?? Operations that cause the disk head to move, such as file unlinks, are called IOPs. Typical hard disks are
limited to about 120 IOPs per second. When sendmail successfully delivers a message it can consume from
10 to 13 IOPs per message.
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406 | Chapter 11: Manage the Queue
processing separate files. Depending on the size of your in-memory disk cache, neither
will likely be able to take advantage of the efficiencies of such caching. In short,
two sendmail daemons processing a deep queue in parallel is worse than a single
sendmail daemon processing that same queue alone.
And if that weren??™t enough, another 10 minutes later a third sendmail daemon starts
to process the queue.
By now, the first sendmail daemon might have finished its preread of the queue and
might have actually begun to send messages. But even if it has, three sendmail daemons
are now processing that single deep queue and a curious thing happens.
Because the disk that holds the queue is finite, the addition of a third sendmail daemon
slows the operation of the first two. The second one, instead of taking 20 minutes
to preread the queue, will now take 30 minutes.
This means that every 10 minutes another sendmail queue-processing daemon is
added to the mix.
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