Download A Modeling Perspective of Image-Based Installation

A Modeling Perspective
of Image-Based
Installation
Enterprise Systems Group (ESG)
Dell OpenManage™
Systems Management
Dell White Paper
By John Zhang and Rick Landau
[email protected]
[email protected]
March 2002
Contents
Introduction ................................................................................................................................. 3
An Analogy Model of Imaged-Based Installation ............................................................... 4
Technical Details ........................................................................................................................ 8
Freeze the Source System for an Effective Snapshot .................................................... 8
Ensure Distortion-Free Image Capture and Deployment ............................................ 9
Personalize Target Systems During Image Deployment ........................................... 12
Make Image-based installation truly remote and unattended .................................. 12
Conclusions ............................................................................................................................... 14
Figures
Figure 1: Photographic Studio Model ....................................................................................................... 4
Figure 2: Image-based Installation System .............................................................................................. 5
Figure 3: Schematic Diagram of an Image-based Installation System.................................................. 5
Tables
Table 1: Comparison of components of PSM and IBI system ................................................................. 6
Table 2: Comparison of Operations of PSM and IBI system .................................................................. 7
Table 3: Methods of Creating a Functional copy of One Computer System's Software on Another
System ..................................................................................................................................................11
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Section
1
Introduction
Conventional software installation is a process that is familiar to most users.
First, insert a floppy or CD into a disk drive. Second, follow the prompts or
installation wizard to enter configuration data for the software. Finally, reboot
the system if necessary. An installer takes inputs from users, checks the existing
system hardware and software, and then copies files to appropriate locations on
the hard disk. More sophisticated installers can also pull files from the network
or use a predefined data file for an unattended operation. User experience with
conventional software installation is not always pleasant. To install an operating
system (OS) on 50 systems, for example, a system administrator has to walk to
every system and run the installers. What often makes the situation worse is that
some systems may not have monitors attached to them, and some may not even
have floppy or CD drives.
An alternative to conventional installations is image-based installation (IBI) that
allows system administrators to dispatch installation commands from a control
console. An IBI system takes a snapshot of a source computer and saves the
image to a storage location. The image can then be deployed to multiple target
systems remotely and monitor-free. Image-based deployment is a fast and
efficient way to clone operating system software, applications, and even data, to
a number of computers to guarantee that they all have an identical setup. A fullservice deployment system includes components to capture and distribute copies
of software and to adjust the target systems so that they can run cooperatively on
a network. Image-based installation systems are best used in the following areas:
March 2002

In corporations where one system administrator handles a large number of like
systems

For high-density servers that may not have peripherals such as floppy disk and
CD drive

In environments such as data centers where physical access to the servers is
minimal

For any situation where a centralized deployment mechanism is required
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Section
2
An Analogy Model of Imaged-Based
Installation
There are a number of products available in the market for using image-based
installation technology to transfers bits, sectors, and files from a source computer
to the hard disks of the target computers without human intervention. This
process has been referred as “cloning,” “provisioning,” “migration,” “copying,”
“deployment,” and “cookie cutting.” Although these words describe to a certain
degree the characteristics of image-based installation, a more precise model is
needed to understand an IBI system. A photographic studio model (PSM) is best
suited for this purpose. The PSM is composed of the following components: a
camera, a source, films, film storage and inventory, photo printer, and photo
papers; see Figure 1.
Negatives
Camera
Photo Printing
Machine
Taking Photo
Printing Photo
Original Photo
Photo paper
…
Photo paper
Figure 1: Photographic Studio Model
Similar to a PSM, an IBI system has the following components: source computer
system, imaging agent, images, image database, deployment agent, and target
computer systems; see Figures 2 and 3. The correlation between the components
in the two systems is shown in Table 1.
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Figure 2: Image-based Installation System
Figure 3: Schematic Diagram of an Image-based Installation System
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Components
Photographic Studio Model
Image-based Installation
System
Source
Original photos
Master computer system to be
copied to other computers
Imaging agent
Camera for taking the photos
A software program (imaging
agent) to capture the files or
sectors of the master
computer system
Images
Negatives of the photos
Image files that store the
contents of hard disks
Copying agent
Photo printing machine
A software program (which
could be the same imaging
agent) to write files or sectors
to the target computer
systems.
Target
Photo paper for printing the
photos
Target computer systems to
write the files to
Table 1: Comparison of components of PSM and IBI system
The photographic studio model can also be used to illustrate the basic operations
of an IBI system. The model has the following basic operations: prepare the
source for photographing, take photos, produce negatives, and develop the
photos. An IBI system has similar operations: prepare master computer, capture
images, transfer images if needed, and deploy images to target systems. The
operational similarities of the two systems are summarized in Table 2.
Operations
Photographic Studio Model
Image-based Installation
System
Prepare source
Remove any marks on the
original photo that should not
go to the copies.
Clean up any identity of the
source computer system such
as event log, hostname, or IP
addresses.
Capture image
Ensure that original photo is
ready to be photographed
(appropriate lightning and
stable).
Ensure that the source
computer system is in a
“capture-able” state. The
computer has to be shut
down.
Bring a camera to take a
picture of the original.
Use an imaging agent to read
the content of the hard disks
of the source computer.
Develop negatives.
Send contents back to the
imaging server for storage.
Transfer images
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Send the film to the studio for
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Transfer image files to other
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Operations
Photographic Studio Model
Image-based Installation
System
development.
images servers for
deployment.
Print images
Use negative to print the
photos.
Assign image to target
systems.
Write image content to the
hard disks of the target
systems.
Table 2: Comparison of Operations of PSM and IBI system
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Section
3
Technical Details
There are four technologies that are central to the operation of an IBI system,
which provides the ability to do the following:

