There are a few cases in that XMail is distributing data randomly to
various destinations (e.g. relay domains or internal spool folders -
the code with "rand() % count"). It's not distributing the data
evenly in theses cases, so you end up with e.g. one server getting or
traffic or one spool folder being filled more than others. Especially
the first case has been problematic for some of our customers.
On a side note: all the different cases use basically the same logic,
so fixing it in one case would actually fix all of them.
A collegue did quite some testing on it and here are his findings:
When using load balancing with more than 2 relays, XMail is unevenly
distributing the load upon the configured relays.
Here is an example of the distribution to 3 mailservers. The data was
captured in a test running for 9 hours with approximately 25500
mails:
1. relay: 55.31%
2. relay: 22.31%
3. relay: 22.36%
The reason for this behaviour is the randomization / shuffling code
that is used for picking a mail relay to deliver an e-mail to.
Let's stick with the setup above using 3 mail relays. In this case
the randomization code will choose only one relay in the list of
available relays (floor(3 / 2)) and swap it with another relay, which
is also chosen at random and completely indepent of the first choice.
Thus, swapping the first with the first relay, the second with the
second, etc. is a possible outcome.
The easiest way to visualize the randomization in this case is the
following table:
1 2 3
1 x
2 x x
3 x x
The rows of this table represent the first chosen index of a relay,
the colums denote the index of the second choice (and thus the relay
that is swapped to the position of the first chosen relay). The 'x'
in the table is set, whenever the first element in the list will not
change (for example, swapping the second and the third relay).
As you can easily see in this table, in 5 out of 9 cases (55%) the
first relay is not swapped to another position and, consequently, 55%
of the e-mails get delivered to the first relay (which is almost
exactly the number we have
measured in our test environment).
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