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Chapter 28 . Emulation and Virtualization 699 A bochs package is included in SUSE Linux. Install it in the usual way, and type the command bochs. A set of text menus appears in which you can set various options. You can write these to disk to create a configuration file for future use. After you have set the options, or bochs has read them from a configuration file, a graphical window will appear. Provided bochs knows how to find its virtual CD drive, you can begin installing an operating system, and you have created and configured a suitable file as a virtual hard disk, you can begin installing an operating system. Because bochs is so slow, unless you have very powerful hardware to run it on, or you have an academic interest in the workings of emulation and the x86 processor family, you will probably conclude that for practical purposes either QEMU or VMWare is a more useful solution, particularly if you want to get real work done with your guest operating system. Figure 28-4 shows the starting of a SUSE installation in bochs. Figure 28-4: Starting a SUSE installation in bochs Virtual Machines Using QEMU QEMU is an open source hardware emulator. It can emulate an x86 system on x86, but can also both emulate and run on some other architectures. In particular, it can emulate and run on the PowerPC architecture.
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698 Part V . SUSE Linux in the Enterprise Note Figure 28-3: A Windows installer running under Wine Due to the complexity and size of the Windows API, Wine is always a work in progress, and all Windows packages do not run correctly under Wine. For information about which Windows applications run correctly under Wine, see http:// appdb.winehq.org. A commercially supported version of Wine known as CrossOver Office is available from CodeWeavers (www.codeweavers.com). This version of Wine provides enhancements and bug fixes that enable it to run many more applications, such as the complete Microsoft Office suite, than the free version of Wine. The CodeWeavers folks are very good about working directly with the Wine community, are the leading commercial sponsor of the Wine project, and push all of their fixes into the open source version of Wine. CrossOver Office is an excellent investment if you need to run Office or other resource-intensive software packages on your Linux system. As a matter of fact, this section of this chapter was written in Word running under CodeWeavers Wine implementation. The bochs PC Emulator The bochs project goes back a long way. It is a free (licensed under the GNU LGPL) PC hardware emulator that provides a complete emulation of PC hardware in software. As is the case with QEMU and VMWare (see later in the chapter), you can install an operating system into bochs. However, bochs does not offer virtualization of the underlying hardware to the guest. This means that it can be built and run on any Unix-like platform on any hardware architecture, but it also means that it is slow. For most practical purposes, if you want to run x86 operating systems as virtual machines on x86 hardware, then either QEMU (free) or VMWare (proprietary and commercial) will probably be a better first choice.
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Chapter 28 . Emulation and Virtualization 697 Windows applications that you need to run because the virtual machine looks like an actual Windows system to the software. However, that seems like overkill when all you need to do is to edit a Word document that contains complex macros or tweak someone s n-dimensional Excel spreadsheet. Wouldn t it be nice to simply be able to run the one or two Windows applications that you need without the overhead and associated storage of a completely emulated Windows system? Providing the ability to run Microsoft Windows applications natively on Linux and other Unix-like systems is the precise goal of the Wine (Wine Is Not an Emulator) Project (www.winehq.com). Wine is an open source implementation of the application programming interface (API) provided by Microsoft Windows and used by all Microsoft Windows applications. Wine runs in the X Window system environment on Linux and Unix. SUSE Linux includes a stable and tested version of Wine to enable many Microsoft Windows applications to install and execute normally on SUSE Linux systems. Wine is one of the most popular and most misunderstood open source projects. Unlike emulators, which simulate the behavior of a given operating system or graphical environment and support its applications within that environment, Wine is a complete re-implementation of the API used by Windows