Wednesday, December 17, 2014

VMWare error: The parent virtual disk has been modified since the child was created

I needed to shrink the registry files on a ESXi guest (guest1), and in order to do so I shut it down and mounted the virtual disk on a separate ESXi guest (guest2). Once it was done, I removed the temporarily shared virtual disk from guest2 and went to boot guest1. I received an error that said "Cannot open disk: .... The parent virtual disk has been modified since the child was created". I then tried reverting to a snapshot and that failed, so for a minute I thought I may have just lost one of my servers. Fortunately this is something that can be fixed, and it wasn't too difficult.

The ESXi guest's virtual hard disk can consist of multiple files. What was happening is that the ID tag(s) saved in those files to tell the system which order to align those files in to create the complete hard disk were getting changed when I mounted it on guest2. To fix the problem I needed to sort through those virtual disk files and correct any mismatched ID tags. Here's how I did that

First, I had to enable SSH access to my ESXi host. You can also get in through the ESXi CLI too if you'd prefer. To enable SSH access to an ESXi host, you can do it from the direct console or from the vSphere Client. From the direct console, log in and go to Troubleshooting Options->Enable SSH. To enable it through the vSphere client, open the host and go to Configuration->Security Profile. Then in the Services section, click Properties, click on SSH, then Options. From there you can set the startup policy, and also start the service.

Once I had SSH access enabled, I logged into the host holding guest1. Now you want to get into the datastore where the virtual hard disk files are stored for the guest, and navigation commands are the same as you'd use in Linux. For me it was cd /vmfs/volumes/datastore1/guest1. Once I was in that directory I could use the ls command to list all the files. What I needed where the guest1.vmdk, guest1-0000001.vmdk, guest1-000002.vmdk, etc files. For my machine, I had guest1.vmdk, and then three add-ons, so four total virtual disk files to look at.

At this point you'll want to use your favorite text editor to open these files. Personally, I use vi, but you can use whatever you'd like that is available. Open each of the virtual disk files and note the value at the top for CID and parentCID, then close and move to the next. The parent CID of guest1.vmdk should be something like ffffffff. Once I had opened each of my files I had this

CID: 32b76102, parentCID: ffffffff

CID: 7d3d984f, parentCID: fa1f4813

CID: 49eb6c66, parentCID: 6e1b350e

CID: fa1f4813, parentCID: 49eb6c66

Now is where you get to solve the puzzle. Which CID or parentCID is incorrect and screwing up your virtual hard disk? I had to draw it out, but what I ended up with is

6e1b350e (???)<-49eb6c66 d3d984f="" fa1f4813="" guest1-000001="" guest1-000002="" guest1-000003="" p="">
For me it was that the parentCID value on guest2 was pointing to an unknown CID. Once I found that out, I opened guest1-000002.vmdk in vi and changed the parentCID value to the CID value of guest1.vmdk, 32b76102. Saved and closed, then booted up guest1 without any other problems.

I did try it a second time just to see, and in that case the same thing happened on the same file. It looks like when I mount the virtual disk in guest2, the CID value on the primary vmdk file gets changed. All the others stay the same, so if you can find the parentCID value that is going nowhere and change it, then you're back in business.

Articles I found that helped me come out with the solution

VMWare KB about hard disks failing to open

VMWare KB about resolving CID mismatch on virtual hard disks

Enabling ESXi Shell or SSH access to ESXi host

Editing VMWare config files

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