The Real (and Virtual) Adventures of Nathan the IT Guy

Oct 7 2009   1:55AM GMT

Understanding VMware Snapshots.

Nathan Simon Nathan Simon Profile: Nathan Simon

I wanted to clear the air on snapshots, everyone is always asking, how much space does it take to remove a snapshot, but no one has a real answer…  whatever the answer may be this information inserted from the Basic System Administration PDF, which is readily available from VMware and can be downloaded from this link here, will help to explain how they work, and what can be done if you run out of space and cannot commit a snapshot. Please take your time and read through the below information and you will get a much better understanding of snapshots. The information below is a copyright of VMware. I have in no way altered its content.

“The Understanding Snapshots section does not include information on delta disks. The section should contain the following content:

To take a snapshot, the state of the virtual disk at the time of taking the snapshot must be preserved. When this occurs, the guest operating system cannot write to the VMDK file. The delta disk is an additional VMDK file where the guest is given write access. The delta disk represents the difference between the current state of the virtual disk and the state at the time of the previous snapshot. If more than one snapshots exist, delta disks might represent the difference (or delta) between each snapshot. Also, the guest can write to every single block of the virtual disk causing the delta disk to grow as large as the base VMDK of the virtual machine.

NOTE To consolidate all snapshots into the base virtual machine, you might need extra disk space, as large as the base VMDK.

When a snapshot is deleted, if a user chooses to merge the changes between the snapshots to the previous disk‐state, all the data from the delta disk that contains the information about the deleted snapshot is written to the parent disk. This might involve a large amount of disk I/O and might reduce the virtual machine performance until consolidation is complete.

If the user chooses to ignore the delta disks, delta consolidation is not required.

See VMware Knowledge Base system for more information on the iterative snapshot deletion behaviour. I’ve Included the details of the (KB article 1003302).

Details

If you try to initiate a Delete All snapshot for a virtual machine using Snapshot Manager, and if that virtual machine is on a datastore that does not have sufficient space for the snapshot, the following message displays in VMware Infrastructure (VI) Client:

msg.hbacommon.outofspace: there is no more space for the redo log of <VMname>-0000xx.vmdk.

You are given the option to abort or retry.

  • If you choose Abort, the virtual machine is powered off, the snapshot is aborted, and a Consolidate Helper snapshot is created. The Snapshot Manager UI displays that Consolidate Helper snapshot. You can delete the Consolidate Helper snapshot after you have made space available.
  • If you click Retry, the Snapshot Manager returns to Consolidate Helper snapshot mode unless you have made more disk space available.

Solution

Free up disk space if possible, or extend the VMFS volume using VI Client.

To extend the VMFS volume:

  1. Select the host on which the virtual machine resides and click the Configuration tab.
  2. Select the datastore on which the virtual machine resides and click Properties.Note: If there is no available storage, a new LUN must be presented to every ESX host that can see the LUN.

  3. In the dialog that appears, click Add Extent and follow the prompts in the Add Extend wizard to add an extent.
  4. Perform a rescan on every ESX host that is being presented the new LUN so that the addition of the extent is detected.
  5. After you have extended the VMFS volume, you can check the Retry option of the Redo log pop-up.

Caution: When using Delete All in the Snapshot Manager, the snapshot furthest from the base disk is committed to its parent, causing that parent snapshot to grow. When that commit is complete, that snapshot is removed and the process starts over on the newly updated snapshot to its parent. This continues until every snapshot has been committed. This can lead to an aggressive use of additional disk space if the snapshots are large. Use care when exercising this option if there is not much space available on the datastore.”

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