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8.2. Choosing the Right Hardware

Sometimes the most expensive machine is not the one that provides the best performance. Your demands on the platform hardware are based on many aspects and affect many components. Let's discuss some of them.

This discussion relies on the specific definitions of various hardware and operating-system terms. Although you may be familiar with the terms below, we have explicitly provided definitions to make sure there is no ambiguity when we discuss the hardware strategies.

A group of machines connected together to perform one big or many small computational tasks in a reasonable time. Clustering can also be used to provide failover, where if one machine fails, its processes are transferred to another without interruption of service. And you may be able to take one of the machines down for maintenance (or an upgrade) and keep your service running—the main server simply will not dispatch the requests to the machine that was taken down.

Load balancing
Say that users are given the name of one of your machines, but it cannot stand the heavy load. You can use a clustering approach to distribute the load over a number of machines (which gives you the advantages of clustering, too). The central server, which users access initially when they type the name of your service into their browsers, works as a dispatcher. It redirects requests to other machines, and sometimes the central server also collects the results and returns them to the users.

Network Interface Card (NIC)
A hardware component that allows your machine to connect to the network. It sends and receives packets. NICs come in different speeds, varying from 10 MBps to 10 GBps and faster. The most widely used NIC type is the one that implements the Ethernet networking protocol.

Random Access Memory (RAM)
The memory that you have in your computer (comes in units of 8 MB, 16 MB, 64 MB, 256 MB, etc.).

Redundant Array of Inexpensive Disks (RAID)
An array of physical disks, usually treated by the operating system as one single disk, and often forced to appear that way by the hardware. The reason for using RAID is often simply to achieve a high data-transfer rate, but it may also be to get adequate disk capacity or high reliability. Redundancy means that the system is capable of continued operation even if a disk fails. There are various types of RAID arrays and several different approaches to implementing them. Some systems provide protection against failure of more than one drive and some ("hot-swappable") systems allow a drive to be replaced without even stopping the OS.

8.2.3. Getting a Fast Internet Connection

You have the best hardware you can get, but the service is still crawling. What's wrong? Make sure you have a fast Internet connection—not necessarily as fast as your ISP claims it to be, but as fast as it should be. The ISP might have a very good connection to the Internet but put many clients on the same line. If these are heavy clients, your traffic will have to share the same line and your throughput will suffer. Think about a dedicated connection and make sure it is truly dedicated. Don't trust the ISP, check it!

Another issue is connection latency. Latency defines the number of milliseconds it takes for a packet to travel to its final destination. This issue is really important if you have to do interactive work (via ssh or a similar protocol) on some remote machine, since if the latency is big (400+ ms) it's really hard to work. It is less of an issue for web services, since it influences only the first packet. The rest of the packets arrive without any extra delay.

The idea of having a connection to "the Internet" is a little misleading. Many web hosting and colocation companies have large amounts of bandwidth but still have poor connectivity. The public exchanges, such as MAE-East and MAE-West, frequently become overloaded, yet many ISPs depend on these exchanges.

Private peering is a solution used by the larger backbone operators. No longer exchanging traffic among themselves at the public exchanges, each implements private interconnections with each of the others. Private peering means that providers can exchange traffic much quicker.

Also, if your web site is of global interest, check that the ISP has good global connectivity. If the web site is going to be visited mostly by people in a certain country or region, your server should probably be located there.

Bad connectivity can directly influence your machine's performance. Here is a story one of the developers told on the mod_perl mailing list:

What relationship has 10% packet loss on one upstream provider got to
do with machine memory ?

Yes.. a lot. For a nightmare week, the box was located downstream of a
provider who was struggling with some serious bandwidth problems of
his own... people were connecting to the site via this link, and
packet loss was such that retransmits and TCP stalls were keeping
httpd heavies around for much longer than normal.. instead of blasting
out the data at high or even modem speeds, they would be stuck at
1k/sec or stalled out...  people would press stop and refresh, httpds
would take 300 seconds to timeout on writes to no-one.. it was a
nightmare.  Those problems didn't go away till I moved the box to a
place closer to some decent backbones.

Note that with a proxy, this only keeps a lightweight httpd tied up,
assuming the page is small enough to fit in the buffers.  If you are a
busy internet site you always have some slow clients.  This is a
difficult thing to simulate in benchmark testing, though.

8.2.5. How Much Memory Is Enough?

How much RAM do you need? Nowadays, chances are that you will hear: "Memory is cheap, the more you buy the better." But how much is enough? The answer is pretty straightforward: you do not want your machine to swap! When the CPU needs to write something into memory, but memory is already full, it takes the least frequently used memory pages and swaps them out to disk. This means you have to bear the time penalty of writing the data to disk. If another process then references some of the data that happens to be on one of the pages that has just been swapped out, the CPU swaps it back in again, probably swapping out some other data that will be needed very shortly by some other process. Carried to the extreme, the CPU and disk start to thrash hopelessly in circles, without getting any real work done. The less RAM there is, the more often this scenario arises. Worse, you can exhaust swap space as well, and then your troubles really start.

How do you make a decision? You know the highest rate at which your server expects to serve pages and how long it takes on average to serve one. Now you can calculate how many server processes you need. If you know the maximum size to which your servers can grow, you know how much memory you need. If your OS supports memory sharing, you can make best use of this feature by preloading the modules and scripts at server startup, so you will need less memory than you have calculated.

Do not forget that other essential system processes need memory as well, so you should not only plan for the web server but also take into account the other players. Remember that requests can be queued, so you can afford to let your client wait for a few moments until a server is available to serve it. Most of the time your server will not have the maximum load, but you should be ready to bear the peaks. You need to reserve at least 20% of free memory for peak situations. Many sites have crashed a few moments after a big scoop about them was posted and an unexpected number of requests suddenly arrived. If you are about to announce something cool, be aware of the possible consequences.

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