I have to agree with the people who say it's parallel delivery overloads
servers. NOt by bogging down performance, but by clogging their
BANDWIDTH!
To put it in simple terms, qmail increases YOUR performance at EVERYONE
ELSES EXPENSE! Most MTAs cannot set limits on connections by a single
server. Therefore, any connection limits are GLOBAL. When q-mail does
its thing, it happilly fills up AS MANY OF THESE CONNECTIONS AS IT CAN
GET. As it finishes one, it nabs it again with ANOTHER connection.
This locks other mail out. A well-behaved MTA wouild simply send one
connection and stream the data, taking into account the fact that OTHER
people need to send mail too. It seems qmail assumes nobody else is
trying to send mail when it is. :) While sendmail is sending it's
messages across the one socket, all the others are free for other
instances of sendmail. q-mail eats up every single connection if it can
manage it.
The only way to keep q-mail from hogging a server is prohibit it from
opening more than one parllel connection, making it's delivery scheme
wasteful when sending to such a site, and turning qmail's strategy from
a performance gain to a performnace LOSS when sending to the site.
If people fixed their MTAs so qmail couldn't break them, then suddenly
qmail would become a worse performer.
Yes, qmail is faster. Yes, it's modular architecture makes sense. Yes,
parellel delivery to different sites makes sense. There's nothing wrong
with using up all of *your* bandwidth and little of everyone elses. But
streaming many connections to a site is a *bad* thing! Every additional
connection qmail streams to a site is a connection that's STOLEN from
the other MTAs. Why do you think people all teh time tell you to only
download oen thign at a tiem from an FTP site? SO others can download
to. The SAME THING applies here.
The bottom line is that qmail is being selfish by trying to monopolise
any site it wants to send mail to simply to increase it's own
performance.
If qmail would simply restrict its parallel connections to a single
server to a reasonable number, it would not be so rude. And after it did
that, if it kept it's connections open, it would send out it's messages
faster.
qmail takes everything it can get, and is built on the assumption that
everyone will let it take as much as it needs to. When this happens, it
performs well. when this does not happen, q-mail hurts beasue of the
overhead of multiple connections. Sendmail takes only what it NEEDS
(one connection) and sends only what it needs to send (one copy of the
message, to be replicated and distributed by the remote MTA)
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