Mirroring the server

The SurgeMail 'Mirror' system allows you to link two systems together and read or deliver Email to either system and both systems will continually 'match' each other. This can be used in several ways:

There are several benefits from mirroring a server.

• Keep a live backup system for 'hot swapping' in case of failure or uprgrade requirements on your live system.
• Keep a live backup system with router based instant failover
  
• Move a system from one geographic location to another with no downtime

What is mirroring?

This is a relatively new invention and as far as we know unique to our systems.  It means that the user's mail folders are mirrored between two systems over a LAN or WAN connection. This means if either system fails, the user can always switch to the other system.  There is no 'single point of failure' in this design. This makes it the only way to have a fully fault tolerant mail system.

How do I turn it on?

Simply setup two mail servers in a similar manner, then configure the global mirror settings (each pointing to the other server and each system with the same shared secret)

How do I know it's working?

In the status window there is a small section:

Mirror out: Que/sent add=612/611 (3343432 bytes) del=612/610   

Mirror  in: Received add=0 del=0 rename=0 failed=0

This shows both halves of the mirroring operation. The "Mirror out:" line shows messages queued to be sent to the other system, and the second number (/611) shows how many have been successfully sent (so one is still queued) and how many delete operations have been queued (612) and how many have been sent (/610), so 2 are still queued.  Obviously these numbers should normally match.

The second line, "Mirror in:" shows how many new items, or deletions have arrived from the other system.

Any Performance Impact?

There is of course some, and the data does need to be sent between the two systems. However, the load is by no means doubled, as the mirroring occurs at the delivery stage after much pre-processing has occurred (e.g. spam & virus filtering). Also, most mail servers will run at about 98% idle, so the extra load is really of no relevance (even for quite large ISP operations).  As a rule of thumb, try running without mirroring first and check the busy % figure on the status output. If it is below 50% then mirroring should cause no trouble, if it's above 50% then reconsider.