VMWare vSAN storage policy configuration

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Creating a vSAN storage policy

To create a vSAN storage policy use following steps:

  1. Go to "Policies and Profiles" from Menu and then go to "VM Storage Policy"
  2. Choose to clone existing policy which matches the requirements closely or create a new policy
  3. Write appropriate name and description for the new policy
  4. On the Policy structure page, select Enable rules for "vSAN" storage, and click Next.
  5. On the vSAN page following options are used more often:
    Availability - Failure to tolerate
    Should be set to at least 1 for anything useful to ensure that disk failure does not lead to data loss.
    Availability - Fault tolerance method
    In case of Hybrid vSAN (Flash:Cache, Magnetic:Capacity), there is only option of RAID 1-Mirroring. But in cases of all flash clusters with enough nodes there can be option of RAID5/6 Erasure coding which is more space efficient then RAID-1 but also more disk intensive (More I/O)
    At least till ESXi 7.x note that if we go for RAID-5/RAID-6 erasure coding putting hosts in maintenance mode either with "Full data evacuation" or with "ensure availability" becomes tricky as there is no RAID-I mirroring so there are no blocks to transfer to other nodes. If we use RAID-5 erasure coding then putting more than one hosts in maintenance mode will create serious issues. Here CLOM_TIMEOUT also may not have much significance. Hence is there is enough storage use RAID-I and avoid RAID-5/RAID-6 as we dont get software style failure tolerance with RAID-5 / RAID-6 erasure coding.
    Storage rules - Space efficiency
    Deduplication and compression are available on all-flash arrays
    Advance policy rules - No. of stripes per object
    This can be used to split an object into stripes before storing so that a single object can be read and written simultaneously to multiple disk groups. For this we should have many disk groups across many hosts. For example in case of 3 disk groups per host and 5 hosts, total 15 host-groups in cluster, stripe-width of 3 makes more sense then stripe width of 1. This depends a lot on application usage patterns also.
    Advance policy rules - IOPS limit
    This is good feature to have to limit IOPS of test VMs, esp. if we are expecting them to have heavy I/O and want to ensure they do not affect other production VMs on same cluster
    Advance policy rules - Object space reservation
    In case of vSAN there is no performance benefit of thick provisioning at all. Hence unless we want to show the disk usage as part of calculations / summary pages, there is no point in going for anything other then "Thin provisioning"
    Advance policy rules - Flash read cache reservation
    Reserve portion of cache for VM. This can be used to improve performance of VMs / disks by applying this policy and ensuring they get higher than usual cache.
    Advance policy rules - Force provisioning
    This can be useful to ensure that vSAN allows more data to be stored / created on vSAN datastore even if the storage compliance cannot be met.
    For example if in a three node cluster with FTT=1, one node has failed and due to partial repair ( https://core.vmware.com/resource/intelligent-rebuilds-vsan# ) all the remaining space on two free nodes is used up. If there is a new request to create a new object (Use additional vSAN storage), it would be accepted only if force provisioning is set to true.
  6. Create the policy as per requirements

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Assigning vSAN storage policy

To assign VSAN policy to a VM or disk of a VM use following steps:

Avoid going to edit settings and changing VM vSAN disk policy from edit settings page. It has been seen to not work properly in a few cases.

  1. Click on the VM
  2. Go to Configure and go to "Policies"
  3. Choose option to "Edit VM storage policy"
  4. Select the desired policy and quite often assign it to all disks and the VM home folder. In rare cases choose a particular disk to be configured with a different policy. For this you may have to enable per-disk policy option from top right corner.
    Note that if you select two different types of policies then VM summary page may randomly show compliant and non-compliant for the VM, even if its all disks are compliant to assigned policy.
  5. After policy is applied we can see compliance status against "VM home folder" and also other VM disks on the same Configure -> Policies page.


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