I agree that logging shouldn't be done at the expense of performace but usually that is dictated by how much logging is done and where in the process, not by the logging format. Apache (as an example) seems to be able to cope with high volumes and a readable log file so it can be done.

Having had to cross corelate faults through logs as diverse as unix system logs, firewall-1 logs, apache logs, mail logs, proxy logs, custom application logs etc. my experience has been that having the date time human readable in some recognisable format is essential. I guess I am too much of a Unix guy to want to have to fire up a GUI just to look at an event log :)

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Until you've lost your reputation, you never realize what a burden it was or what freedom really is. -Margaret Mitchell