Take an effective snapshot of the state of a system. It must be possible for copies
of that snapshot to run on other computer systems.

Capture the contents of the system and distribute to other computers on the
network.

Change the identities of multiple copies of a system, so that they can function as
new individuals on the computer networks where they will reside.

Accomplish all the necessary operations on both the source and target systems
with no human intervention required. That is, all these operations must be
performed a) remotely over the network; b) with no human at the source or
target system; and c) with no head (monitor, keyboard, and mouse) attached to
either system. In addition, a practical operation must be able to perform these
tasks on the network securely, to avoid the possibility of outside interference
with deployment operations.
Freeze the Source System for an Effective Snapshot
In general, a running operating system maintains much of its state in memory
rather than on disk, and therefore a snapshot of the disk state does not capture
the state of the entire system. Also, the operating system may constantly update
the hard disk to change its content. Therefore image capturing must be done as
an off-line activity, relative to the source computer system. The IBI system must
take control of the source computer system in order to capture its state through
the following steps.

Shut down the normal operating system on the source computer.

Run the imaging agent on the target computer.

Instruct the agent to read the disk contents and transfer them to an intermediate
master file. And

Restart the normal operating system.
This interruption of service is currently unavoidable, until operating systems
implement functions to cooperate in this snapshot process.
Also, it is possible that some modification of the operating system is necessary, to
"button-up" the system before capture to make deployment easier. For example,
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it may be necessary to turn off networking in the operating system, so that
deployed copies do not accidentally use the source computer's identity before
they have been given new identities.
Ensure Distortion-Free Image Capture and Deployment
One of the core technologies of an image-based deployment system is the ability
to make a functional copy of one computer system's software on another system.
How does the system copy a running system from one computer to another? A
disk containing a runnable operating system, and maybe applications and data,
can be copied in several different ways and still run correctly; see Table 3.
The first, most obvious method is to copy all the binary data of the source disk to
the target disk. Basically, the capturing agent reads the entire source disk sector
by sector and records the contents in the intermediate master file. Then the
deploying agent takes the contents from the master file and writes them sectorby-sector onto the target disk.
By copying the disk contents this way, all the data and the partition structure of
the source disk are duplicated precisely on the destination disk: the output disk
is an exact, bit-for-bit copy of the input disk.
This method has several disadvantages, however. First, the capturing and
deploying phases both take a long time, since they must read and write the entire
surface of the disk. Second, the master file is very large. And last, the process
requires that the destination disk be very similar to the source disk, in order to
retain valid partition boundaries.
A modification of this technique can overcome some of the disadvantages. If not
all the partitions of the source system disk are in use, then the capturing agent
can record only the contents of the used partitions, plus the partition layout of
the disk. Then the deploying agent can recreate the partition layout properly on
the destination disk, and restore the contents of the used partitions, sector by
sector.
The result is a destination disk that carries all the useful information of the
source: the contents of the used partitions on the output disk are exact copies of
the comparable portions of the input disk.
This modification ("partition-based sector mode copying") is a considerable
improvement on full-disk copying. Reading only part of the disk results in faster
transfers and smaller master files. And, since the partition layout is created on
the destination disk using native tools, the destination disk's geometry does not
need to match that of the source disk.
A further improvement in performance can be made at the cost of some fidelity
in the copying process. It is possible for the capturing agent to read only the
directory and file contents of the source disk, and to record the file system, the
logical contents of the disk, in the master file. One very important characteristic
of this process is that the capturing and deploying agents must be able to read
and write the native file system accurately. This is a simple task for simple file
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systems such as FAT16 and FAT32; more complex for ext2 and ext3; and more
