Saturday, April 11, 2009

Understanding the snapshot - how to check size of a snapshot

When creating a snapshot, the existing vmdk file is locked and a new vmdk is created, a delta file. If there are multiple vmdk's attached to the VM, seperate delta files will be created for each vmdk. If vmdks are placed on other LUNs than where the .vmx file is residing, then all delta files will be placed on the same LUN as the .vmx file. All changes made after the snapshot is taken are added to the new vmdk file(s). The delta vmdk files can grow until they reach the size of the original vmdk file. If a snapshot exists for too long, this can generate problems as the SAN LUN can run out of disk space. If this happens, the VM’s will start to crash. Therefore, as a general rule of thumb, snapshots should not be left unattended for more than one or two weeks unless it is ensured that there is sufficient space on the data store. If the snapshot is needed for a longer period, it is recommended to make a clone instead.

To check the size of the snapshot, simply browse datastore and look for a numbered vmdk file, e.g.


If a second snapshot is taken, it is named:


And so forth…

Below is a number of screenshots where you can see how files are created as snapshots are made:

1. This first sreenshot, VM is just created, no snapshot:

2. Just after first snapshot taken – no further action taken

A new vmdk file is created which is about 18 MB in size when no changes has been made yet. Remark filename, jnrrsnaphosttest-000001.vmdk

3. After installation of a couple of applications

As changes are made, the new vmdk file increases in size. In this case it increases from initial 18 MB to 198 MB. See same file as above.

4. After second snapshot taken:

When a second snapshot is created yet another vmdk file is created (e.g. server123-0000002.vmdk) and so forth...


  1. There is indeed a size limit on how big a snapshot can grow and it can never exceed the size of the original vmdk. Please refer to the vmware documentation for more details.

    Thank you.

  2. Thanks for pointing that out, I've update the post and added a little more info in relation to multiple vmdk's on different LUNs


Note: Only a member of this blog may post a comment.