After you enable and configure VMware vSAN, you can create storage policies that define the virtual machine storage characteristics. Storage characteristics specify different levels of service for different virtual machines.

The default storage policy tolerates a single failure and has a single disk stripe. Use the default policy. If you configure a custom policy, vSAN should guarantee its application. However, if vSAN cannot guarantee a policy, you cannot provision a virtual machine that uses the policy unless you enable force provisioning.

VMware vSAN Policy Options

A storage policy includes several attributes, which can be used alone or combined to provide different service levels. Policies can be configured for availability and performance conservatively to balance space consumed and recoverability properties. In many cases, the default system policy is adequate and no additional policies are required. Policies allow any configuration to become as customized as needed for the application’s business requirements.

Policy Design Background

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 1. VMware vSAN Policy Options
Capability Use Case Default Value Maximum Value Comments
Number of failures to tolerate Redundancy 1 3 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+1 copies of the disk are created, and 2n+1 ESXi hosts contributing storage are required.

A higher n value indicates that more replicas of virtual machines are made, which can consume more disk space than expected.

Number of disk stripes per object Performance 1 12 A standard RAID 0 stripe configuration used to increase performance for a virtual machine disk.

This setting defines the number of HDDs on which each replica of a storage object is striped.

If the value is higher than 1, increased performance can result. However, an increase in system resource usage might also result.

Flash read cache reservation (%) Performance 0% 100% Flash capacity reserved as read cache for the storage is a percentage of the logical object size that is reserved for that object.

Only use this setting for workloads if you must address read performance issues. The downside of this setting is that other objects cannot use a reserved cache.

VMware recommends not using these reservations unless it is absolutely necessary because unreserved flash is shared fairly among all objects.

Object space reservation (%) Thick provisioning 0% 100% The percentage of the storage object that will be thick provisioned upon VM creation. The remainder of the storage will be thin provisioned.

This setting is useful if a predictable amount of storage will always be filled by an object, cutting back on repeatable disk growth operations for all but new or non-predictable storage use.

Force provisioning Override policy No - Force provisioning forces provisioning to occur even if the currently available cluster resources cannot satisfy the current policy.

Force provisioning is useful in case of a planned expansion of the vSAN cluster, during which provisioning of VMs must continue. VMware vSAN automatically tries to bring the object into compliance as resources become available.

By default, policies are configured based on application requirements. However, they are applied differently depending on the object.

Table 2. Object Policy Defaults
Object Policy Comments
Virtual machine namespace Failures-to-Tolerate: 1 Configurable. Changes are not recommended.
Swap Failures-to-Tolerate: 1 Configurable. Changes are not recommended.
Virtual disks User-Configured Storage Policy Can be any storage policy configured on the system.
Virtual disk snapshots Uses virtual disk policy Same as virtual disk policy by default. Changes are not recommended.
Note: If you do not specify a user-configured policy, vSAN uses a default system policy of 1 failure to tolerate and 1 disk stripe for virtual disks and virtual disk snapshots. To ensure protection for these critical virtual machine components, policy defaults for the VM namespace and swap are set statically and are not configurable. Configure policies according to the business requirements of the application. By using policies, vSAN can adjust the performance of a disk on the fly.

Policy Design Recommendations

Policy design starts with assessment of business needs and application requirements. Use cases for VMware vSAN must be assessed to determine the necessary policies. Start by assessing the following application requirements:

  • I/O performance and profile of your workloads on a per-virtual-disk basis
  • Working sets of your workloads
  • Hot-add of additional cache (requires repopulation of cache)
  • Specific application best practice (such as block size)

After assessment, configure the software-defined storage module policies for availability and performance in a conservative manner so that space consumed and recoverability properties are balanced. In many cases the default system policy is adequate and no additional policies are required unless specific requirements for performance or availability exist.

Table 3. Design Decisions on the vSAN Storage Policy

Decision ID

Design Decision

Design Justification

Design Implication

SDDC-VI-Storage-SDS-012

When using a single availability zone, use the default VMware vSAN storage policy.

Provides the level of redundancy that is needed in the management cluster.

Provides the level of performance that is enough for the individual management components.

You might need additional policies for third-party VMs hosted in these clusters because their performance or availability requirements might differ from what the default VMware vSAN policy supports.

SDDC-VI-Storage-SDS-013

When using two availability zones, add the following setting to the default vSAN storage policy:

Secondary Failures to Tolerate = 1

Provides the necessary protection for virtual machines in each availability zone, with the ability to recover from an availability zone outage.

You might need additional policies if third-party VMs are to be hosted in these clusters because their performance or availability requirements might differ from what the default VMware vSAN policy supports.

SDDC-VI-Storage-SDS-014

Leave the default virtual machine swap file as a sparse object on VMware vSAN.

Creates virtual swap files as a sparse object on the vSAN datastore. Sparse virtual swap files only consume capacity on vSAN as they are accessed. As a result, you can reduce the consumption on the vSAN datastore if virtual machines do not experience memory over-commitment which requires the use of the virtual swap file.

None.