VMware : Snapshots - Part II

Hi Everybody,

This is the continuation post to VMware : Snapshots - Part I, please keep on reading.

From VMware Knowledge Base :

When creating a imagesnapshot, there are several options you can specify:

  • Name – used to identify the snapshot.

  • Description – 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.

The following is a high level overview of how create, remove, or revert snapshot requests are processed within the VMware environment:

  1. A request to create, remove, or revert a snapshot for a virtual machine is sent from the client to the server using the VMware API.
  2. The request is forwarded to the VMware ESX host which is currently hosting the virtual machine in question.
    Note: This only occurs if the original request was sent to a different server such as vCenter which is managing the ESX host.
  3. If the snapshot includes the memory option, the ESX host writes the memory of the virtual machine to disk.
    Note: The length of time the ESX host takes to write the memory onto the disk is relative to the amount of memory the virtual machine is configured to use.
  4. If the snapshot includes the quiesce option, the ESX host requests the guest operating system to quiesce the disks via the VMware tools.
    Note: Depending on the guest operating system, the quiescing operation can be done by the sync driver, the vmsync module, or Microsoft's Volume Shadow Copy (VSS) service. For more information on quiescing, see Troubleshooting Volume Shadow Copy quiesce related issues (1007696) for VSS or A virtual machine can freeze under load when you take quiesced snapshots or use custom quiescing scripts (5962168) for the SYNC driver.
  5. The ESX host makes the appropriate changes to the virtual machine's snapshot database (.vmsd file) and the changes are reflected in the snapshot manager of the virtual machine.
    Note: The operation to remove the snapshot entity in the snapshot manager completes before the changes are made to the child disks. The snapshot manager does not contain any snapshot entries while the virtual machine continues to run from the child disk.  For more information, see Committing snapshots when there are no snapshot entries in the snapshot manager (1002310).
  6. The ESX host calls a function similar to the Virtual Disk API functions to make changes to the child disks (-delta.vmdk and .vmdk files) and the disk chain.
    Note:  During a snapshot removal, if the child disks are large in size, the operation may take a long time. This could result in a timeout error message from either VirtualCenter or the VMware Infrastructure Client. For more information about timeout error messages, see vCenter operation times out with the error: Operation failed since another task is in progress (1004790).

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