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Advantages of VVol Storage Snapshots, and a word of caution about Memory Snapshots

RajShekarChelur

One of the many benefits of VVols is that snapshots of a VVol VM are offloaded to the storage array. With HPE Nimble Storage arrays, you can create a large number of space-efficient snapshots and retain them for longer.

Unlike VM Snapshots on a VMFS datastore, VVol snapshots on Nimble do not have any impact on disk IO performance. No penalty on reads for a VM with any number of snapshots. And most importantly, no penalty for deleting the VM snapshots. The deletions are instantaneous. There is no snapshot consolidation process that needs to read the data from redo logs and write them to base disks!

You can also snapshot a VVol VM with the option "Snapshot the virtual machine's memory" selected. Such VMs can be restored to the powered on state it was in when the snapshot was taken. Without this option, the VMs would be restored to a powered off state.

Picture1.jpgCreate VM Snapshot in vCenter Client

This is great, but …

 "Snapshot the virtual machine's memory” does not use storage array snapshots. It uses a new VVol disk, and counts against the total number of VVol volumes supported by the storage array!

Let’s break this down a little more:

  1. When you create a VVol VM, a Nimble volume is created for every VMDK (Data VVol). And one Nimble volume is created to hold the configuration of VM (Config VVol). When the VM is powered on, a Swap VVol is also created.
  2. When you create a snapshot of a VVol VM, a Nimble volume snapshot is created for every data VVol.
  3. And if you chose to snapshot the VM memory? 
    • A new Nimble volume is created to hold the VM memory contents.
    • The size of this volume is the size of the VM’s Memory. So, a VM with 16 GB RAM would result in a 16 GB Nimble volume to be created.

The VVol volumes are shown in the Nimble management UI, with the volume created for memory snapshots highlighted.

Picture2.jpgVVol Volumes created on Nimble Array

The Nimble volume created for holding the memory contents is Thick Provisioned, and typically all of the data in a RAM is in use, so entire contents are written to the storage volume. 

 Storage compression reduces the actual storage capacity used to a good extent, but YMMV. Average compression observed from InfoSight for VM memory VVols is around 4X. If multiple such VM snapshots are created, a new volume is created for each memory snapshot.Picture3.jpgUsage and savings on a memory snapshot VVolIn conclusion, for VVol VM deployments: 

  • Each VM snapshot with memory results in a Nimble volume, thereby reducing the number of VVol VMs you can create on the storage array
  • The volume size is the same as the VM RAM, the volume is thick provisioned and typically uses quite a bit of storage space even after compression
  • Snapshots take quite a bit of time to be created, although this is not specific to VVols

It is advisable not to select "Snapshot the virtual machine's memory” as a default option for all VM snapshots. Use them sparingly when absolutely necessary.

About the Author

RajShekarChelur

VMware Integration Architect at Nimble Storage

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