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Cloned Image: Create Restore Image

Once you have prepared and cleaned your model system volume, it’s finally time to create a restorable image from the model. The method covered in this section is essentially the “manual” method for creating a restorable image of a cloned system.

Create the Cloned Disk Image

Before you create the image, it’s a good idea to use Disk Utility’s Repair Disk feature on the model system volume. This will clean up potential volume structure issues that could have cropped up while you were building your system volume. If Disk Utility reports issues, you should run the Repair Disk process again until no issues are reported.

As you learned in Chapter 2, “Deploying Individual Items and Containers,” it’s fairly easy to create a disk image using the contents of a folder or volume. You can create disk images in the graphical interface using the Disk Image application and at the command line using the hdiutil command.

Here are a few tips to keep in mind when creating a cloned system disk image:

  • It’s usually best to choose a new Disk Image from Folder and then select the model system volume as the source. This will always build an image with zero additional free space, so you can restore it to any drive that’s larger than the system image contents.
  • Make sure to specify just the model system image volume, not the physical drive, as the source for the cloned system image.
  • The cloned system image format must be read-only (optionally compressed) and not encrypted.
  • If you choose to create a compressed image, make sure to have roughly twice the amount of free space available on the creation volume as the amount of used space on the model system volume.
  • Using separate physical drives for the model system volume and the destination for the cloned system image will greatly speed up the disk image creation process.
  • If you are unable to create the cloned system image because the system is reporting Resource Busy errors, unmount, but do not eject, the model system volume and then remount it again to create the cloned system image.

Preparing the Cloned System Image for Deployment

There is one last step to prepare your cloned system image for deployment. The cloned system image must be scanned in preparation for the Apple Software Restore (ASR) process. As you will learn in greater detail in Chapter 5, “Deploying System Images,” the ASR process is used to restore the system image contents to a storage volume for deployment. The image scanning process can be performed by the Disk Utility application or the asr command.

The image scan process calculates additional data checksums for the cloned system image and stores this information in the disk image file. This checksum data is later used to verify the system restoration process. The image scan process will also reorder the cloned system disk image file if it is not properly ordered for the network ASR process. If the disk image file needs to be reordered, the system will have to rewrite the file to the same volume. This reordering process is why you need roughly twice the free space on the creation system to prepare the cloned system image for deployment.

To scan your cloned system image from the Disk Utility application:

  1. Open Disk Utility and choose Images > Scan Image for Restore.
  2. In the Open dialog that appears, select your cloned system image.
  3. Authenticate as an administrative user to initiate the scan process.

    Depending on the size of your system image and the speed of your hardware, it can take anywhere from a few minutes to a few hours to complete the image scan process. During this time Disk Utility will show a progress bar dialog. When the process is complete, this dialog will let you know whether the image scan process was successful.

Scanning your cloned system image from the asr command is nearly as easy. The syntax is sudo asr imagescan -source followed by the path to the cloned system image. In the following example, Michelle scans the cloned system image she previously created, ClonedSystem.dmg, on the external Storage volume. The asr command also displays image scan progress and success messages.

MyMac:~ michelle$ sudo asr imagescan -source /Volumes/Storage/ClonedSystem.dmg
Password: (password not shown)
Block checksum: ....10....20....30....40....50....60....70....80....90....100
asr: successfully scanned image "/Volumes/Storage/ClonedSystem.dmg"

Updating a Cloned System Image

One of the primary disadvantages of a cloned system image is that updates and changes are not easily made. Once a deployable system image is created, the image file itself can never be changed. Essentially, you have to replace your outdated cloned system image with a new cloned system image.

This process of having to build new cloned system images with every change is especially frustrating when you are testing and trying to work the kinks out of a new system deployment. Even the smallest change requires that you completely rebuild the entire system image.

There are three methods that you can use to update an existing cloned system image and still use your previous work as a starting point:

  • Retain the original model system volume and apply your updates to the model system. Once the model system is updated, create a new cloned system image as outlined previously in this section.
  • Restore the cloned system to a bootable volume as outlined in Chapter 5, “Deploying System Images.” Treat this restored volume as the new model system and apply your updates. Once your updates are complete, create a new cloned system image.
  • Convert the cloned system image to a read/write disk image as outlined in Chapter 2, “Deploying Individual Items and Containers.” Mount the new read/write disk image that contains your cloned system and apply the changes to the mounted volume. Once your updates are complete, convert the system disk image back to a read-only format and then rescan the image for restore.
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