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A disk snapshot is a copy of the virtual machine (VM) disk file recorded at a specific time, much like a Windows restore point. The snapshot preserves the original VM disk file by disabling writes to the original disk file - all new writes are made to the snapshot version of the VM. If you create more than one snapshot of your virtual machine (VM), then you'll have multiple restore points available to revert to. When you create a snapshot, what was currently writable becomes read-only from that point on. Using in-file delta technology, new files are created that contain all changes (delta) to the original disk files.

The size of a snapshot file can never exceed the size of the original disk file. When requests are made to change a block on the original disk, it is instead changed in the delta file. If the previously changed disk block in a delta file is changed again it will not increase the size of the delta file because it simply updates the existing block in the delta file.

Though snapshot files are small in size initially, they will grow as writes are made to the VM's disk files. If the number and size of the snapshot files grow significantly over time, they might end up eroding considerable datastore space, thereby choking VM operations. To conserve disk space, administrators need to continuously track snapshot growth per VM, identify 'heavy-weight' snapshots that may not be of use any longer, and purge them. The VM Snapshots test helps administrators achieve the same. While the measures reported by the test capture the snapshot file count per VM and the total size of the snapshot files of a VM, the detailed diagnosis reveals the size of each snapshot, thus enabling administrators to quickly spot those snapshot files that are too large in size.

Measurement	Description	Measurement Unit	Interpretation
Number_of_snapshot	Indicates the number of snapshot files of this VM that are currently available.	Number	A number of snapshots of a VM provides administrators with multiple restore points. On the flipside though, a high number of snapshots can also be considered a waste of valuable disk space, especially if many of the snapshots hold less critical, but heavy-weight changes/writes to the disk. To accurately identify those snapshots that are consuming disk space excessively, and to learn when they were created, who their parents are, and their current consistent file system state, use the detailed diagnosis of this measure.
Total_snapshot_size	Indicates the total size of all snapshots of a VM.	MB	Snapshots typically grow in 16 MB increments to help reduce SCSI reservation conflicts. Though small in size initially (16 MB), snapshots can grow with time, but can never grow beyond the original disk file size. If a marked increase is noticed in the value of this measure over time, it could indicate that one/more snapshots are rapidly growing in size. To know which snapshots are contributing to this phenomenon, use the detailed diagnosis of the Number of snapshots measure. The rate of growth of a snapshot will be determined by how much disk write activity occurs on your server. Servers that have disk write intensive applications, such as SQL and Exchange, will have their snapshot files grow rapidly. On the other hand, servers with mostly static content and fewer disk writes, such as Web and application servers, will grow at a much slower rate.
Num_Large_snapshot	Indicates the number of snapshots that are of a size more than the configured SIZELIMIT	Number	Snapshots typically grow in 16 MB increments to help reduce SCSI reservation conflicts. Though small in size initially (16 MB), snapshots can grow with time, but can never grow beyond the original disk file size. If a marked increase is noticed in the value of this measure over time, it could indicate that a number of snapshots are rapidly growing in size. To know which snapshots are growing beyond the size limit set, use the detailed diagnosis of this measure.
Num_Aged_snapshot	Indicates the number of snapshots that are of an age over the configured AGELIMIT	Number	Use the detailed diagnosis of this measure to identify the old snapshots, so that you can figure out whether they deserve to be retained or not. While many snapshots provide essential restore points for VMs, many others hold less critical information. The 'less useful' snapshots can be eliminated to save disk space.