It is important that you understand some of the basics of how PMDF works. This section attempts to present a very basic overview.

When PMDF receives a message, PMDF writes the message as one or more disk files in the PMDF queue directories. These files represent copies of the message: at least one copy for each channel to which the message must be enqueued. It is crucial that the received message be written to a non-volatile medium such as a magnetic disk file: were the system to crash before the message could be sent on to its final destination, then the message might be lost. For this reason, PMDF always writes received messages to disk before giving a positive acknowledgement of receipt to the transmitter of the message. After a message is received, an entry is made in the queue cache database.

Once PMDF has received a message, it attempts to deliver it. This is done by submitting a processing job for each channel to which the message is enqueued. These jobs attempt to send the message to wherever it is next bound (as determined by PMDF's domain rewriting rules). If a job is successful, then the message copy it was handling is deleted and the corresponding entry removed from the queue cache database. If not successful, then the message copy is left on disk for a subsequent delivery attempt.

So, the normal mode of operation is: A message is received, it is written to a file, a record is added to the queue cache database, a processing job is started, the job reads the message file, the job then deletes the file, the record is removed from the queue cache database.

Given this basic scenario, several things should be clear:

• Increased throughput can be realized by:
  • decreasing disk write and read times,
  • increasing the internal memory buffer size for processing jobs to decrease the use of temporary buffer files for large messages,
  • increasing the number of simultaneous processing jobs and any resources they might require,
  • decreasing processing job overhead,
  • decreasing per message processing job overhead by increasing the number of messages handled per job,
  • ensuring that the number of SMTP server processes available for accepting incoming SMTP over TCP/IP messages is appropriate for the level of message traffic,
  • for general SMTP over TCP/IP channels, used to send SMTP messages to multiple different destinations, decreasing per connection overhead for outgoing SMTP over TCP/IP messages by collecting and sending messages to the same destination host in one connection,
  • for daemon SMTP over TCP/IP channels, commonly used to send SMTP messages to single specific relay systems such as mailhubs or firewalls, using multiple threads for outgoing connections, and
  • tuning the queue cache database.
• Using a virtual RAM disk for the message store is a very bad idea. Should your system crash, mail can be lost or corrupted.¹
• Keeping the message store on a shadowed disk can hurt performance: whereas shadowset reads are on the average faster, writes are on the average slower. Since usually only one read will be required, the decreased read time will not be sufficient to compensate for the increased write time.