MTBF interpretations (Re: ZFS performance)
On Fri, 11 Nov 2022 14:05:33 -0500 Dan Ritter <dsr@randomstring.org> wrote:
> Claimed MTBF: 1 million hours. Believe it or not, this is par
> for the course for high-end disks.
>
> 24 hours a day, 365 days a year: 8760 hours per year.
> 1000000/8760 = 114 years.
>
> So, no: MTBF numbers must be presumed to be malicious lies.
With your interpretation every single drive would not be allowed to fail before its MTBF value. That's wrong. MTBF is a mean value for all drives of this type, not a guaranteed minimum value for a single drive.
If there are one million drives, statistically with every hour one drive will fail. That means, even with correctly claiming an MTBF of 1 million hours, one of those million drives will statistically(!) run only one hour before it fails.
Some more practical explanation:
If you have one thousand drives then you have to expect one drive failure for every one thousand hours (MTBF=1,000,000). Statistically you have to expect nearly nine of those drives to fail per year (if I did my math correctly). With higher MTBF values this value will be smaller, with lower MTBF values you'd have to expect more drives to fail.
But this is only a pure statistical value. If you are the one of a million, your single drive will fail within the first hour...
(and on top there are other details, like: the MTBF is calculated for specific environmental conditions; if your environment is more rogue, then you'd have to expect a lower MTBF than the one declared by the vendor... and vice versa)
regards
hede
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