How to resolve slow download speeds for attachments from the Cognos reportserver

What are the possible reasons why an output of type CSV, PDF or MHT could take extremely long to download over the network between the Cognos report server and client computer?

There are a few important factors that will influence the download time of output files. Here are some of the most important considerations when troubleshooting download speeds:

1) All of the components in the network make up the path between the server and the client. This means that the slowest link in the chain will be the maximum speed of the network. For instance, if there is a 100 Mb/s LAN connection between the client PC and the network Hub, then the Hub is most likely connected to another Hub or Switch, and follows a path of connectivity all the way back to the server. In short, if a 100 Mb/s hub has 8 ports, then the 100 Mb/s traffic is shared equally over all 8 ports, which in effect gives only 12 Mb/s per connection. This will allow a maximum of 1.5 Mb/s downloads in between two PC's connected on the same hub. Theoretically this would allow a file of +- 150 Mb to be copied in under 2 minutes.

If the Cognos report server is located in a different region, and a WAN connection between the two regions in question is 8 Mb/s, then this bandwidth will be shared between all clients between both regions. Therefore to test this scenario, one should download a test file from the remote server, and compare the different download speeds.

The use of Switches, or intelligent Hubs, can help to improve performance, as such network devices can spread network traffic to the clients that need it most. Compared to a Hub, the Switch can allot up to 50% of the total maximum speed to a single port, where a Hub will give an exact and equal fraction based on the number of ports available. This is roughly 12.5% of the total bandwidth on an 8 port Hub.

2) If the network speed is not the problem, the second biggest problem in download speed can be linked to anti-virus software. The anti-virus scanner on the server and client may be running in "on-access" mode. This will cause the virus scanner to scan any file once it is accessed. This would increase hard disk activity and slow down the download process.

This behavior can be tested by disabling anti-virus software on the server and client, but please take care when doing this as this will reduce your protection against virus threats and care should be taken that the anti-virus is enabled as soon as possible after the test.

3) IIS could have a proxy or firewall enabled, that will intercept all communications between the web server and client. An example is the use of urlscan. This could slow down the download processing time by increasing the CPU load on the web server. This can be tested in a similar fashion as mentioned in A, by comparing download speeds from a file-share on the Cognos server, versus the speed when using a hyper-link to download a file via the IIS server.

4) Encryption on either side could have an effect on the download speed. This includes the use of server certificates, or connection encryption like a VPN. Here one would need to consult the Network Administrator to find out how the connection between the Cognos server and the client site is secured.

5) The access to the Content Store database has a direct influence on the download speed, as the reports are drawn from the Content Store Database and then made available for download. If the connection speed between the Cognos gateway and Content Store Database is hampered, one will see a direct effect on download speeds.

6) The hardware inside the client and server will affect the transfer speed. If hard disks in the client or server is under high load, or is a slow disk, it will be the bottleneck in the download performance. One would need to eliminate all the components in the line between the Cognos server and the client. This is best accomplished with the help of the Network Administration and Server Administration teams.