applications. It therefore does not need to translate Windows API calls into equivalent Linux or X Window system calls on the fly. For example, Windows applications running under WINE call the same graphical APIs that they always called, but the implementation of those APIs immediately performs specific X Window system functions rather than the Windows functions that one would see if the application was running on a Microsoft Windows system. Very cool! Wine is installed on SUSE Linux as part of the Office applications package. To install a Windows software package on Linux, execute the wine command, followed by the name of the installer for that package. For example, to install the Windows NX Client package on your Linux system, you would execute the following command: $ wine nxclient-1.4.0-92.exe Figure 28-3 shows a Windows installer running on a SUSE system under Wine. The first time that you run it on your Linux system, Wine creates a mock Windows directory hierarchy, as indicated by the message in the Konsole window in Figure 28-3. This mock Windows directory structure is located in the directory ~/.wine, and contains a configuration directory, a dosdevices directory that provides DOS/Windows drive letter mappings, a drive_c directory that mirrors the directory structure that Windows expects to find on its boot drive, and three files that contain various system and user information and represent the Windows registry. After you have installed a software package under Wine, you can execute any of the applications that it contains by specifying the wine command followed by the full pathname of the program that you want to run, as in the following example: $ wine ~/.wine/drive_c/Program Files/NX Client for Windowsnxclient.exe
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696 Part V . SUSE Linux in the Enterprise dosbox The dosbox package is similar to dosemu, although it is designed essentially for playing games. The web page at http://dosbox.sourceforge.net includes a long list of DOS games with comments on how well they work under dosbox. dosbox can run both real-mode and protected-mode DOS games, and its graphics support is remarkably good. When dosbox starts, you are presented with a DOS prompt showing the drive letter Z:. dosbox contains an internal mount command, so you can type, for example: Z:> mount C /home/roger/DOSPROG Then when you switch to drive C: under dosbox, you will be able to run the DOS programs that you have saved in that directory. In Figure 28-2, dosbox is seen running a graphical DOS program, which fails under dosemu. Figure 28-2: dosbox running Stefan Weber s Penrose tiling generator Running Microsoft Windows Applications Using Wine Even the most rabid Linux fanatic has to recognize that there are times when you must run Microsoft Windows applications for compatibility reasons with other applications or your co-workers. As discussed elsewhere in this chapter, one solution is to run software such as VMWare that emulates an entire Windows system. Within the context of this virtual machine, you can then install and execute the
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Chapter 28 . Emulation and Virtualization 695 DOS Emulation Using dosemu and dosbox SUSE contains packages for two DOS emulators, dosemuand dosbox. They are similar, with the difference that dosboxis intended as a platform for running DOS games. dosemu The dosemu package that comes with SUSE includes a version of FreeDOS, and free software DOS clone. However, it is possible to replace this with MS-DOS or another DOS variant if you want to. After the package is installed, all you need to do to run it is type either dosemu or xdosemu, and you will see a window with a DOS prompt (the first time you run it you are asked to confirm the path to the DOS version that it is going to run and to confirm that you accept the license). The virtual DOS C: drive that appears is in fact rooted at ~/dosemu/freedos/ and is copied from the master copy at /opt/ dosemu/share/dosemu/freedos/. The virtual D: drive is simply your home directory (however, because DOS expects filenames to be in 8.3 format, long filenames are garbled). If you create a directory DOSPROG under your home directory and copy into it a DOS program that you want to run, you will then be able to run it simply by switching to the directory D:DOSPROG within dosemu. To exit from dosemu, type exitemu at the prompt, and the DOS session will close. Figure 28-1 shows dosemudisplaying a typical DOS directory listing. Figure 28-1: dosemu showing a directory listing Many DOS programs will work fine in dosemu, but games and other programs that produce graphical output may fail.
