Virtual SAN Policy Design
After Virtual SAN is enabled and configured, storage policies that define the virtual machine storage characteristics must be created. These characteristics allow configuration of different levels of service to be provided to a virtual machine. If no specific policy is applied, a default policy tolerating a single failure and having a single disk stripe is applied. VMware recommends that the default policy be used, unless there are specific business requirements that need to be met. Creating multiple policies is one of the major benefits of Virtual SAN, which is that it allows for policies to be defined on a per virtual machine basis, and changed on the fly.
Virtual SAN enforces policies that have been configured. Therefore, if a policy cannot be met, virtual machine provisioning will fail, unless force provisioning has been set. Almost any combination of the possible attribute capabilities can be set in a policy. An understanding of the policies that can be set allows the administrator to avoid configuring policies that do not make sense.
Some additional benefits of Virtual SAN policy application include the ability to configure policies at any time and to switch policies on the fly for virtual machines.
In this section, the following topics are discussed:
Virtual SAN policy options
General policy design recommendations
Virtual SAN Policy Options
Virtual SAN allows you to set several different policy attributes in a storage policy. These attributes can be used alone or combined to provide different service levels.
Before making design decisions, understand the policies and the objects to which they can be applied. The policy options are listed in the following table.
Table 71. Virtual SAN Policy Options
| Capability | Use Case | Value | Comments |
|---|---|---|---|
| Number of failures-to-tolerate | Redundancy | Default 1 | This is a standard RAID 1 mirrored configuration that provides redundancy for a virtual machine disk. The higher the value, the more failures can be tolerated. For n failures tolerated, n+1copies of the disk are created, and 2n+1 hosts contributing storage is required. |
| Number of disk stripes per object | Performance | Default 1 | This is a standard RAID 0 stripe configuration used to increase performance for a virtual machine disk. |
| Flash read cache reservation (%) | Performance | Default 0 | Flash capacity reserved as read cache for the storage is a percentage of the logical object size that will be reserved for that object. |
| Object space reservation (%) | Thick provisioning | Default 0 | The percentage of the storage object that will be thick provisioned upon VM creation. The remainder of the storage will be thin provisioned. |
| Force provisioning | Override policy | Default: | Force provisioning allows for provisioning to occur even if the policy configured cannot be satisfied by the currently available cluster resources. |
By default, policies are configured based on the application requirements. However, they are applied differently depending on the object.
Table 72. Object Policy Defaults
| Object | Policy | Comments |
|---|---|---|
| Virtual machine namespace | 1 Failures-to-Tolerate | Configurable. However, changes are not recommended. |
| Swap | 1 Failures- to-Tolerate | Configurable. However, changes are not recommended. |
| Virtual disk(s) | User-Configured Storage Policy | Can be any storage policy configured on the system. |
| Virtual disk snapshot(s) | Uses Virtual Disk Policy | Same as virtual disk policy by default. Changes are not recommended. |
| If no user-configured policy is configured the default system policy of 1 failure to tolerate and 1 disk stripe is used for virtual disk(s) and virtual disk snapshot(s). | |
|---|---|
Policy defaults for the VM namespace and swap are set statically (and are not configurable) so there is appropriate protection for these critical virtual machine components.
| Policies must be configured based on the application’s business requirements. This feature gives Virtual SAN its power because it can adjust how a disk performs on the fly, based on the policies configured. | |
|---|---|
Table 73. Policy Selection – Design Decisions
| Decision ID | Design Decision | Design Justification | Design Implication |
|---|---|---|---|
| For this design, <Customer> has made the following decisions listed in this table. | |||
| Use the default policy. |