Difference between snapshot sizes reported by vCenter and Aria Operations

Products

• VMware Aria Suite

Issue/Introduction

When checking the size of a snapshot present on a virtual machine, differences are noticed in the sizes reported by the vCenter and Aria Operations snapshot metrics

Environment

Aria Operations 8.x

Cause

What is a Snapshot:

A snapshot preserves the state and data of a virtual machine at a specific point in time.

• The state includes the virtual machine’s power state (for example, powered-on, powered-off, suspended).
• The data includes all of the files that make up the virtual machine. This includes disks, memory, and other devices, such as virtual network interface cards.

 

Child Disks and Disk Usage:

When a snapshot is created a <disk-name>--delta.vmdk file is created for the corresponding <disk-name>--.vmdk file.  

A collection of .vmdk and --delta.vmdk files for each virtual disk is connected to the virtual machine at the time of the snapshot.

These delta files can be referred to as child disks.

These child disks can later be considered parent disks for future child disks. (In the case that a further snapshot is taken)

From the original parent disk, each child constitutes a delta pointing back from the present state of the virtual disk, one step at a time, to the original.

If a virtual machine is running on a snapshot, it is making changes to a child disk. The more write operations made to this disk, the larger it grows. This is true for each delta.

The Disk Chain:

Generally, when you create a snapshot for the first time, the first child disk is created from the parent disk. Successive snapshots generate new child disks from the last child disk on the chain. 

The image below shows a simplified version of snapshot creation and removal:

 

 

 

Resolution

Snapshot sizes in vCenter Vs Aria Operations:

Snapshot space measurements in Aria Operations and vCenter are conceptually different.

Using the diagram above as an example, vCenter considers the size of the snapshot to be the size of Parent (vm.vmdk), while Aria Operations considers the size of the snapshot to be the size of the Child (vm--delta.vmdk)

From the point of view of Aria Operations, snapshot size is the space which will be potentially reclaimed when the snapshot is deleted.