Grr, I think I messed up the threshold detection ("> progress_goal") on the
denominator.
Take 2:
expr: |
(*(progress_goal - progress_current) > 0)* /
(( predict_linear(*progress_current*{fstype!~"fuse.*|nfs.*"}[12h],
604800) *> progress_goal*) - *progress_current*) * 604800
On Thursday, 18 May 2023 at 08:57:57 UTC+1 Brian Candler wrote:
> > I have seen a bunch of examples for predicting when a disk will fill,
> but these all seem to rely on the assumption that a gauge is trending
> downwards, and we are predicting when it meets zero.
>
> Here is a better recipe as a starting point, using predict_linear():
> https://groups.google.com/g/prometheus-users/c/PCT4MJjFFgI/m/kVfOW069BQAJ
>
> > How can we write an expression that will tell us the time in seconds
> until progress_current meets *progress_goal*?
>
> In principle, I think you should just be able to replace the metric in
> that expression with (progress_goal - progress_current) and see when that
> hits zero. Something like this:
>
> expr: |
> (*(progress_goal - progress_current) > 0)* / (*(progress_goal -
> progress_current)* -
> ((*progress_goal -*
> predict_linear(*progress_current*{fstype!~"fuse.*|nfs.*"}[12h],
> 604800)) < 0)) * 604800
>
> (That's syntactically valid, but otherwise untested. Obviously you should
> remove or change the label selectors on progress_current to suit the metric
> in question).
>
> I see +progress_goal and -progress_goal in the denominator so it should be
> possible to simplify it further, I think to this:
>
> expr: |
> (*(progress_goal - progress_current) > 0)* /
> (( predict_linear(*progress_current*{fstype!~"fuse.*|nfs.*"}[12h],
> 604800) *- progress_current*) > *progress_goal*) * 604800
>
> The value of this expression is intended to return *the time until the
> goal is reached*, using linear interpolation over the past 12 hours, to
> predict where it will be in 7 days. If the goal isn't expected to be
> reached in 7 days then it will return no value.
>
> I can attempt to justify that expression graphically:
>
> | C2 ^
> | / |
> | / |
> G +.^......x.....|....
> | | / D
> | N / |
> | | / |
> | v C1 v
> |
> +---0d------7d----> time
>
> G is the progress_goal, which I'm assuming is constant
> C1 is the value of progress_current now (time = 0d), which is less than G
> C2 is the future predicted value of progress_current (at time = 7d), which
> is greater than G
>
> N (numerator) is G - C1
> D (denominator) is C2 - C1
>
> And geometrically, I think the ratio of the crossover time (0d...x) to the
> full time (0d...7d) is the same as the ratio of N to D. Hence (N/D)*7d
> gives the time to crossover.
>
> Give it a go. Note that if progress_goal is a metric, rather than a
> constant, then it will need to have exactly the same label set as
> progress_current; or you will need to add some on(...) or ignoring(...)
> clauses as appropriate.
>
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