Daniele Tosatto

Load balancing uses a number of algorithms, called load balancing methods, to determine how to distribute the load among the servers. When a Load balancer is configured to use the least packets method, it selects the service with the least number transmitted and received packets.

Least Packets Load balancing allows you to distribute client requests across multiple servers to optimize resource utilization. Load balancers improve server fault tolerance and end-user response time. In a scenario with a limited number of servers providing service to a large number of clients, a server can become overloaded and degrade server performance. Load balancing is used to prevent bottlenecks by forwarding the client requests to the servers best suited to handle them. Thus, balancing the load.

In a load balancing setup, the load balancers are logically located between the client and the server farm. Load balancing is used to manage traffic flow to the servers in the server farm. The network diagram shows the topology of a basic load balancing configuration. Load Balancing can be performed on HTTP, HTTP, SSL, FTP, TCP, SSL_TCP, UDP, SSL_BRIDGE, NNTP, DNS, ANY, SIP-UDP, DNS-TCP, and RTSP.

The following example shows how a Load Balancer works using the least packets method. The load balancer selects the server by using the value (N) of the following expression:
N = Sum of the number of packets transmitted and received in last 14 seconds.

The load balancer delivers the requests as follows:

• Server-3 receives the first request because it has the lowest N value.
• Server-1 and Server-3 have the same N value, so round robin is performed.
• Server-1, Server-2 and Server-3 have the same N value, so round robin is performed until one of the N values changes.

Whether it’s load balancing XenApp Web Interface, iPhone/iPad resources, websites, linux servers, windows servers, e-commerce sites, or enterprise applications, NetScaler is the perfect choice. NetScaler, available as a network device or as a virtualized appliance, is a web application delivery appliance that accelerates internal and externally-facing web application up to 5x, optimizes application availability through advanced L4-7 traffic management, increases security with an integrated application firewall, and substantially lowers costs by increasing web server efficiency.

Load balancing uses a number of algorithms, called load balancing methods, to determine how to distribute the load among the servers. When a Load balancer is configured to use the token method, it selects a server based on the value of a token extracted from the client request. You can configure the location and size of the token. For subsequent requests with the same token,the load balancer chooses the same server that handled the initial request. As you can imagine, this provides a level of persistence for the client requests.

The following example shows how a Load Balancer works using the token method:

• Requests with “AA” in the URL are sent to Server-1.
• Requests with “BB” in the URL are sent to Server-2.
• Requests with “CC” in the URL are sent to Server-3.

Whether it’s load balancing XenApp servers, iPhone/iPad resources, websites, linux servers, windows servers, e-commerce sites, or enterprise applications, NetScaler is the perfect choice. NetScaler, available as a network device or as a virtualized appliance, is a web application delivery appliance that accelerates internal and externally-facing web application up to 5x, optimizes application availability through advanced L4-7 traffic management, increases security with an integrated application firewall, and substantially lowers costs by increasing web server efficiency.

Custom Load balancing allows you to distribute client requests across multiple servers and optimize resource utilization. Load balancers improve server fault tolerance and end-user response time. In a scenario with a limited number of servers providing service to a large number of clients, a server can become overloaded and degrade server performance. Load balancing is used to prevent bottlenecks by forwarding the client requests to the servers best suited to handle them. Thus, balancing the load.

Load balancers are logically located between the client and the server farm. Load balancing is used to manage traffic flow to the servers in the server farm. The network diagram shows the topology of a basic load balancing configuration. Load Balancing can be performed on HTTP, HTTP, SSL, FTP, TCP, SSL_TCP, UDP, SSL_BRIDGE, NNTP, DNS, ANY, SIP-UDP, DNS-TCP, and RTSP.

Load balancing uses a number of algorithms, called load balancing methods, to determine how to distribute the load among the servers. When a load balancer is configured to use a custom load balancing method, it calculates a load metric for the servers (N) based on server CPU usage, memory and/or response time.

The following example shows how a Load Balancer works using the custom method. The load balancer selects the server by using the value (N) of the following expression:
N = A custom metric calculated on the server for CPU Usage, memory and/or response time.

• Server-1 is using 20 MB of memory.
• Server-2 is using 70 MB of memory.
• Server-3 is using 80 MB of memory.

