If you have antivirus software installed on the same system as your Aspera transfer server it is important to ensure that the antivirus program does not interfere with Aspera functionality and performance.
Certain antivirus programs intercept the system calls Aspera software must make, or conduct file scanning while transfers are running which can slow down performance considerably. Transfers may be unable to run altogether with an incompatible antivirus program on the system.
The most common reason for these types of issues is using an antivirus program meant for desktop systems rather than for file server systems. Desktop antivirus software is extremely prevalent (such as Symantec or McAfee), but can cause problems in server environments which have unique needs. File server antivirus programs are more sensitive to work being done on the server and its network, and are designed to minimize impact on system performance.
A way to test whether poor transfer performance on a system is due to your antivirus program is to completely uninstall the antivirus program and attempt some transfers, to see whether performance improves.
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If you determine that the antivirus program is negatively impacting your transfers, you can either leave the antivirus program off entirely, or ensure you install a product geared towards file server systems and not desktop systems. For example, Kaspersky and AVG offer file server antivirus software options. If you are still having issues due to your antivirus software, contact the technical support for your antivirus program.