It hasn't been a good week for cloud computing. Social bookmarking service Ma.gnolia finally admitted defeat after unsuccessfully trying to recover its damaged database. Gmail went offline for hours after some rogue code ate the service.

So much for the cloud.

Can we really rely on web-based services and software? If you're expecting us to say no, surprise! The answer is yes - albeit with some very important qualifications.

In the great scheme of things Google's downtime wasn't much; six outages in six months sounds like a lot, but it averages out at 15 minutes a month. Annoying, sure, but hardly significant: our ISP's mail servers are borked more often and for longer periods than that, and we lose more than 15 minutes a month to computer crashes.

Ma.gnolia? People lost a bunch of bookmarks. Nobody's going to die because they can't find that really funny web page.

That doesn't mean that cloud computing has nothing but silver linings, though. When even high street banks can teeter on the edge of bankruptcy, it's very silly to assume that your chosen cloud computing provider - your hosting firm, your photo library, your online document storage or anything else you might stick on the internet - is going to be around forever, or that they won't employ the odd idiot who hits the wrong button and zaps something important.

There's also the problem of the small print, especially with free services. Storing all your photos on Facebook? If you break the terms and conditions, or if Facebook thinks you have, the site can kill your account and delete all your stuff.

Even if none of these things happen, there's damage to your own stuff to consider. Do you keep your photos on your PC and a backup on Flickr? Imagine having to manually download each and every image because your hard disk's crashed. See you in a decade!

Take all those things together and one word should be looming above your PC in fifty-foot letters of fire: backup. If you need it, make sure you've got a copy of it.

Cloud computing from reputable providers can be as reliable as, and in some cases more reliable than, any other utility, product or service - but like anything else things can and occasionally do go wrong.

If you think super internet power makes providers immune from crises, cock-ups or catastrophes, it's not just your data that's in the clouds.

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