Many clients can read from the same NetBoot image, but when a client needs to write
anything (such as print spools, browser caches, and other temporary files) back to its
startup volume, NetBoot automatically redirects the written data to the client’s shadow
file. These shadow files are separate from regular system and application software files,
and they preserve the unique identity of each client during the entire time the client is
running off a NetBoot image.
NetBoot also transparently maintains changed user data in
the shadow files, while reading any unchanged data from the system image. The shadow
files are re-created at each startup, so any changes that the user makes to the startup volume
are lost at restart.
This behavior preserves the condition of the environment the administrator set up when
creating the NetBoot image, which is ideal in lab and kiosk situations where you want to
ensure that users never alter the startup volume. If you’re using NetBoot for deployment
purposes only, this is also an ideal approach, as your NetBoot system is only there to act as a
conduit for your deployment mechanism and shouldn’t be modified by the client computer.
This behavior has other important implications, however.
For example, if a user saves
a document to the startup volume, after a restart that document is gone. However,
this potential problem can be remedied with network-based user accounts; then when
network-based users log in, they can store documents and preserve preferences in
their network home folders.
The default location for the shadow files also presents a problem for NetBoot deployment
scenarios. By default, to provide the best performance, the NetBoot shadow files
are stored on the client computer’s local internal drive in the /private/var/netboot/.com.
apple.NetBootX folder. Because this folder, and the volume it resides on, is being used by
the NetBoot system, you cannot perform any meaningful system deployment tasks on the
computer’s local internal drive.
To resolve this issue, NetBoot supports a diskless booting mode wherein the shadow files
are stored on an AFP share point hosted from the NetBoot server. The shadow file share is
named NetBootClientsn in the /Library/NetBoot folder on your NetBoot server, where n
is the number of the share point as it was configured by the Server Admin tool.
With diskless booting enabled, NetBoot enables you to operate client computers that
are literally diskless and gives you full access to the computer’s local internal hard drive,
allowing you to perform deployment or maintenance tasks. Further, you can individually
select the diskless booting option for each standard NetBoot image.
NetInstall images use diskless booting by default, albeit in a slightly different form. Instead
of requiring an additional share point for shadow files, the NetInstall boot process uses
the local system RAM for scratch disk space. This allows you to host as many simultaneous
NetInstall clients as you need without worrying about AFP limitations.
Creating Simple NetBoot Images
In this section you will learn how to create simple NetBoot and NetInstall system images.
Once you have created your NetBoot images, you can host them from a NetBoot server.
System Image Utility is the application you will use to create Mac OS X
v10.5 NetBoot and NetInstall images. This application has been significantly
reengineered for Mac OS X v10.5, and it replaces the Network
Image Utility used with previous versions of Mac OS X. System Image
Utility is located in the /Applications/Server folder on your Mac OS X
v10.5 Server computer. This application can also be installed as part of the Server Admin
Tools v10.5 on any Mac OS X v10.5 computer.
Choosing NetBoot Image Sources
Although Mac OS X Server v10.5 supports hosting NetBoot images with previous versions
of Mac OS, the System Image Utility application can create NetBoot images of
only Mac OS X v10.5.x or Mac OS X Server v10.5.x. Therefore, to acquire older NetBoot
images you must use previous versions of the Mac OS X Server administration tools.
Further, versions of Mac OS X prior to version 10.4.7 are not universal, and images must
be created separately for Intel- and PowerPC-based Macs.
As when creating a standard system image, you must have a source from which to create
your NetBoot images. The System Image Utility can create NetBoot and NetInstall images
from several different sources that contain Mac OS X or Mac OS X Server. Whether you
choose to create a standard NetBoot or NetInstall image will affect how System Image
Utility creates the image.
Viable System Image Utility NetBoot image sources are:
Mac OS X installation media - Standard NetBoot images created using installation
media contain an installed “clean” version of the operating system that contains minimal
configuration. NetInstall images created using the install media replicate the experience of
starting from the installation media, thus allowing you to install the standard system
software via the network.
Nonbooted system volumes - When a mounted nonbooted system volume is selected
as a source, the entire contents of the volume, including the operating system, configuration
files, and applications, are copied to the NetBoot image. If you choose to create
a standard NetBoot image, the client experience will be identical to that of starting
up from the source system volume. If you choose to create a NetInstall image, a compressed
restorable disk image of the source system volume is created. A client starting
up from this NetInstall image will be presented with an interface similar to the standard
Mac OS X installation interface. Starting this installation process, however, will restore
the source system volume to a Mac client instead of the standard installation image.
Your model Mac, when set to target disk mode and plugged into your
NetBoot creation computer, certainly qualifies as a valid nonbooted system volume
from which you can create a NetBoot or NetInstall image.
Disk images - System Image Utility treats the contents of a mounted disk image identically
to that of a standard volume. Thus, if the disk image volume’s contents constitute
a copy of the Mac OS X installation media, the contents will be treated as the
original media. Further, if the disk image volume’s contents are that of an installed
and configured system, the volume will be treated as a nonbooted system volume.
The justification for using NetBoot as a network-based deployment mechanism is clear.
Using the System Image Utility, you can easily create a NetInstall image that will, via the
network, start your target Macs and restore your cloned or modular system image to the
computer’s local hard drive. Even though the user experiences the Mac OS X Installer
interface when started from a NetInstall image, the Apple Software Restore (ASR) mechanism
is used to restore your system image. | 
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