Measuring spare capacity

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I am trying to get a handle on the amount of spare capacity in my MS
boxes.

We are running two MS which handle about 250,000 messages by day (MRTG
count by recipient). The 'Max Unscanned Messages Per Scan' and Max
Unsafe Messages Per Scan' are both set at 50 and max children at 15.

I isolated a day's worth of maillog entries out of a rotated file and
looked at some stats. Counting all instances of 'Found X messages
waiting' I found that 75% had just one message, 17% two, 5% three and
the more populous batches hardly registering at all. Doing the same for
'Scanning X messages' I found a slightly wider spread but tapering off
quickly after 15.

These are the distributions for the first 15 counts:

New Batch: Scanning $a messages, 
1       36153 
2       8187 
3       2188 
4       767 
5       319 
6       139 
7       70 
8       45 
9       18 
10      11 
11      17 
12      6 
13      4 
14      3 
15      5 

Found $a messages waiting 
1       0 
2       7545 
3       7019 
4       5126 
5       3484 
6       2535 
7       1724 
8       1238 
9       915 
10      618 
11      489 
12      372 
13      260 
14      211 
15      161

This surprised me. I was expecting the batch size to grow during busy
periods. It seems that the batch size is generally a single message,
even though more messages are waiting to be processed. Looking at log
snippets in the mailing list archives confirms that this is common.
Looking at the time distribution of (rather rare) larger batches I found
these spread randomly over the day.

We regularly get a peak in the incoming messages queue of a few
thousands of messages. This makes me believe that there is not that much
slack in the capacity. During these peaks the number of messages per
batch does go up to the maximum.

Is there a way to measure how many more messages per day a given system
can take?

Thanks for any ideas.

Bart...

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