The chief information officer of the Commonwealth Bank has acknowledged that the bank is "embarrassed every single time" its computer systems fail, as they did last week when the CBA's Eftpos and ATM networks were unavailable for several hours.Michael Harte, who has led the bank's billion-dollar core systems' revamp project, told a Sydney audience yesterday that the "five-nines" (99.99999 per cent) reliability sought from the bank's mission-critical applications meant that the bank should endure downtime of no more than 5.68 minutes a year.  "[But] you have to pay for that," Harte acknowledged. He added that it was not easy to achieve this for all systems.Of the A$1.2 billion spent by the bank on information technology each year, Harte said that $650 million was invested in "keeping the systems alive"."It's really, really expensive, and it's really, really hard," he said, adding that the bank continued to push computer vendors on the issue of delivering a "five-nines" level of resilience.Harte said systems' reliability had improved markedly in recent years. Where six years ago, CBA had endured 80 Severity 1 class systems' failures (a problem for which there is no obvious workaround) each year, these had now been cut to eight a year. In the same period, during which much of the bank's core baking platform has been replaced, the IT group tripled the system's capabilities. "If you do the maths, that's pretty good," he said.However, for both the Australian Prudential Regulation Authority, which recently signalled its frustration with ongoing bank computer crashes, and customers, who have grown accustomed to the benefits of 24x7 bank access, there is a rising tide of intolerance for any systems problems, whatever the maths involved.