This article may be helpful when you encounter these issues:

  • Virtual machines are not responding or cannot start due to broken parent and child virtual disk dependencies.
  • Virtual machines are not responding or do not start due to redo logs residing on datastores that do not have free space.
  • Snapshot creation takes too long when specifying the memory snapshot option.
  • Snapshot delete or remove operations result in the vSphere or VMware Infrastructure (VI) Client timing out.
  • Backups fail while quiescing during a snapshot operation.

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 VM still running on snapshots, and the user may not notice until the datastore is full.

In vSphere 4.x, an alarm can be created to indicate if a virtual machine was running in snapshot mode. For more information, seeConfiguring 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 <memory> 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.
  • Quiesce: If the <quiesce> 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 systems 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.

When a snapshot is created, it is comprised of these files:

  • <vm>-<number>.vmdk and <vm>-<number>-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 <number> value may not be consistent across all child disks from the same snapshot. The file names are chosen based on filename availability.
  • <vm>.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.
  • <vm>Snapshot<number>.vmsnThese files are the memory state at the time of the snapshot.

Note: The above files will be placed in the working directory by default in ESX/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, seeChanging the location of snapshot delta files for virtual machines in ESXi 5.0 (2007563)

What products use the snapshot feature?

In addition to being able to use snapshot manager to create snapshots, snapshots are used by many VMware and third-party products and features. Some VMware products that use snapshots extensively are:

  • VMware Data Recovery
  • VMware Lab Manager
  • VMware vCenter and the VMware Infrastructure Client (Snapshot Manager, Storage vMotion)

Note: This is not an exhaustive list.

See more details on VMware KB