One of the great advantages of using Rackspace Cloud Servers is the flexibility that you have to purchase only the amount of computing power that you need. When business is good and you need to increase your server capacity, you can scale your implementation horizontally by distributing your traffic over multiple servers by using Cloud Load Balancers.

One way to find out how much computing power you need is to install and test your application on a few implementations of different sizes. Then, load test your application while simulating traffic to your site.

We recommend that you test your site from a URL that does more than just retrieve a static web page. For example, access a page that uses PHP and makes a database query to make the test more representative of normal traffic.

This article shows you some of the standard tools that you can use to view your server’s performance, and helps you determine whether the server size that you chose is up to the task.

Considerations

Rackspace Cloud Servers are virtual partitions of larger physical machines that allocate resources based on a process called central processing unit (CPU) scheduling. As a result, they don’t perform exactly like a dedicated machine with similar resources. You can learn more about CPU scheduling by reading the “Performance” section of the Cloud Servers FAQ.

OnMetal Cloud Servers are also available. OnMetal servers are single-tenant, bare metal servers that you provision through the same OpenStack® API as a Rackspace cloud server. Because you can create or delete them as quickly as virtual machines (VMs), they offer the agility of multi-tenant environments with the performance of single-tenant hardware.

Also consider that cloud servers come in the following flavors or server types:

• General Purpose Compute optimized

• Memory optimized

• I/O optimized servers

The Memory, Compute, and I/O flavors offer faster disk access and network speed than General Purpose flavors. Disk size and virtual CPU allocation are different for equivalent flavors. Compare the offerings based on the performance needs that you identify in the following sections.

The following table shows flavor classes for different workloads:

(Prototype)	(Scale)	(Optimize)
General purpose virtual servers	General purpose or Workload-optimized virtual servers	General purpose or Workload-optimized OnMetal servers
	Description
VMs running on multi-tenant hosts. Smaller sizes, balanced resources, and CPU and network burst capability provide the lowest price points and best value.	VMs running on multi-tenant hosts. Smaller sizes and workload-specific designs allow for price-performance optimization for your particular application.	API-driven, instantly provisioned, single-tenant, bare metal servers. Full host and workload-specific designs provide large-scale cost efficiencies, as well as maximum and consistent performance.
General purpose	Workload optimized	Workload optimized
Class name: General Purpose v1	Class name: Compute v1	Class name: OnMetal General Purpose
Use cases: - Testing and development - Low to medium-traffic web servers - Batch processing - Network appliances - Small to medium databases	Use cases: - Medium to large-traffic web servers, application servers, batch processing, and network appliances	Use cases: - Large-traffic web servers, application servers, batch processing, and network appliances
	I/O optimized	I/O optimized
	Class name: I/O v1	Class name: OnMetal I/O
	Use cases: - Medium to large relational databases and NoSQL data stores	Use cases: - Large-scale online transaction processing (OLTP), relational databases, and NoSQL data stores
	Memory optimized	Memory optimized
	Class name: Memory v1	Class name: OnMetal Memory
	Use cases: - Medium to large caches, search indexes, and in-memory analytics	Use cases: - Large caches, search indexes, and in-memory analytics

Performance testing in Linux

If your application is running on a Linux® system, there are many utilities that you can use to determine how well your server is handling the load. The main statistics that you should examine are the load average on the server and the available memory while your application is running.

You can use the following utilities to monitor server performance:

• free: This is a quick and easy-to-use monitoring utility that gives you a snapshot view of the amount of available memory on your server. Adding the -m switch to the command shows you the available memory in megabytes (as opposed to the default measurement, which is kilobytes).

• top: This utility does more than just check available memory. You can also view the load average on the server and the processes that are using the most resources on your server.

• iotop: You can use the iotop command to monitor disk I/O on a per-process basis.

• dstat: The dstat command shows you the I/O statistics and other information with more versatility than other commands.

Note: You might need to install extra packages on your server to use the iotop and dstat commands.

If you use these tools while you run your web application and see an excessive load average or excessive memory usage, you either need to tune your application or choose a more powerful server flavor on which to run it.