Why are Write Latencies High Even When the EMC Unity Appliance has Plenty of System Cache Memory and Fast Cache SSDs?
If you are experiencing high write latencies in VMs when using an EMC Unity Hybrid array, it could be because the caching and tiering solutions in Unity don’t cache random small block writes, and your applications might be generating large volumes of such storage IO.
The big difference between the two is that VSA caches only 2GB of reads from the Master VM1,2. VirtuCache caches reads + writes from all server & desktop VMs, and it can cache to TBs of in-host SSD/RAM, so that all storage IO is serviced from in-host cache.
More details in the table below.
How to Select SSDs for Host Side Caching for VMware – Interface, Model, Size, Source and Raid Level ?
In-VMware Host Caching – Datrium’s Appliance Specific Host Cache Versus Virtunet’s Host Cache for Any Storage.
Dell's PowerEdge VRTX hyper-converged appliance can either have all hard drive datastores or all SSD datastores, but you can't have SSDs act as tiering or caching media for VRTX volumes. That's where VirtuCache comes in.