understanding_snapshoting

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.
A virtual machine provides several operations for creating and managing snapshots and snapshot chains. These operations let you create snapshots, revert to any snapshot in the chain, and remove snapshots. You can create extensive snapshot trees.

In VMware Infrastructure 3 and vSphere 4.x, the virtual machine snapshot delete operation combines the consolidation of the data and the deletion of the file. This caused issues when the snapshot files are removed from the Snapshot Manager, but the consolidation failed. This left the virtual machine still running on snapshots, and the user may not notice until the datastore is full with multiple snapshot files.

In vSphere 4.x, an alarm can be created to indicate if a virtual machine was running in snapshot mode. For more information, see Configuring VMware vCenter Server to send alarms when virtual machines are running from snapshots (1018029).

In vSphere 5.0, enhancements have been made to the snapshot removal. In vSphere 5.0, you are informed via the UI if the consolidation part of a RemoveSnapshot or RemoveAllSnapshots operation has failed. A new option, Consolidate, is available via the Snapshot menu to restart the consolidation.
 
Creating a snapshot
When creating a snapshot, there are several options you can specify:

Name: This is used to identify the snapshot.
Description: This is used to describe the snapshot.
Memory: If the flag is 1 or true, a dump of the internal state of the virtual machine is included in the snapshot. Memory snapshots take longer to create, but allow reversion to a running virtual machine state as it was when the snapshot was taken. This option is selected by default. If this option is not selected, and quiescing is not selected, the snapshot will create files which are crash-consistent, which you can use to reboot the virtual machine.

Note: When taking a memory snapshot, the entire state of the virtual machine will be stunned. For more information, see Taking a snapshot with virtual machine memory renders the virtual machine to an inactive state while the memory is written to disk (1013163).
 
Quiesce: If the flag is 1 or true, and the virtual machine is powered on when the snapshot is taken, VMware Tools is used to quiesce the file system in the virtual machine. Quiescing a file system is a process of bringing the on-disk data of a physical or virtual computer into a state suitable for backups. This process might include such operations as flushing dirty buffers from the operating system's in-memory cache to disk, or other higher-level application-specific tasks.

Note: Quiescing indicates pausing or altering the state of running processes on a computer, particularly those that might modify information stored on disk during a backup, to guarantee a consistent and usable backup. Quiescing is not necessary for memory snapshots; it is used primarily for backups.
When a snapshot is created, it is comprised of these files:

-.vmdk and --delta.vmdk

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 files can be referred to as child disks, redo logs, or delta links. These child disks can later be considered parent disks for future child disks. From the original parent disk, each child constitutes a redo log pointing back from the present state of the virtual disk, one step at a time, to the original.

Note:
The value may not be consistent across all child disks from the same snapshot. The file names are chosen based on filename availability.
If the virtual disk is larger than 2TB in size, the redo log file is of --sesparse.vmdk format.
.vmsd

The .vmsd file is a database of the virtual machine's snapshot information and the primary source of information for the Snapshot Manager. The file contains line entries which define the relationships between snapshots as well as the child disks for each snapshot.
 
Snapshot.vmsn

The .vmsn file includes the current configuration and optionally the active state of the virtual machine. Capturing the memory state of the virtual machine lets you revert to a turned on virtual machine state. With nonmemory snapshots, you can only revert to a turned off virtual machine state. Memory snapshots take longer to create than nonmemory snapshots.
Notes:
The preceding files will be placed in the working directory by default in ESXi/ESX 3.x and 4.x. This behavior can be changed if desired. For more information on creating snapshots in another directory, see Creating snapshots in a different location than default virtual machine directory (1002929).
In ESXi 5.x and later, snapshots descriptor and delta VMDK files will be stored in the same location as the virtual disks (which can be in a different directory to the working directory). To change this behavior, see Changing the location of snapshot delta files for virtual machines in ESXi 5.0 (2007563).
For more information, see:
Taking Snapshots of a Virtual Machine
Take a Snapshot in the vSphere Web Client
Take a Snapshot in the vSphere Client