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Increasing the Destaging Window for Dirty Write Cache
In Write-Back caching policy, VirtuCache caches all writes from VMware host to in-host SSD or RAM. A write acknowledgement is sent to VMware by VirtuCache once the write is committed to in-host SSD / RAM. This happens before the write is committed to the backend storage array. In this way writes are accelerated by VirtuCache.
Review this link for details on how VirtuCache Write-Back caching policy works. As this link mentions, VirtuCache caps the maximum amount of dirty writes (write cache not yet flushed to backend SAN) to 2 minutes (destaging window).
To increase the destaging window from the default value of two minutes, use the below VNXCLI command
set-vm-win <VM> [<destaging time in mins: 2> :default = 2]
We don’t recommend increasing the destaging window in your production environment. The 2-minute window ensures that if an ESXi host fails which then triggers VMware HA and VMs restart on other hosts in the cluster, that the dirty write cache for all the VMs on the failed host gets synced with the backend array from the mirrored write cache on other hosts in under 2 minutes, and before the VMs restart on other hosts. Increasing the destaging window will result in delayed restart of VMs in case of a host failure event, hence we don’t recommend increasing the destaging window beyond the default value of 2 minutes