Summary: VM's were migrated off of an existing server to a new VxRail appliance and the expected gains in reduced storage consumption were not realized.
Article Content
| Issue | VM's were migrated off of an existing server to a new VxRail appliance and the expected storage savings were not realized. This article addresses an all flash appliance since Dedupe and compression are only supported on an all flash appliance. Here is a real world example: A legacy Dell ESXi Hosts/Dell EqualLogic SAN had a capacity of 14TB with only 1TB free. A new VxRail had a capacity of 26.5TB. After migrating the VM s from the legacy Dell ESXi Hosts and migrating the data from the data stores (about 10TB), only 8TB of free space was left on the VxRail. The Vxrail had 26.5 TB of free storage minus 10 TB of from the new VM's should have left 16.5 TB's free, but only 8TB's was left free? |
| Cause | The VxRail appliance uses ESXi VSAN which essentially uses a RAID 1 like protection mechanism for it's data. In a true RAID 1 configuration blocks of data are duplicated across a mirror disk. In VSAN blocks of data are duplicated across multiple nodes. If the defaults are used there are two mirrored blocks for every source block On the existing server in the example above the system was using RAID 5 which can result in a ratio of storage consumption of 1.25. VSAN has a default ratio of 2. So in the above example assuming 10TB's were to be moved at a cost of 1.25 per Raid 5 then 12.5 TB's were actually moved. RAID 1 with 2 mirrors means 25 TB's were actually consumed which leaves 1TB free. In the example above 8 TB's were left free. Since deduplication and compression were turned on in the VSAN a savings of 7 TB's was realized. |
| Resolution | With an all flash appliance you can selectively turn on Raid 5/6 policies per VM. Caution should be taken when doing this since there will be a performance hit. It is recommended this only be done on VM's that don't access the disk a lot. This VMWare Youtube video explains how to create a Raid 5/6 policy in VSAN: vSAN in 3 Minutes: Configuring RAID 5/6 Policies: This VMWare Youtube video explains how to set a Raid 5/6 policy on a VM: Working with Virtual SAN Storage Policies |