vSAN storage policies 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 VMware Virtual SAN datastore is created and a default storage policy is assigned to the datastore. The default policy has Failures to tolerate set to 1, a single disk stripe per object, and a thin-provisioned virtual disk. Each virtual machine deployed to vSAN datastores is assigned at least one virtual machine storage policy. You can assign storage policies when you create or edit virtual machines.
Storage Policy Rules
1. Failures to tolerate (FTT)
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.
If fault domains are configured, 2n+1 fault domains with hosts contributing capacity are required. A host which does not belong to a fault domain is considered its own single-host fault domain.
You can select a data replication method that optimizes for performance or capacity. RAID-1 (Mirroring) uses more disk space to place the components of objects but provides better performance for accessing the objects. RAID-5/6 (Erasure Coding) uses less disk space, but performance is reduced.
For instance, if you set the failures to tolerate setting to 1, each piece of written data is stored twice. For failure prevention, storing virtual machine objects requires twice as much storage. For instance, a VSAN cluster would need 200 GB of storage space to accommodate a 100 GB virtual disk.
2. Site disaster tolerance
In a stretched cluster, this rule defines the number of additional host failures that an object can tolerate after the number of failures defined by FTT is reached.
None - standard cluster is the default value. For a stretched cluster, you can choose to keep data on the Preferred or Secondary site for host affinity.
Host mirroring - 2 node cluster defines the number of additional failures that an object can tolerate after the number of failures defined by FTT is reached. vSAN performs object mirroring at the disk group level. Each data host must have at least three disk groups to use this rule.
Site mirroring - stretched cluster defines the number of additional host failures that an object can tolerate after the number of failures defined by FTT is reached.
3. 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. For an all-flash configuration, the striping is across flash devices that make up the capacity layer. Make sure that your vSAN environment has sufficient capacity devices present to accommodate the request.
4. Force provisioning
If the option is set to Yes, the object is provisioned even if the 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.
5. 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. The following options are available:
Thin provisioning (default)
25% reservation
50% reservation
75% reservation
Thick provisioning
6. 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.
7. 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.
8. 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. Use this option 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 supported only for hybrid configurations. You must not use this attribute when defining a VM storage policy for an all-flash cluster. Default value is 0%. Maximum value is 100%.
I hope this article help us to understand various vSAN storage policy rulesets.
Thanks for Reading!
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