAN datastores: one for the management VMs (vsanDatastore) and one for the workload VMs (WorkloadDatastore). Both datastores share the same underlying storage devices and consume from the same pool of free space. Additional clusters created in the SDDC include only a WorkloadDatastore.

Each virtual machine deployed to a vSAN datastore is assigned at least one virtual machine storage policy. You can assign storage policies when you create or reconfigure virtual machines.

Availability Attributes for vSAN VM Storage Policies

Site disaster tolerance
Defines the data redundancy method used by stretched clusters to handle a site failure. This attribute applies to stretched clusters. If you have a standard vSAN cluster, choose None (standard cluster).
The options are:
  • None (standard cluster)
  • Dual-site mirroring (stretched cluster)
  • None - Keep data on primary (stretched cluster)
  • None - Keep data on secondary (stretched cluster)
Failures to tolerate
Defines the number of host and device failures that a virtual machine can tolerate. You can choose to have no data redundancy, or select a RAID configuration optimized for either performance (Mirroring) or capacity (Erasure Coding).
  • RAID-1 uses more disk space but provides better performance.
  • RAID-5/6 (Erasure Coding) uses less disk space, but the performance is reduced.
Table 1. RAID Configurations, FTT, and Host Requirements
RAID Configuration Failures to Tolerate (FTT) Minimum Hosts Required
RAID-1 (Mirroring) This is the default setting. RAID-1 1 3
RAID-5 (Erasure Coding) 1 4
RAID-1 (Mirroring) 2 5
RAID-6 (Erasure Coding) 2 6
RAID-1 (Mirroring) 3 7
Important: Use of a VM storage policy with FTT = 0 (No Data Redundancy) is not recommended and can cause data loss if there is a host failure or if the VM becomes unresponsive.

Advanced Attributes for vSAN VM Storage Policies

Number of disk stripes per object
Defines the minimum number of storage devices across which each replica of a virtual machine object is striped. A value higher than 1 might result in better performance, but also makes more use of system resources. Default value is 1. Maximum value is 12. Change the default value only when recommended by VMware support.
IOPS limit for object
Defines the IOPS limit for an object, such as a VMDK. IOPS is calculated as the number of I/O operations, using a weighted size. If the system uses the default base size of 32 KB, a 64-KB I/O represents two I/O operations.

When calculating IOPS, read and write are considered equivalent, but cache hit ratio and sequentiality are not considered. If a disk’s IOPS exceeds the limit, I/O operations are throttled. If the IOPS limit for object is set to 0, IOPS limits are not enforced.

vSAN allows the object to double the rate of the IOPS limit during the first second of operation or after a period of inactivity.

Object space reservation

This setting defines the percentage of the logical size of the virtual machine disk (vmdk) object that must be reserved (provisioned) when deploying virtual machines. The default reservation value in VMware Cloud on Public Cloud is 0% (Thin provisioning). You can specify Thick provisioning to reserve capacity for larger-than-expected vSAN writes, but the underlying vmdk structure remains the same as it is in the Thin provisioning configuration, and is not the same as the Thick provision eager zeroed provisioning model available on-premises.

You should consider setting the Object Space Reservation (OSR) advanced policy setting to Thin provisioning. OSR controls only the space reservation and has no performance impact. Although capacity management is often critical for on-premises data centers, VMware Cloud on Public Cloud Elastic DRS ensures the cluster will not run out of free space.

Flash read cache reservation
This setting is ignored in VMware Cloud on Public Cloud. In Hybrid vSAN deployments, it designates how much flash capacity is reserved as read cache.
Disable object checksum
If the option is set to No, the object calculates checksum information to ensure the integrity of its data. If this option is set to Yes, the object does not calculate checksum information.

vSAN uses end-to-end checksum to ensure the integrity of data by confirming that each copy of a file is exactly the same as the source file. The system checks the validity of the data during read/write operations, and if an error is detected, vSAN repairs the data or reports the error.

If a checksum mismatch is detected, vSAN automatically repairs the data by overwriting the incorrect data with the correct data. Checksum calculation and error-correction are performed as background operations.

The default setting for all objects in the cluster is No, which means that checksum is enabled.

Force provisioning
If the option is set to Yes, the object is provisioned even if the Primary level of failures to tolerate, Number of disk stripes per object, and Flash read cache reservation policies specified in the storage policy cannot be satisfied by the datastore. Use this parameter in bootstrapping scenarios and during an outage when standard provisioning is no longer possible.

The default No is acceptable for most production environments. vSAN fails to provision a virtual machine when the policy requirements are not met, but it successfully creates the user-defined storage policy.