Sun Fire Systems Design and Configuration Guide
- Understanding a Running System
- Design Rules of Thumb
- Analyzing an Existing System
- Designing for RAS
- A Logical Design Specification
Now that you have completed your statement of requirements you can work on the first half of designing a Sun Fire systemdesigning the logical server. By the end of this chapter, you will have completed a logical design containing a list of how many of each of the components you need, and a listing of your RAS requirements. Then, you can apply this configuration in Chapter 4 when you choose the physical system in which to place your design.
Systems design is done in this somewhat "backwards" manner for two important reasons:
To make sure your requirements are clearly stated and met.
Multiple servers can be located inside one physical chassis because Sun Fire systems support domains.
Following this process will also help ensure that you purchase a system with enough room for future expansion.
This chapter covers the following topics, which describe the logical design process:
Understanding a Running System
Design Rules of Thumb
Analyzing an Existing System
Designing for RAS
A Logical Design Specification
Understanding a Running System
This section reviews the basics of a computer system. While this is likely all "refresher" material, many people misunderstand the real roles of computer components somewhat. As such, an analogy to a reception desk is used to help better illustrate the different role each major component plays. In this analogy we follow a receptionist answering various types of incoming calls to show how a computer manages the requests it receives.
Every computer system has three main components that can be configured:
Of course, a Sun Fire system has many other components too, including repeater boards, the Fireplane, and so on. However, in the Sun Fire system (as with most computer systems), these are part of the fundamental architecture of the machine and cannot be configured by the customer. This fact means that to design your system, you should pay close attention to the decisions you make regarding CPUs, memory, and I/O because these decisions will directly affect the effectiveness of your design.
Notice the use of the term CPUs. Because the Sun Fire system board is sold with a minimum of two processors, it is not possible to buy a single-CPU Sun Fire system. All Sun Fires are multiprocessor systems.
The Sun Fire system uses the PCI bus for all I/O. The I/O is what allows you to do anything productive with the system. Without I/O, you would have no keyboard, no network connection, no disks, and so forth.
Understanding the impact I/O has on the system is important. When something has to be done with I/O, an interrupt is generated. The CPUs must handle this interrupt. Frequently, I/O is the single-biggest resource sink on a system. This fact is especially true when you have multiple types of I/O running heavy loads concurrently, which generates a large number of interrupt requests.
For example, consider a backend database server that is front-ended by a dozen or more concurrent web servers. When a web server needs some dynamic data, it has to make a request via the network to the server, which then must do the appropriate database selects and retrieve the data from its local disk, finally shuffling the reply back across the network to the web server that requested it. This can result in a number of I/O interrupts, as the system must handle all of the network packets as well as all of the disk seeks to get the database information off disk.
When you multiply one request times a dozen or more web servers, each request times a dozen or more clients, you can see that the database server could easily become swamped with I/O interrupts, which excludes the computing power needed to run the operating system, manage memory, and run the database itself.
To tie everything together, think of I/O as each individual phone call received by a receptionist. Each phone call generates an interrupt that the receptionist must handle. Depending on the request, it may result in a lot of data transfer (talking) back to the caller. More calls generate more interrupts. Eventually, the phone system (server) hits a limit either in the amount of concurrent requests that it can handle (memory), the speed with which the requests can be fulfilled (by the CPUs), or how fast the caller and the CPUs can communicate (I/O speed).
The CPU is actually responsible for much more than computation. Anything that puts a load on the system, including databases, web servers, email, NFS, NTP, and general network and user traffic, requires a lot of CPU power. The CPU does not do as much thinking as it does handling. Any time the system must do anything, it must ask the CPU, which has to prioritize the task, schedule it, and allocate resources for it, and do so in a way that allows all the other multitude of things going on to continue running too.
In this way, the CPU can be thought of as a busy receptionist. The receptionist has a number of standard routines. These may include forwarding calls to employees, taking messages, setting up appointments, and even providing direct responses to simple requests, "What is your address?" When an incoming phone call is received, the receptionist executes the proper routine, and completes the request if possible. If the request cannot be fulfilled in a reasonable amount of time, the receptionist may have to place the caller on hold temporarily to handle some other tasks and free up some time.
In some cases, the receptionist may receive a request that is too complex to be handled by standard routines. For example, the receptionist may receive a call that the boss is running late, and that several meetings need to be rescheduled. Here, the receptionist must do some thinking to determine which meetings can be moved to when. At the same time, the receptionist must still pay attention to other incoming calls, to ensure an important request is not missed.
If things get too busy for one receptionist to handle, you may need two or more receptionists. Some callers may even get frustrated and hang up. Even for those that do get through, there will likely not be enough time to properly answer their queries.
So, it is important to consider not only the difficulty of each request, but the volume too. In our analogy, each incoming request requires a certain baseline of time to handle properly. Typically, the receptionist will have to press a button to pick up the appropriate line, answer the call with a greeting, listen and analyze the request, then prioritize it and complete it appropriately. Even if a request consists of nothing more than "Is Mr. Johnson in?", it still takes a certain amount of time to fulfill the request.
In the Sun Fire system, the system memory is dynamic random access memory (DRAM). The system uses memory to store things that it is using actively such as the operating system, programs, and their data.
When asked to execute a program, the system must allocate space in memory to hold an image of the program and its associated data. This space can grow or shrink as the program runs, since its resource requirements may change. In reality, most applications grow over time because they do a poor job of cleaning up after themselves.
When a system is under a very heavy load, it may run out of room in memory to hold all the information it needs. In this case, it uses predetermined disk space, known as swap space, to temporarily store lesser-used things from memory temporarily to make room for other things. This is known as paging, since it involves selectively moving specific data out of memory in sections know as pages. When those pages are needed, the system incurs a page fault, and the data is moved from disk back into memory.
In extreme situations, the system may undergo swapping. In this case, memory images of entire programs are moved from memory out to disk. This is a significant performance hit, and if the system starts swapping, some serious problems may occur. Unfortunately, the terms paging and swapping are often used interchangeably, perhaps because the disk storage is called "swap space," but they are really very different.
Do not undervalue how important memory is to a running system. Not having enough memory is perhaps the single greatest cause of performance problems.
With the receptionist analogy, you can think of memory as the number of incoming phone lines available. Even if you have five receptionists (CPUs), it will not help the situation if you only have four phone lines (memory). The phone system will still be slow, since you have a bottleneck in the amount of requests you can handle concurrently. To accept another call, the current caller will have to be placed on hold (page-out) in order to get back to the first caller (page-in).
If the load gets too heavy for the phone system, and no more lines can be put on hold, calls will have to be disconnected (swap-out) to make room for others. The receptionists will then have to call the person back (swap-in), a much more time-intensive process.