Design Issues




Design Issues

 Making the multiplicity of processors and storage devices transparent to the users has been a key challenge to many designers. Ideally, a distributed system  should look to its users like a conventional, centralized system. The1 user interface of a transparent distributed system should not distinguish between local and remote resources. That is, users should be able to access remote resources as though these resources were local, and the distributed system should be responsible for locating the resources and for arranging for the appropriate interaction. Another aspect of transparency is user mobility It would be convenient to allow users to log into any machine in the system rather than forcing them to use a specific machine.

A transparent distributed system facilitates user mobility by bringing over the user's environment (for example, home directory) to wherever she logs in. Both the Andrew file system from CMU and Project Athena from MET provide this functionality on a large scale; NFS can provide it on a smaller scale. Another design issue involves fault tolerance.

We use the termfault tolerance in a broad sense. Communication faults, machine failures (of type fail-stop), storage-device crashes, and decays of storage media should all be tolerated to some extent. A fault-tolerant system should continue to function, perhaps in a degraded form, when faced with these failures. The degradation can be in performance, in functionality, or in both. It should be proportional, however, to the failures that cause it.

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 A system that grinds to a halt when only a few of its components fail is certainly not fault tolerant. Unfortunately, fault tolerance is difficult to implement. Most commercial systems provide only limited fault tolerance. For instance, the DEC VAX cluster allows multiple computers to share a set of disks. If a system crashes, users can still access their information from another system. Of course, if a disk fails, all systems will lose access.

But in this case, RAID can ensure continued access to the data even in the event of a failure (Section 12.7). Still another issue is scalability—the capability of a system to adapt to increased service load. Systems have bounded resources and can become completely saturated under increased load. For example, regarding a file system, saturation occurs either when a server's CPU runs at a high utilization rate or when disks are almost full.

Scalability is a relative property, but it can be measured accurately. A scalable system reacts more gracefully to increased load than does a nonscalable one. First, its performance degrades more moderately; and second, its resources reach a saturated state later. Even perfect design cannot accommodate an ever-growing load. Adding new resources might solve the problem, but it might generate additional indirect load on other resources (for example, adding machines to a distributed system can clog the network and increase service loads).

Even worse, expanding the system can call for expensive design modifications. A scalable system should have the potential to grow without these problems. In a distributed system, the ability to scale up gracefully is of special importance, since expanding the network by adding new machines or interconnecting two networks is commonplace. In short, a scalable design should withstand high service load, accommodate growth of the user community, and enable simple integration of added resources. Fault tolerance and scalability are related to each other.

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A heavily loaded component can become paralyzed and behave like a faulty component. Also, shifting the load from a faulty component to that component's backup can saturate the latter. Generally, having spare resources is essential for ensuring reliability as well as for handling peak loads gracefully

An inherent advantage of a distributed system is a potential for fault tolerance and scalability because of the multiplicity of resources. However, inappropriate design can obscure this potential. Fault-tolerance and scalability considerations call for a design demonstrating distribution of control and data. Very large-scale distributed systems, to a great extent, are still only theoretical.

 No magic guidelines ensure the scalability of a system. It is easier to point out why current designs are not scalable. We next discuss several designs that pose problems and propose possible solutions, all in the context of scalability. One principle for designing very large-scale systems is that the service demand from any component of the system should be bounded by a constant that is independent of the number of nodes in the system.

Any service mechanism whose load demand is proportional to the size of the system is destined to become clogged once the system grows beyond a certain size. Adding more resources will not alleviate such a problem. The capacity of this mechanism simply limits the growth of the system. Central control schemes and central resources should not be used to build scalable (and fault-tolerant) systems. Examples of centralized entities are central authentication servers, central naming servers, and central file servers.

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Centralization is a form of functional asymmetry among machines constituting the system. The ideal alternative is a functionally symmetric configuration; that is, all the component machines have an equal role in the operation of the system, and hence each machine has some degree of autonomy. Practically, it is virtually impossible to comply with such a principle. For instance, incorporating diskless machines violates functional symmetry, since the workstations depend on a central disk. However, autonomy and symmetry are important goals to which we should aspire.