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694 Part V . SUSE Linux in the Enterprise FAUmachine can be regarded as emulating Linux under Linux, because the guest Linux system runs as a process on top of Linux. Virtualization implies (at least) that a virtual machine exists: in other words that there is a software environment which emulates (at least some aspects) of a real hardware machine. If you can emulate an entire hardware platform, then you are doing virtualization. The term is used most often in cases where there is also some mechanism for transferring some of the resources directly to the virtual environment, which is a requirement that the virtual machines must provide to ensure decent performance. It is possible to emulate an entire hardware platform. For example, the bochs project emulates an entire PC environment. You can then install an operating system into this virtual PC. The operating system thinks that it is being installed into real hardware. When you start the emulator, the guest operating system boots within it and you have a virtual machine running on top of your real one. This is the approach used by bochs, VMWare, QEMU, and Xen, but there are significant differences in how it is achieved. To get decent performance in a virtual machine, there needs to be a way of sharing some of the real hardware resources with the guest operating system. As noted previously, the term virtualization is usually used with the implication that as well as providing a virtual machine capability, some of the resources of the real hardware are virtualized and offered to the guest operating system. This is not the case with bochs, which is why it is so slow, although it provides an entire emulated PC environment. However, bochs can run on any Unix system on any hardware platform. VMWare, QEMU, and Xen do have the ability to virtualize real resources and hence provide better performance. To be able to virtualize real system resources in this way requires that the emulated system have the same architecture as (or in the case of Xen, very similar to) that of the real hardware. The Xen virtualization system is creating a great deal of interest at the moment, as it is a way of running multiple virtual systems on a single server with minimal loss of performance. It also has the very powerful feature of allowing running systems to be migrated from one physical server to another with downtime measured of only fractions of a second. This makes it a serious competitor with VMWare s high-end server products. The only disadvantage is that at present Xen requires a slightly modified version of the operating system in order to be able to run it. This is not a problem for Linux (or indeed any open source system); current versions of SUSE already include the necessary modifications. However it does mean that at present it is not possible to run Windows operating systems within Xen. Future versions of Xen (3.0 onwards) running on new Intel and AMD processors (with Intel s VT and AMD s Pacifica extensions) will be able to run other operating systems unmodified, so soon Windows guest operating systems on Xen will also be possible.
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28 CHAPTER Emulation and Virtualization …. Since the earliest days of Linux, there has been interest in running software designed for other systems on a Linux system. For obvious historical reasons, many people wanted ways to run applications written for MS-DOS (or other DOS variants) or Windows on Linux. There has also been an interest in finding ways of running Linux on top of Linux. Initially the motivation for these attempts largely had to do with enabling desktop usage of DOS or Windows programs in a Linux environment. However, as time has gone by, and hardware has become more powerful, the emphasis has partly changed. There is now great interest in the idea of consolidating server loads by moving multiple existing systems into a virtual environment running on top of Linux. Emulation versus Virtualization We look at two main approaches in this chapter: emulation and virtualization. To some extent these two terms are interchangeable, and many forms of emulation involve a certain amount of virtualization. Emulation refers to the idea of creating an environment (as a running program) that behaves in terms of input and output like the target system. For example, the dosemuprogram is an emulation of a DOS system. Although it is a program running under Linux, it responds to interactions from users just as a real DOS system would. Wine s Windows emulation behaves slightly differently. Strictly speaking, Wine s approach is not emulation (indeed the name Wine stands for Wine is not an emulator ) because there is not a process running that pretends to be a Windows system. What Wine does is to intercept calls by Windows programs to the operating system and libraries and replace them with calls to its own infrastructure, which results in the right behavior. User Mode Linux and In This Chapter Emulation versus virtualization DOS emulators Using Wine Working with bochs and QEMU Creating virtual machines with Xen User mode Linux and FAUmachine Other emulators ….
