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 Why not to run Anti-Virus Software actively against your Post Office.
On any storage based server, the bottleneck for performance and concurrency is the disk I/O subsystem. In addition to this, the major contributor to the reliability of the overall system is the stability of the on-disk structures. The FirstClass server achieves its high levels of reliability and performance by careful control of the way it uses the disk subsystem.
Virus scanning software usually works in two ways. One method walks the disk file system, opening each file it finds, reading the contents and checking for viruses. This method causes two problems for the FirstClass core server:
1. Additional load is placed upon the most critical subsystem
2. The scanner may cause contention for a file that the server needs, leading to corruption.
The second method instruments the base OS file system and hooks the create and write operations. The scanning is done either serially (blocks the operation) or by scheduling a scheme similar to the first method. This method again adds delay to the disk subsystem and because external code runs as part of the I/O, it puts the core server at risk of strange behavior. The FirstClass server performs extremely intense I/O via a pool of disk worker threads. In addition the calls are made using async calls. It is unlikely that the virus software has ever run under such extreme conditions, thus exposing the possibility of strange concurrency bugs into the I/O system. These bugs usually do not crash the server, but can cause data to be lost or changed as it is written to the collaborative store. The result is a corrupt post office.
It is for these reasons that we recommend against running any kind of software which dynamically alters the behaviour of the base operating system. We do not test our software under these conditions and we do not guarantee its performance or reliability if such software is employed.
Finally, since FirstClass does not store files on disk in their native image format it is unlikely that virus scanners would detect them. Direct scanning of the disk often does not work with any system which does not directly store the files as raw in the file system.
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