The practical approximation of symmetric and autonomous configuration is clustering, in which the system is partitioned into a collection of semiautonomous clusters. A cluster consists of a set of machines and a dedicated cluster server. So that cross-cluster resource references are relatively infrequent, each cluster server should satisfy requests of its own machines most of the time.

 Of course, this scheme depends on the ability to localize resource references and to place the component units appropriately. If the cluster is well balanced —that is, if the server in charge suffices to satisfy all the cluster demands—it can be used as a modular building block to scale up the system. Deciding on the process structure of the server is a major problem in the design of any service. Servers are supposed to operate efficiently in peak periods, when hundreds of active clients need to be served simultaneously.

A single-process server is certainly not a good choice, since whenever a request necessitates disk I/O, the whole service will be blocked. Assigning a process for each client is a better choice; however, the expense of frequent context switches between the processes must be considered. A related problem occurs because all the server processes need to share information.

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One of the best solutions for the server architecture is the use of lightweight processes, or threads, which we discussed in Chapter 4. We can think of a group of lightweight processes as multiple threads of control associated with some shared resources.

Usually, a lightweight process is not bound to a particular client. Instead, it serves single requests of different clients. Scheduling of threads can be preemptive or nonpreemptive. If threads are allowed to run to completion (nonpreemptive), then their shared data do not need «to be protected explicitly. Otherwise, some explicit locking mechanism must be used. Clearly, some form of lightweight-process scheme is essential if servers are to be scalable.



