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Delete all Snapshots and Consolidate Snapshots feature FAQ (1023657)

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Purpose

This article clarifies questions related to the changes introduced in the Delete all snapshots process. This article also describes the impact on the delta disks of the "Delete all snapshots" process also known as the "Consolidation process"

Note: A "Consolidate" feature has been added in vSphere 5.0. This feature's impact on the snapshot delta files will be identical to what is described in this KB article. For more information, see Consolidating snapshots in vSphere 5.x (2003638).

Resolution

Which patches change this feature?

What are the changes?

Before these patches, committing more than one snapshot using the Delete all option from the Snapshot manager would require additional space to perform the operation. The amount of extra space required was directly related with the amount of snapshots and their size.
 
The patches modify the Delete all snapshots operation to commit every snapshot of the chain directly to the Base Disk(s) of the virtual machine. With this new algorithm:
  • If the Base Disk is preallocated (thick provision), no extra space is required for the Delete all operation. The Base Disk will not grow as it is preallocated or thick.
  • If the Base Disk is non-preallocated (thin provision), the base disk will grow only on committing information from the snapshots. Each thin provision disk may grow up to its maximum size as mentioned in the Provisioned Size option in the virtual machine settings for the disk.
You can check the Summary tab of the virtual machine to know how much the virtual machine disk utilization can grow. The difference between Provisioned Storage and Used Storage in the right panel indicate how much the Base Disks can grow.

However, if the virtual machine has disks in different datastores, this amount is shared between them.
  • For versions prior to VMware ESX 4.0 Update 2, the task of consolidating all snapshots (Remove All Snapshots task) caused unique changes stored only in the second snapshot delta disk to be copied upward through the snapshot chain and into the first snapshot, or its parent. This effect is recursive for each preceding parent file.

    For example, assume you have a base disk of size 8 GB and 2 levels of snapshots, each of 4 GB each. During a Remove All Snapshot Tasks, the first snapshot delta disk file can grow to a maximum of 8 GB because all new blocks from the second snapshot are written. Any common changes stored in both snapshot levels do not require additional space.

  • From ESX 4.0 Update 2 and later, the snapshot mechanism has changed. VMware ESX now incorporates improved consolidation procedures which lessen the demand of free space. You are able to consolidate virtual machine delta disks even while minimal free space on your datastore is available.

What files get locked during the Delete all operation?

All flat and delta files that are used by the chain of snapshots are locked.

How to monitor the status of the Consolidation Process?

 
There is no way to accurately determine the progression of the commit process and the progression bar in the vSphere Client may not report the accurate percentage and/or may appear to be stuck. As long as the files are being read /written and the time stamps on the files continue to update, the process is working.
 

How long is the consolidation going to take?

It is not easy to accurately determine how long the process will take. Many factors will directly and indirectly impact the time required to complete the process such as size/number of delta files, load/performances on the SAN, etc. However, you can use one of these techniques to estimate or get a worst case scenario of the time required.
  1. If the process is already started, you may monitor the throughput via esxtop or looking at the vCenter performance charts. You can then estimate the time based on the aggregated size of the delta files.
  2. Before you start the process, you may simulate a disk clone operation. This technique might not give you the most accurate estimation but it should provide a worst case scenario. If the delta files are numerous / large, it is the safest option as it does not require to start the process and no changes are made into the base disk. However, it requires the virtual machine to be shutdown.

Can I change the type of the virtual disks when it has snapshots?

You cannot change the type of a virtual disk with snapshots directly. You can perform one of these operations to change the type of disks:
  • When you do Storage vMotion you can change the type of disks, but not having snapshots is a requirement even for Storage vMotion in ESX/ESXi3.x and ESX/ESXi4.x.
  • Storage vMotion with snapshots is supported in ESXi5.0 and above.
  • When you clone the virtual machine you can change the type of disks on destination virtual machine. However, it will be the same type for all the disks.
  • If you clone from Service Console, you can select any type of disk for the destination. However, you cannot clone from the Service console if the virtual machine is running.

If cloning snapshots to a new virtual disk, what If I clone a disk that uses a VMware Paravirtual controller?

If you cloned the virtual disk using the host CLI you might run in to issues. Running the vmkfstools -i command creates a cloned drive with an LSI controller, even if the source disk is using VMware Paravirtual. This results in the virtual machine failing to boot. To resolve this issue, change the controller type to the same as the source.

For related information, see:
About VMware Paravirtual SCSI Controllers in the vSphere 5 Documentation Center

What is the difference between running the process while the virtual machine is powered on and when it is powered off?

If the virtual machine is running when you click Delete all from the interface or run vmware-cmd <cfg> removesnapshots, an additional snapshot is created to accommodate the incoming I/O while all the other snapshots get committed to the Base Disk. The size of the snapshot can grow depending on the I/O activity. It is ideal to reduce the I/O activity to facilitate the process.
 
That last snapshot will commit to the Base Disk at the very end of the process. After this, all the snapshot files are deleted.

Note: If the virtual machine is powered off, no additional free space is required and the process will complete faster as there is no need track writes and the additional snapshot and delta file are not created as part of the process.

Will each snapshot file get deleted as soon as it gets committed to the base disk, releasing space on the datastore?

No. All files are deleted together for particular virtual disk after completing the process. Once all the snapshots are committed for one virtual disk, all the files for that virtual disk will be deleted at the same time. The consolidation process, however, may still continue to run for other virtual disks till the process gets completed for the second virtual disk and so on.

What is the performance impact?

The snapshot consolidation process will generate IOPS on the underlying storage array. If the process is started while the virtual machine is powered on and if the virtual machine requires intensive I/O, performance degradation may be noticed. This is because, additional read and writes are required to consolidate the delta disks to the base disk while the Guest OS continues its normal activity. If the application in use fits this description and is a critical production server, it would be advisable to start the process after-hours or during a period of low activity.

After the process is started, is it possible to stop it?

No. Once the process is started, it cannot be canceled or paused. The process has to complete before any further operations can be done on the virtual machine including power operations.

Additional Information

This article discusses only the impact to the snapshot delta disk files and does not discuss the other snapshot files such as the snapshot descriptor (.vmsd) and memory snapshots (.vmsn) files. For more detail regarding this, please see Understanding virtual machine snapshots in VMware ESXi and ESX (1015180).

Tags

delete-snapshots snapshot-data

See Also

Update History

05/20/2013 - Added the question how to monitor snapshot process 04/04/2014 - Modified "What are the changes" and added information regarding space required to consolidate snapshots.

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