Virtual SAN storage polices define storage requirements for your virtual machines. These policies determine how the virtual machine storage objects are provisioned and allocated within the datastore to guarantee the required level of service.

When you enable Virtual SAN on a host cluster, a single Virtual SAN datastore is created and a default storage policy is assigned to the datastore.

When you know the storage requirements of your virtual machines, you can create a storage policy referencing capabilities that the datastore advertises. You can create several policies to capture different types or classes of requirements.

Each virtual machine deployed to Virtual SAN datastores is assigned at least one virtual machine storage policy. You can assign storage policies when you create or edit virtual machines.

Note:

If you do not assign a storage policy to a virtual machine, it uses a default Virtual SAN policy with a Number of failures to tolerate set to 1, a single disk stripe per object, and a thin-provisioned virtual disk.

Table 1. Storage Policy Attributes

Capability

Description

Number of disk stripes per object

The minimum number of capacity devices across which each replica of a virtual machine object is striped. A value higher than 1 might result in better performance, but also results in higher use of system resources.

Default value is 1. Maximum value is 12.

Do not change the default striping value.

In a hybrid environment, the disk stripes are spread across magnetic disks. In the case of an all-flash configuration, the striping is across flash devices that make up the capacity layer. Make sure that your Virtual SAN environment has sufficient capacity devices present to accommodate the request.

Flash read cache reservation

Flash capacity reserved as read cache for the virtual machine object. Specified as a percentage of the logical size of the virtual machine disk (vmdk) object. Reserved flash capacity cannot be used by other objects. Unreserved flash is shared fairly among all objects. This option should be used only to address specific performance issues.

You do not have to set a reservation to get cache. Setting read cache reservations might cause a problem when you move the virtual machine object because the cache reservation settings are always included with the object.

The Flash Read Cache Reservation storage policy attribute is not supported for an all-flash cluster and you must not use this attribute when defining a VM storage policy. This attribute is supported only for hybrid configurations.

Default value is 0%. Maximum value is 100%.

Note:

By default, Virtual SAN dynamically allocates read cache to storage objects based on demand. This feature represents the most flexible and the most optimal use of resources. As a result, typically, you do not need to change the default 0 value for this parameter.

To increase the value when solving a performance problem, exercise caution. Over-provisioned cache reservations across several virtual machines can cause flash device space to be wasted on over-reservations and not being available to service the workloads that need the required space at a given time. This space wasting and unavailability might lead to performance degradation.

Number of failures to tolerate

Defines the number of host and device failures that a virtual machine object can tolerate. For n failures tolerated, each piece of data written is stored in n+1 places, including parity copies if using RAID 5 or RAID 6.

When provisioning a virtual machine, if you do not choose a storage policy, Virtual SAN assigns this policy as the default virtual machine storage policy.

Default value is 1. Maximum value is 3.

If fault domains are configured, 2n+1 fault domains with hosts contributing capacity are required. A host, which is not part of any fault domain is considered its own single-host fault domain.

Default value is 1. Maximum value is 3.

Note:

If you do not want Virtual SAN to protect a single mirror copy of virtual machine objects, you can specify the Number of failures to tolerate=0. However, the host might experience unusual delays when entering maintenance mode. The delays occur because Virtual SAN must evacuate the object from the host for the maintenance operation to complete successfully. Setting the Number of failures to tolerate=0 means that your data is unprotected, and you might lose data when the Virtual SAN cluster encounters a device failure.

Note:

If you create a storage policy and you do not specify a value for Number of failures to tolerate, by default, Virtual SAN creates a single mirror copy of the virtual machine objects and tolerates only one failure. However, if multiple component failures occur, your data might be at risk.

Force provisioning

If the option is set to Yes, the object is provisioned even if the Number of failures to tolerate, Number of disk stripes per object, and Flash read cache reservation policies specified in the storage policy is not satisfiable 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. Virtual SAN fails to provision a virtual machine when the policy requirements are not met, but it successfully creates the user-defined storage policy.

Object space reservation

Percentage of the logical size of the virtual machine disk (vmdk) object that must be reserved, or thick provisioned when deploying virtual machines.

Default value is 0%. Maximum value is 100%.

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.

Virtual SAN 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, Virtual SAN repairs the data or reports the error.

If a checksum mismatch is detected, Virtual SAN 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.

Failure tolerance method

Specifies whether the data replication method optimizes for Performance or Capacity. If you select RAID-1 (Mirroring) - Performance, Virtual SAN uses more disk space to place the components of objects but provides better performance for accessing the objects. If you select RAID-5/6 (Erasure Coding) - Capacity, Virtual SAN uses less disk space, but the performance is reduced. You can use RAID 5 by applying the RAID-5/6 (Erasure Coding) - Capacity attribute to clusters with four or more fault domains, and set the Number of failures to tolerate to 1. You can use RAID 6 by applying the RAID-5/6 (Erasure Coding) - Capacity attribute to clusters with six or more fault domains, and set the Number of failures to tolerate to 2.

For more information about RAID 5 or RAID 6, see Using RAID 5 or RAID 6 Erasure Coding.

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.

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

When working with virtual machine storage policies, you must understand how the storage capabilities affect the consumption of storage capacity in the Virtual SAN cluster. For more information about designing and sizing considerations of storage policies, see Designing and Sizing a Virtual SAN Cluster.