difficult yet for the several versions of the Windows NTFS file system. Since the
agents are not, in general, running as part of the operating system being captured
or deployed, correct file system access requires additional, usually proprietary,
technology.
The output disk in this case is not a copy of any part of the input disk. It contains
the logical contents of the input disk, but in different locations on the disk. The
partition layout and boot information must be copied and reproduced separately
from the file contents. Note that an additional, accidental benefit of this method
is that the file system has been defragmented.
Since most real disks include some empty space, this "file mode" transfer results
in even faster capture and deployment and smaller intermediate master files.
And since the partition layout is also reproduced logically, the destination disk's
geometry does not need to match that of the source disk.
Another technology combines some of the advantages of the other approaches.
In this case, the imaging agent records only the disk sectors that have been used
in the active partitions. The agent examines the allocation tables of the file
system in the several partitions and reads only the sectors that have been
allocated to files within the file system. The agent need not know the directory
or file structure; only how to read the allocation tables or bitmaps. As usual,
partition layout and boot information must be copied separately. Such a
technique is often called "smart sector copying," since the sector contents are still
copied in binary mode, but only the needed sectors are copied.
The output disk in this case is very similar to the input disk. Within a partition,
all the important sectors -- those allocated to files, directories, etc. -- match the
corresponding input sectors; the rest of the sectors are unimportant by this
definition, and no guarantee need be made about their contents.
The performance of smart sector copying, both in speed and intermediate master
file size, is very similar to that of file mode copying. An additional advantage is
that the agents do not have to understand the file systems fully, but only be able
to determine the sector allocations.
Method of copying
Process
Advantages
Disadvantages
Full disk sector
mode
Read/write all the
sectors of the disk.
Output copy is really
identical to input.
Takes a long time to
read/write the whole
disk.
Intermediate master
file is very large.
Output disk geometry
must match input
disk.
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Method of copying
Process
Advantages
Disadvantages
Partition sector
mode
Read/write all the
sectors of some (or
all) partitions
Output copy is nearly
identical to input.
Still takes a long time
to read/write sectors
that are empty space.
If you can avoid
copying some
partitions, then the
process is faster and
the intermediate
master file is smaller.
Output disk geometry
does not need to
match input.
File mode
Read/write the file
contents of some (or
all) partitions.
Very fast.
Intermediate master
file is as small as
possible.
Agents must be able
to read/write the
native file systems
accurately.
Output disk is
defragmented.
Smart sector mode
Read/write only the
allocated blocks of
some (or all)
partitions.
Performance similar
to file mode, very fast.
Intermediate master
file is small.
Target file system has
the same
fragmentation and
performance as the
source.
Agents do not need to
understand the native
file system fully, only
the allocation
methods.
Table 3: Methods of Creating a Functional Copy of One Computer System's
Software on Another System
There is one additional technology that is normally used in capturing the
contents of the source disk and storing them in the intermediate master file: data
compression. In general, the contents of the source disk sectors, or file data, are
compressed by the capturing agent before the data is sent to the master file. This
reduces the amount of data sent from the agent to the master file, and often
increases the overall speed of the transfer. Compression algorithms are highly
compute-intensive, and it is possible that compression of the data will saturate
the CPU where the capturing agent is running. Often the user is given a choice
of several different compression algorithms, including no compression, to
accommodate varying speeds of source system CPUs.
Regardless of the algorithm used, the capturing agent compresses the data from
the source disk before sending it to the master file. During deployment, the
master file contents are transmitted to the deployment agent, which decompresses the data and restores it to the correct place on the disk. De-
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compression of data requires much less computing than compression, so there is
little chance of saturating the CPU of the target system. Capturing agents may
use industry standard compression algorithms such as ZIP, gzip, LZ, LZW, etc.,
or they may use proprietary algorithms, but only lossless algorithms can be used.
The effectiveness of compression varies, depending on the nature of the data.
