Posts Tagged 'Physics'

October 31, 2007


"ah - I don't need backups."
"Too busy to do backups - I'll get to that later."
"Backups? It costs too much."
"I don't need backups - MTBF of a Raptor is 1.2 Million hours."
"Oops - I forgot about doing backups."

Backups are one of the most commonly forgotten tasks of a system administrator. In some cases, they are never implemented. In other cases, they are implemented but not maintained. In other cases, they are implemented with a great backup and recovery plan - but the system usage or requirements change and the backups are not altered to compensate.

A hard drive really is a fairly reliable piece of IT equipment. The WD 150GB Raptor has a rating of 1.2 Million hours MTBF. With that kind of mean time between failures, you would think that you would never have to worry about a hard drive failing. How willing are you to take that chance? What if you double your odds by setting up two drives in a RAID 1 configuration? Now can you afford to take that chance? How willing are you to gamble with your data?

What if one of your system administrators accidentally deletes the wrong file? Maybe it's your apache config file. Maybe it's a piece of code you have been working on all day. Or, maybe your server gets compromised and you now have unknown trojans and back doors on your server. Now what do you do?

Working in a datacenter with thousands of servers, there are thousands and thousands of hard drives. When you see that many hard drives in production, you are naturally going to see some of them fail. I have seen small drives fail, large drives fail, and I have even seen RAID 1 mirrors completely fail beyond recovery. Is it bad hardware? Nope. Is it Murphy's Law? Nope. It's the laws of physics. Moving parts create heat and friction. Heat and friction cause failures. No piece of IT equipment is immune to failure.

That 1.2 million hours MTBF looks pretty impressive. For a round number, let's say there are 15,000 drives in the SL datacenter. 1,200,000 hours / 15,000 drives = 80 hours. That means that every 80 hours, one hard drive in the SL datacenter could potentially fail. Now how impressive is that number?

Ultimately, regardless of the levels of redundancy you implement, there is always a chance of a failure - hardware or human - that results in data loss. The question is - how important is that data to you? In the event of a catastrophic failure, are you willing to just perform an OS reload and start from scratch? Or, if a file is deleted and unrecoverable, are you willing to start over on your project? And lastly, how much downtime can you afford to endure?

Regardless of how much redundancy you can build into your infrastructure with the likes of load balancers, RAID arrays, active/passive servers, hot spares, etc, you should always have a good plan for doing backups as well as checking and maintaining those backups.

Have you checked your backups lately?


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