Frequently Asked Questions

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Ans: Robustness A distributed system may suffer from various types of hardware failure. The failure of a link, the failure of a site, and the loss of a message are the most common types. To ensure that the system is robust, we must detect any of these failures, reconfigure the system so that computation can continue, and recover when a site or a link is repaired. view more..
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Ans: We survey two capability-based protection systems. These systems vary in their complexity and in the types of policies that can be implemented on them. Neither system is widely used, but they are interesting proving grounds for protection theories view more..
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Ans: Revocation of Access Rights In a dynamic protection system, we may sometimes need to revoke access rights to objects shared by different users view more..
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Ans: Design Issues Making the multiplicity of processors and storage devices transparent to the users has been a key challenge to many designers. Ideally, a distributed system should look to its users like a conventional, centralized system. The1 user interface of a transparent distributed system should not distinguish between local and remote resources. That is, users should be able to access remote resources as though these resources were local, and the distributed system should be responsible for locating the resources and for arranging for the appropriate interaction. view more..
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Ans: Design Principles Microsoft's design goals for Windows XP include security, reliability, Windows and POSIX application compatibility, high performance, extensibility, portability, and international support. view more..
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Ans: Input and Output To the user, the I/O system in Linux looks much like that in any UNIX system. That is, to the extent possible, all device drivers appear as normal files. A user can open an access channel to a device in the same way she opens any other file—devices can appear as objects within the file system. The system administrator can create special files within a file system that contain references to a specific device driver, and a user opening such a file will be able to read from and write to the device referenced. By using the normal file-protection system, which determines who can access which file, the administrator can set access permissions for each device. Linux splits all devices into three classes: block devices, character devices, and network devices. view more..
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Ans: Communication Protocols When we are designing a communication network, we must deal with the inherent complexity of coordinating asynchronous operations communicating in a potentially slow and error-prone environment. In addition, the systems on the network must agree on a protocol or a set of protocols for determining host names, locating hosts on the network, establishing connections, and so on. view more..
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Ans: Naming and Transparency Naming is a mapping between logical and physical objects. For instance, users deal with logical data objects represented by file names, whereas the system manipulates physical blocks of data stored on disk tracks. Usually, a user refers to a file by a textual name. view more..
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Ans: Stateful Versus Stateless Service There are two approaches for storing server-side information when a client accesses remote files: Either the server tracks each file being accessed byeach client, or it simply provides blocks as they are requested by the client without knowledge of how those blocks are used. In the former case, the service provided is stateful; in the latter case, it is stateless. view more..
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Ans: Computer-Security Classifications The U.S. Department of Defense Trusted Computer System Evaluation Criteria specify four security classifications in systems: A, B, C, and D. This specification is widely used to determine the security of a facility and to model security solutions, so we explore it here. The lowest-level classification is division D, or minimal protection. Division D includes only one class and is used for systems that have failed to meet the requirements of any of the other security classes. For instance, MS-DOS and Windows 3.1 are in division D. Division C, the next level of security, provides discretionary protection and accountability of users and their actions through the use of audit capabilities. view more..
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Ans: An Example: Windows XP Microsoft Windows XP is a general-purpose operating system designed to support a variety of security features and methods. In this section, we examine features that Windows XP uses to perform security functions. For more information and background on Windows XP, see Chapter 22. The Windows XP security model is based on the notion of user accounts. Windows XP allows the creation of any number of user accounts, which can be grouped in any manner. Access to system objects can then be permitted or denied as desired. Users are identified to the system by a unique security ID. When a user logs on, Windows XP creates a security access token that includes the security ID for the user, security IDs for any groups of which the user is a member, and a list of any special privileges that the user has. view more..
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Ans: An Example: Networking We now return to the name-resolution issue raised in Section 16.5.1 and examine its operation with respect to the TCP/IP protocol stack on the Internet. We consider the processing needed to transfer a packet between hosts on different Ethernet networks. In a TCP/IP network, every host has a name and an associated 32-bit Internet number (or host-id). view more..
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Ans: Application I/O interface In this section, we discuss structuring techniques and interfaces for the operating system that enable I/O devices to be treated in a standard, uniform way. We explain, for instance, how an application can open a file on a disk without knowing what kind of disk it is and how new disks and other devices can be added to a computer without disruption of the operating system. Like other complex software-engineering problems, the approach here involves abstraction, encapsulation, and software layering. Specifically we can abstract away the detailed differences in I/O devices by identifying a fewgeneral kinds. Each general kind is accessed through a standardized set of functions—an interface. The differences are encapsulated in kernel modules called device drivers that internally are custom-tailored to each device but that export one of the standard interfaces. view more..
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Ans: Transforming I/O Requests to Hardware Operations Earlier, we described the handshaking between a device driver and a device controller, but we did not explain how the operating system connects an application request to a set of network wires or to a specific disk sector. Let's consider the example of reading a file from disk. The application refers to the data by a file name. Within a disk, the file system maps from the file name through the file-system directories to obtain the space allocation of the file. For instance, in MS-DOS, the name maps to a number that indicates an entry in the file-access table, and that table entry tells which disk blocks are allocated to the file. In UNIX, the name maps to an inode number, and the corresponding inode contains the space-allocation information. How is the connection made from the file name to the disk controller (the hardware port address or the memory-mapped controller registers)? First, we consider MS-DOS, a relatively simple operating system. The first part of an MS-DOS file name, preceding the colon, is a string that identifies a specific hardware device. For example, c: is the first part of every file name on the primary hard disk view more..
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Ans: STREAMS UNIX System V has an interesting mechanism, called STREAMS, that enables an application to assemble pipelines of driver code dynamically. A stream is a full-duplex connection between a device driver and a user-level process. It consists of a stream head that interfaces with the user process, a driver end that controls the device, and zero or more stream modules between them. view more..
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Ans: Performance I/O is a major factor in system performance. It places heavy demands on the CPU to execute device-driver code and to schedule processes fairly and efficiently as they block and unblock. The resulting context switches stress the CPU and its hardware caches. I/O also exposes any inefficiencies in the interrupt-handling mechanisms in the kernel. view more..
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Ans: Multiple-Processor Scheduling Our discussion thus far has focused on the problems of scheduling the CPU in a system with a single processor. If multiple CPUs are available, load sharing becomes possible; however, the scheduling problem becomes correspondingly more complex. Many possibilities have been tried; and as we saw with singleprocessor CPU scheduling, there is no one best solution. Here, we discuss several concerns in multiprocessor scheduling. We concentrate on systems in which the processors are identical—homogeneous—in terms of their functionality; we can then use any available processor to run any process in the queue. (Note, however, that even with homogeneous multiprocessors, there are sometimes limitations on scheduling. Consider a system with an I/O device attached to a private bus of one processor. view more..
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Ans: Structure of the Page Table In this section, we explore some of the most common techniques for structuring the page table. view more..




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