After you enable and configure 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 unless your environment requires policies with non-default behavior. If you configure a custom policy, vSAN will guarantee it; 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

Value

Comments

Number of failures to tolerate

Redundancy

Default 1

Max 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 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

Default 1

Max 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

Default 0

Max 100%

Flash capacity reserved as read cache for the storage is a percentage of the logical object size that will be 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

Default 0

Max 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

Default:

No

Force provisioning allows for 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 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.

Note:

If you do not specify a user-configured policy, 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 to ensure appropriate protection for these critical virtual machine components. Policies must be configured based on the application’s business requirements. Policies give VMware vSAN its power because it can adjust how a disk performs on the fly based on the policies configured.

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. Policy Design Decision

Decision ID

Design Decision

Design Justification

Design Implication

ROBO-VI-STORAGE-SDS-009

Use the default VMware vSAN storage policy.

The default vSAN storage policy provides the level of redundancy that is needed for the management workloads within the consolidated cluster.

Additional policies might be needed for workloads because their performance or availability requirements might differ from what the default VMware vSAN policy supports.

ROBO-VI-STORAGE-SDS-010

Configure the virtual machine swap file as a sparse objects on vSAN .

Enabling this setting creates virtual swap files as a sparse object on the vSAN datastore. Sparse virtual swap files will only consume capacity on vSAN as they are accessed. The result can be significantly less space consumed on the vSAN datastore, provided virtual machines do not experience memory over commitment, requiring use of the virtual swap file.

Administrative overhead to enable the advanced setting on all ESXi hosts running VMware vSAN.