[mythtv-users] 2 TB Hard Drive Recommendations
mythtv-list at dinkum.org.uk
Mon Dec 6 10:50:47 UTC 2010
On 6 Dec 2010, at 07:54, Simon Hobson wrote:
> Jean-Yves Avenard wrote:
>> so two drives out of 6 dying in 18 months, so close to each other
>> isn't that great.
> That's one of the problems where the idea of RAID breaks down. You buy a set of disks (same make and model, possibly even from the same manufacturing batch), stuff them in an enclosure, and run them for the same power on hours, doing the same usage profile - ie they are run under near identical conditions. That means the failures are liable to be less distributed temporally than you might be thinking.
> So you've massively increased the chance of two (or more) simultaneous failures.
I've seen this so many times, "we can buy cheap drives because they are in a raid array" buy two identical drives at the same time and I've often seen adjacent serial numbered drives arrive. The raid controller manufacturers tell you, you must have identical drives with the same firmware! To me that's asking for trouble, I've often told clients to buy one extra drive, check it works then store it away for 6 months. After 6 months change one of the disks in the array for the stored one and keep the original disk safe. That way you break the identical usage pattern and have a compatible drive available for _when_ a failure occurs.
A friend who works for a place that has serious storage, entire rooms full of spinning disks! He tells me that when they purchase disks that must be identical they specify to the manufacturer that the drives must come from at least two different manufacturing plants and be labelled as such. They do buy rather a lot of disks and no it's not Google.
> I saw that in one of our Unix servers at my last job. A drive failed, and it took our maintainers a while to get a replacement because they struggled to understand that they can't supply any old "9GB drive" and it must be at least the same number of blocks as the smallest in the array. By the time it did get replaced, two more were showing an increasing error count and I think we replaced 3 in an array of 4 ! For that reason, I've always setup my arrays to use a bit less than the full disk to allow for a slightly smaller drive as a replacement.
> So I think there is an argument (if possible) to at least spread your purchases and build the array over several months - if you use all the same drive, that should at least give you a chance of them coming from different batches and reduce the chance of a common defect.
> Simon Hobson
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