When new end user VMs are provisioned in Horizon VDI and these VMs use Instant Clones and/or App Volumes, the end user VMs get assigned the same UUID for a brief period (duplicate UUID), as a result cache operational state of ‘Write-Back’ is not applied to these VMs, only ‘Write-Through’…
Write-Back Caching
VMware VMs have two UUIDs – BIOS UUID and instance UUID. VirtuCache doesn’t support Write-Back policy where multiple VMs in the same vCenter share the same BIOS UUID, even if these VMs have different Instance UUIDs. When using the cloning feature in Datrium (now end of lifed), the cloned VM…
Because VirtuCache uses different APIs for ESXi 6.x and 7.x, VirtuCache cannot establish peer connections between hosts that are on ESXi 6.x and 7.x, as a result, VirtuCache cannot replicate (mirror) the write cache between hosts that are on ESXi 6.x and 7.x, and so VirtuCache’s ‘Write-Back 1 Replica’ policy…
In rare cases, one or more VMDKs on a VM could have its ‘sharing’ field (drop-down field in ‘Edit Settings’ at the VM level) set to multi-writer, in which case it is not possible to move that VM to Write-Back caching mode. Such VMs can only be set to ‘Write-Through’…
If some VMs are not in Write-Back operational state when caching policy is set as ‘Write-Back 1 Replica’, it could be because of VirtuCache is not claiming the LUN when it was installed. You can verify this symptom by issuing ‘list-vms’ command in vnxcli (type vnxcli in ESXi CLI) for…
If VirtuCache driver build numbers on hosts in the same ESXi cluster are different, then VirtuCache doesn’t create peer connections between hosts with mismatched build numbers, and hence any VirtuCache tasks that require such a connection over the replication network, don’t happen. So read directs in case of write-through caching…
If the write-replication network fails, then VirtuCache keeps trying to replicate writes for 30seconds, during which time writes are not being acknowledged. After the 30 second period, VirtuCache times out the writes. Then VirtuCache freezes IO till the write cache is synced with backend SAN. Since we keep upto 2…
‘Write-Back One Replica’ Caching Policy ensures that a single host failure in a cluster doesn’t result in data loss. Now if you have a 2-host cluster and one host fails, then the VMs that are in ‘Write Back’ operational state on the host that is still up and running will…
Caching policy of ‘Write-Back 1 Replica’ is assigned to VMs or Datastores, but the operational state remains ‘Write-Through’. The reasons could be one or more listed below. The VirtuCache replication network is the vmotion network and the vmotion network might be using the vmotion stack that precludes all other traffic from…
VM might show operational state of Write-Through even though its caching policy is Write-Back, because the VM might be on local storage or NAS storage and not on SAN storage. In this situation the caching policy will show as Write-Through, but no caching will actually happen, because VirtuCache doesn’t cache…