The requests are delivered as follows:

• Server-1 receives the first, second, third, fourth and fifth request because it has the lowest N value.
• Server-1 and Server-2 then have the same N value, so round robin is performed. Server-2 receives the sixth request, Server-1 receives the seventh request.
• Server-1, Server-2, Server-3 then have the same N value, so round robin is performed.

Whether it’s load balancing XenApp Web Interface, iPhone/iPad resources, websites, linux servers, windows servers, e-commerce sites, or enterprise applications, NetScaler is the perfect choice. NetScaler, available as a network device or as a virtualized appliance, is a web application delivery appliance that accelerates internal and externally-facing web application up to 5x, optimizes application availability through advanced L4-7 traffic management, increases security with an integrated application firewall, and substantially lowers costs by increasing web server efficiency.

Citrix has released version 3.2 of Profile Management and it can now be downloaded from Citrix.com.

You’ll find it under the XenApp and XenDesktop product lists!

For details and a full list of fixed issues see the following links:

About Profile Management 3.2
http://support.citrix.com/proddocs/index.jsp

Profile Management 3.2 – Issues Fixed in This Release
http://support.citrix.com/article/ctx126527

With the recent release of the Hyper-V / Windows 7 / XenDesktop whitepaper I thought it would be a good time to go over just a few points on how you can leverage the scalability reports that have been released by Citrix and other vendors.

Citrix intentionally follows the same process for scalability testing so that the results can be used for comparing hardware and hypervisor platforms. Since it is impractical to create tests that match each customer’s environment, a standard configuration is selected and the performance data published. The choice of the Login VSI medium user workload allows for a consistent, repeatable, and slightly random workload to be run across different configurations to determine performance. Also, by using a readily available third-party tool for the workload, customers can easily reproduce the test results with an unbiased workload.

Sizing a XenDesktop environment should neither be done blindly nor should it rely solely on published scalability results. If you want to size your environment correctly, you must conduct a pilot with actual desktop users and their applications. The pilot results can then be used in combination with published data to determine the expected number of desktops a given hardware platform can support in your environment. Most initial pilots should focus on two key areas: CPU and storage activity. These two areas have a significant impact on a desktop virtualization project and are historically the ones that are undersized in larger environments. Certainly other areas can be monitored such as RAM and network activity, but these areas are not as difficult to size as CPU and storage.

In most cases, scalability results published by any vendor will not meet the requirements of your environment making it impossible to size your farm using the reported numbers. For instance, the Login VSI medium workload which uses Microsoft Office, IE, and Adobe generates about 5 IOPS per user when running in the steady-state loop. What is also true is that in order to prevent any network latency from affecting the response times, IE is using cached web pages for viewing. If a user was actually browsing web pages, both the network and storage activity would increase as the pages are read and cached by the browser. Recent tests have shown that a heavy IE workload can generate as much as 20-30 IOPS per session. Therefore, testing with your anticipated workload is paramount to obtaining realistic sizing data.

With a bit of logic, you can integrate the results from published reports with your pilot results to anticipate how the proposed XenDesktop environment will perform. For instance, the published results from the Citrix whitepaper show a BL460C G6 blade with 64GB RAM is capable of hosting about 75 Windows 7 users on Hyper-V with the Login VSI medium workload. Suppose you run a pilot with a similar hardware configuration but with your user workload and at the end of the pilot determine the optimal number of users per server is 50. Taking both pieces of data into account you can derive that your pilot workload requires approximately 50% more server capacity than the Login VSI medium workload.

Knowing this relationship allows you to leverage the studies published with the Login VSI medium workload. To determine how your workload would perform using the environment in the study, whether it be a different hypervisor or hardware, just adjust the Login VSI sizing results reported by 50% for your environment. Similar conclusions can be drawn regarding CPU usage or storage I/O operations (IOPS) when comparing published reports to pilot results.

Citrix Inc.

Citrix releases Access Gateway VPX Express : you can download it here for free.

Access Gateway VPX Express is the easiest and the fastest way for customers to try out Citrix’s premier secure access solution – all at no cost to you. Available as a virtual appliance, it allows customers to deploy on their existing server hardware. With the Express, customers will receive a license for 5 concurrent users that’s valid for 12 months.