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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 March 2002 Page 2 Dell Enterprise Systems Group 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 Page 3 Dell Enterprise Systems Group 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. March 2002 Page 4 Dell Enterprise Systems Group Figure 2: Image-based Installation System Figure 3: Schematic Diagram of an Image-based Installation System March 2002 Page 5 Dell Enterprise Systems Group 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 March 2002 Send the film to the studio for Page 6 Transfer image files to other Dell Enterprise Systems Group 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 March 2002 Page 7 Dell Enterprise Systems Group 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, March 2002 Page 8 Dell Enterprise Systems Group 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 March 2002 Page 9 Dell Enterprise Systems Group 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. March 2002 Page 10 Dell Enterprise Systems Group 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- March 2002 Page 11 Dell Enterprise Systems Group 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 March 2002 Page 12 Dell Enterprise Systems Group 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. March 2002 Page 13 Dell Enterprise Systems Group 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. . This White Paper is for informational purposes only. DELL MAKES NO WARRANTIES, EXPRESS OR IMPLIED, IN THIS WHITE PAPER. Dell cannot be responsible for errors in typography or photography. Dell, PowerEdge, and OpenManage are trademarks of Dell Computer Corporation. Other trademarks and trade names may be used in this document to refer to either the entities claiming the marks and names or their products. Dell disclaims proprietary interest in the marks and names of others. ©Copyright 2002 Dell Computer Corporation. All rights reserved. Reproduction in any manner whatsoever without the express written permission of Dell Computer Corporation is strictly forbidden. For more information, contact Dell. Information in this document is subject to change without notice. March 2002 Page 14 Dell Enterprise Systems Group