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Idea from ISC DHCP, cf dhcpd.8:
The log-threshold-high and log-threshold-low statements
The log-threshold-low and log-threshold-high statements are used to control when a message is output about pool usage. The value for both of them is the percentage of the pool in use. If the high threshold is 0 or has not been specified, no messages will be pro- duced. If a high threshold is given, a message is output once the pool usage passes that level. After that, no more messages will be output until the pool usage falls below the low threshold. If the low threshold is not given, it default to a value of zero.
A special case occurs when the low threshold is set to be higer than the high threshold. In this case, a message will be generated each time a lease is acknowledged when the pool usage is above the high threshold.
Note that threshold logging will be automatically disabled for shared subnets whose total number of addresses is larger than (264)-1. The server will emit a log statement at startup when threshold logging is disabled as shown below:
"Threshold logging disabled for shared subnet of ranges:
This is likely to have no practical runtime effect as CPUs are unlikely to support a server actually reaching such a large number of leases.
From this I like the idea to have a hook library which performs a simple action (log is an example) when a threshold is crossed in a reasonably sized pool.