Modern operating systems, file systems, and database systems often have some
compression built in, so that further compression is less effective than expected.
Personalize Target Systems During Image Deployment
Networked computer systems must meet a few requirements to be “good
network citizens.” Computers in a network are mutually exclusive, meaning that
none of them can have the same identity. Each computer on a network must
have at least a unique name or address or both. For some local area networks,
the unique address is derived from the hardware that attaches to the network.
But more commonly the name and/or address are set in the operating system
software.
The source system at some point had a network identity. The identity might
have been removed during the process of freezing or “buttoning-up” the system
for capture. So the intermediate master file might or might not still have a
network identity, depending on the details of the deployment system. In any
case, the deployed copies of the source system must not be allowed to speak on
the network with their old identities. On IP networks, for instance, this would
result in “duplicate IP address” errors.
On some operating systems, typically UNIX-based systems, it is possible to
replace the network identity by changing a few well-known configuration files.
Other systems, such as Windows, are more complex. In these cases, the
deployment system must run a start-up agent on the target system to change the
network identity using the system's native tools.
It is also possible that the target systems will need other individual identifying
information, such as encryption keys, security certificates, etc.
Make Image-based Installation Truly Remote and Unattended
A major technology that makes image-based installation truly remote and
unattended is Preboot Execution Environment (PXE) developed by Intel on top
of standard protocols such as DHCP, TFTP, and TCP/IP. Because PXE provides a
mechanism for executables to be downloaded to system memory without going
thorough the running operating system, it can be used for a) remote setting up
systems where there is no operating system; b) remote booting system for
diagnostic purpose; and c) remote network boot when there is no local storage.
When a PXE-enabled system boots up, it requests for an IP address by
broadcasting a DHCPDISCOVER message with a PXE extension. The DHCP
server or Proxy DHCP server sends the target system an IP address and the
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address of a TFTP server(s). The target system then downloads the Network
Bootstrap Program from the TFTP server and executes it. For an image-based
installation system, the imaging or deploying agent or is a program that can be
downloaded through PXE to target the system’s memory for execution. Once the
agent is downloaded, it runs on the target system to read from or write to the
hard disk and send the messages back the imaging server.
If a target system is not PXE-enabled, an agent has to be pre-installed in the
target system to enable remote deployment. For example, a special partition can
be created on a hard disk to store the agent. Once a system is booted to the
special partition, the agent contacts the imaging server for deployment or
capturing commands. If there are no deployment commands from the server, the
agent changes the boot order and reboot to normal hard disk.
When there is no PXE support and an agent is not pre-installed, certain human
interaction is needed to carry out image-based installation. This might include
using a Floppy disk or CD to emulate PXE or to load the expected agent.
Important Note:
Operating system images are not completely transportable between machines.
Differences in system configuration can cause an operating system from one
machine not to run on another. Major differences typically include peripheral
disk subsystem controllers such as RAID and SCSI technologies versus IDE disk
subsystems, or internal components such as power control and DMA systems.
IBIs do not contain any magic technology that can make up for major differences
in hardware. However, most operating systems will accommodate minor
differences, such as CPU speeds, memory sizes, and network attachments.
Additionally, IBIs can smooth over differences in disk sizes, manufacturers,
models, and speeds. If a user needs to clone an operating system image within a
set of similar machines, the results are usually highly successful.
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Section
4
Conclusions
IBI systems provide a mechanism to distribute large operating systems and
applications remotely and monitor-free. It has the advantage of fast-deployment
and centralized control. The persistent agents running on target systems can also
expand an IBI system’s capability in updating a computer’s BIOS, RAID, and
applications. Despite some constraints such as license control and PXE support,
the IBI system could become a major low-cost solution for remote and
unattended software deployment to computers with minimal peripherals and
high modularity.
.
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