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Everything You Know About Disks Is Wrong
modapi writes "Google's wasn't the best storage paper at FAST '07. Another, more provocative paper looking at real-world results from 100,000 disk drives got the 'Best Paper' award. Bianca Schroeder, of CMU's Parallel Data Lab, submitted Disk failures in the real world: What does an MTTF of 1,000,000 hours mean to you? The paper crushes a number of (what we now know to be) myths about disks such as vendor MTBF validity, 'consumer' vs. 'enterprise' drive reliability (spoiler: no difference), and RAID 5 assumptions. StorageMojo has a good summary of the paper's key points."
MT[TB]F has become a completely BS metric because it is so poorly understood. It only works if your failure rate is linear with respect to time. Even if you test for a stupendously huge period of time, it is still misleading because of the bathtub curve effect. You might get an MTBF of say, two years, when the reality is that the distribution has a big spike at one month, and the rest of the failures forming a wide bell curve centered at say, five years.
Suppose a tire manufacturer drove their tires around the block, and then observed that not one of the four tires had gone bald. Could they then claim an enormous MTBF? Of course not, but that is no less absurd than the testing being reported by hard drive manufacturers.
Or she forgot to put in the part that Enterprise drives are replaced on a schedule BEFORE they fail. At Comcast I used to have 30 some servers with 25-50 drives each scattered about the state. every hard drive was replaced every 3 years to avoid failures. These servers (Tv ad insertion servers) made us between $4500-13,000 a minute they were in operation in spurts of 15 minutes down 3-5 minutes inserting ad's. Downtime was not acceptable so we replaced them on a regular basis.
Most enterprise level operations that relies on their data replace drives before they fail. In fac tthe replacement rate was increased to every 2 years not for failure prevention but for capacity increases.
Do not look at laser with remaining good eye.
What's interesting to me is that neither of these papers mentions the issue of pre-installation handling. The good folks over at Storage Review seem to be of the opinion that the shocks and bumps that happen to a drive between the factory and the final installation are the most significant factor in drive reliability (much more than brand, for example).
The google paper talks a bit about certain drive "vintages" being problemmatic, but I wonder if they buy drives in large lots, and perhaps some lots might have been handled roughly during shipping. If they could trace back each hard drive to the original order, perhaps they could look to see if there's a correlation between failure and shipping lot.
-R
Well, the article actually says that drives don't have a spike of failures at the beginning.
Hmm, the Google paper says they do, from 3-6 months (Figure 2).
Which leaves us with confirmation that 50% of all studies are wrong.
My God, it's Full of Source!
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