VSAN-Troubleshooting-Reference-Manual

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Diagnostics and Troubleshooting Reference Manual – Virtual SAN Very large VMDK considerations With Virtual SAN 6.0, virtual machine disk sizes of 62TB are now supported. However, consideration should be given as to whether an application actually requires this size of a VMDK, and serious consideration should be given to the size of the VMDK chosen for a virtual machine and application. Once again, although Virtual SAN might have the aggregate space available on the cluster to accommodate this large size, it will depend on where this space is available and whether or not this space can be used to meet the requirements in the VM storage policy. For example, in a 3 node cluster which has 200TB of free space, one could conceivably believe that this should accommodate a VMDK with 62TB that has a NumberOfFailuresToTolerate=1 (2 x 62TB = 124TB). However if one host has 100TB free, host two has 50TB free and host three has 50TB free, then this Virtual SAN is not going to be accommodate this request, i.e. it will not move components around to ensure that there are two hosts with greater than 62TB free. This is another reason for recommending uniformly configured ESXi hosts. This will allow Virtual SAN to balance hosts from a usage perspective, and allow an even distribution of components across all hosts in the cluster. If for some reason, there is an imbalance in the usage, an administrator can try to use the proactive rebalance mechanism introduced in version 6.0 through RVC. Changing a VM Storage Policy dynamically It is important for Virtual SAN administrators to be aware of how Virtual SAN changes a VM Storage Policy dynamically, especially when it comes to sizing. Administrators need to be aware that changing policies dynamically may lead to a temporary increase in the amount of space consumed on the Virtual SAN datastore. When administrators make a change to a VM Storage Policy and then apply this to a virtual machine to make the change, Virtual SAN will attempt to find a new placement for a replica with the new configuration. If Virtual SAN fails to find a new placement, the reconfiguration will fail. In some cases existing parts of the current configuration can be reused and the configuration just needs to be updated or extended. For example, if an object currently uses NumberOfFailuresToTolerate=1, and the user asks for NumberOfFailuresToTolerate =2, if there are additional hosts to use, Virtual SAN can simply add another mirror (and witnesses). In other cases, such as changing the StripeWidth from 1 to 2, Virtual SAN cannot reuse the existing replicas and will create a brand new replica or replicas without impacting the existing objects. This means that applying this policy change will increase the amount of space that is being consumed by the virtual machine, albeit temporarily, and the amount of space consumed will be determined by the V M W A R E S T O R A G E B U D O C U M E N T A T I O N / 1 4 4
Diagnostics and Troubleshooting Reference Manual – Virtual SAN requirements placed in the policy. When the reconfiguration is completed, Virtual SAN then discards the old replicas. Provisioning with a policy that cannot be implemented Another consideration related to VM Storage Policy requirements is that even though there may appear to be enough space in the Virtual SAN cluster, a virtual machine will not provision with certain policy settings. While it might be obvious that a certain number of spindles is needed to satisfy a stripe width requirement, and that the number of spindles required increases as a NumberOfFailuresToTolerate requirement is added to the policy, Virtual SAN does not consolidate current configurations to accommodate newly deployed virtual machines. For example, Virtual SAN will not move components around hosts or disks groups to allow for the provisioning of a new replica, even though this might free enough space to allow the new virtual machine to be provisioned. Therefore, even though there may be enough free space overall in the cluster, most of the free space may be on one node, and there may not be enough space on the remaining nodes to satisfy the replica copies for NumberOfFailuresToTolerate. A well balanced cluster, with unform storage and flash configurations, will mitiage this issue significantly. What happens when a threshold is reached? There is only one capacity threshold to consider with Virtual SAN and that is the physical disk drive threshold. This is set at 80%, and if any single disk drive reaches this threshold (i.e. more than 80% of the drive is consumed), Virtual SAN attempts an automatic rebalance of components across the cluster to bring the amount of space consumed below this threshold. In version 5.5, components will be moved to disks whose capacity is below 80% used. VSAN will not move components to a disk that is already 80% full. If all of the disks go above the 80% threshold, rebalance will stop since there are no target disks that can be moved to. In version 6.0, Virtual SAN will continue to do rebalancing to achieve better resource utilization, even if all the disks are above the 80% capacity mark. Also in version 6.0, there is a new proactive rebalance utility that can be run from RVC. There are no thresholds set on the Virtual SAN datastore. Note however that the capacity usage shown against the Virtual SAN datastore is the average fullness of all disks in the Virtual SAN cluster. V M W A R E S T O R A G E B U D O C U M E N T A T I O N / 1 4 5
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