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692 Part V . SUSE Linux in the Enterprise Using DRBD If the primary node in your DRBD cluster goes down, you will need to promote the secondary node to primary status so that you can mount the filesystem and start your applications. When the primary node comes back up, any data written to the secondary node is replicated to the primary node, and you can then reverse their roles and carry on normal operation. To change the status of a node, you need to use the drbdsetup command. The drbdsetup command can be used to change the DRBD runtime environment, including the definition of host parameters. To change from a secondary to a primary node in the event of a primary node failure, you need to tell DRBD to set the DRBD device to primary. bible:~ # drbdsetup /dev/nb0 primary This tells DRBD on the secondary node that it is safe to use the device and also that this device should be marked as the master when it comes to the owner of the data. This is important because if the old primary node comes back to life, you must maintain where the most recent copy of the data is. If you have managed to fix the broken (old primary) node, the data from the new primary node is automatically transferred to the now secondary node. When this is complete, you need to set the new primary to secondary and the old primary to primary so that normal operation can resume. To set the new primary to secondary, you need to use the drbdsetup command again: bible:~ # drbdsetup /dev/nb0 secondary When the roles have been reversed on both nodes, and the machine you originally designed as the primary is restored to its former glory, you can carry on to use its services as normal. Finally, the enterprise is somewhere where your IT services must work, must be maintainable, and must be transparent to the end user. Saying that Linux is enterprise-ready means that at least those points must be satisfied, and only recently has this happened. Now that customers are happy to use Linux in this enterprise space, things can only get better. More money into the Linux market will help to boost its scalability, stability, and usability in the long run! …
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Chapter 27 . Enterprise Architecture 691 Each protocol version provides a different profile for writing data depending on your network and disk latency and also whether you need to guarantee your data has been saved to the remote disk or not. . If you are using a high-latency connection, you may need to use protocol A. This effectively removes any issues you have with the network as the kernel returns a write operation as soon as it has been written to the local disk and sent to the network buffer. This is the fastest protocol you can use if your application needs near-local write efficiency. . If you are using a low-latency network, protocol C guarantees data has been written to both machines disks before a write is returned by the kernel. If you notice that your disk writes are taking too long when using this protocol, you may want a mid point between protocol A and C. . Protocol B returns a write only when data has been written to the local disk and has been received by the remote network buffer. This decreases the amount of time it takes for local writes but also gives you some guarantee that your network is on the way to being written to the remote disk. Choosing the best protocol is something that you must be comfortable with because it severely impacts the speed of local writes to the disk, thus slowing down the application that relies on the data being replicated. As we are more concerned with the integrity of the data and not the speed of the transfer, we have chosen protocol C. Defining Your Hosts When your network and protocol have been defined, you need to specify the hosts that will be taking part in the replication. In Listing 27-1, we have defined node1 first, telling DRBD that this is the primary node. You need to define the IP address of the node, the raw device to replicate, and the name of the device as it will be used by the node itself. After DRBD has been configured for a device, a DRBD device is created to be used as the device to be replicated. We have defined that /dev/sdb1 should be replicated and that we will use the device /dev/nb0 to access the replicated device. For example, we can use mkreiserfs/dev/nb0to create a Reiser filesystem that is replicated to the secondary node automatically. For the sake of verbosity, we have also specified in the listing the TCP port that we will use for sending DRBD data over the network. And we have also made a similar configuration with the configuration that is specific to the secondary node. When saved, you can copy the drbd.conf file to the secondary node as is and start DRBD with rcdrbdstart. When DRBD has started, check /proc/drbd for information on what DRBD is doing and whether the node is in primary or secondary mode. Note
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690 Part V . SUSE Linux in the Enterprise Listing 27-1 (continued) } # DR DB Server on node2 { device = /dev/nb0 disk = /dev/sda8 address = 192.168.0.3 port = 7788 } } We have defined two machines, node1 and node2; node1 is the primary node in the partnership (the first node defined). As synchronization takes place over the network, it is a good idea to specify how much bandwidth can be used when sending data between machines. If you have to share your network bandwidth with other applications which is usually true over a wide area network (WAN) link to a DR site you do not want to saturate your connection for the sake of one DR configuration. The net section enables you to configure any network-based operations that DRBD will use during synchronization. For example, here we have defined a maximum synchronization rate of 30 megabytes per second (Mbps) so as not to saturate our 100 Mbps link. DRBD Protocols A very important aspect of DRBD is the protocol used for synchronization. DRBD defines three protocols that can be used. They are listed in Table 27-1. Table 27-1 DRBD Protocol Versions Protocol Version Description B Data is marked as written when it has been written to the local disk and an acknowledgment has been received from the secondary node s network buffer. A Data is marked as written when it has been written to the local disk and the network buffer. C Data is marked as written when it has been written to the local disk and an acknowledgment has been received when data has been written to the remote disk.
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