Disks volume capacity alert thresholds based on volume size

The basic cheat of this method is the same one I use to produce percentages from arbitrarily scaled numbers:  x * n - s

'n' is the number you have.  'x' scales it out.  's' shifts it back to the start of the range you want to end up with.

if n is 1 - 50 but you need a graph to show it wider to line up with another metric that is 1-100:

2 * 50 = 100 (top of the other graph)

but since this also raise the 1 to a 2 at the bottom of the graph, -1 brings it back down to 1-99... which is pretty close to 1-100 and plays more nicely in a graph together.

We know we want 10% at smaller sizes and 2% at larger sizes.  We just have to cheat the scale up and down to match those sizes we want based on the volume size.  I'm going to try to figure out how to drive that scale up and down more simply based on a property... that way, we can treat azure and vsphere disks differently due to the differences in how you have to resize them... and different applications that need extra freespace can just be a device level property adjustment.

Time to play :)

Final numbers... floor of 2 and ceiling of 10:

8000 / (200 * [math]::Log($size,2) - 1000) - 6

Darnit... can't edit... From there, we twiddle the expand/contract values so that 1024 stays at 2 where it is, and 128 grows to 10 where we want it... then set a ceiling of 10 and use the resulting value to calculate the new threshold.

That's the math... I just have to figure out how to apply it.

This is very close:

$sizes    = @(128,256,512,1024)
$expand   = 200
$contract = 1000

# X=log2(Y)

foreach ( $size in $sizes) {
    $newPercent = 2000 / ($expand * [math]::Log($size,2) - $contract)
    write-host $size`t$newPercent
}

I've been giving this some thought as well and was debating writing a new DS for it... since the real concern is making sure there's enough freespace on a disk... but we're tracking that using the inverse (Space Consumed rather than Space Free) and doing it with a percentage that doesn't scale correctly.  I was going to figure out how to produce a metric threshold that operates as a sliding value that would automatically produce a threshold based on both disk size and freespace that wasn't just a static number or a dynamic value based on normal usage... more of a consistent, predictable value.

I don't have if figured out yet.

It's been brought up a few times in the forums over the years. Your thought should work and might be the easiest but I personally would look to make a DataSource change to keep it a bit cleaner. I do see a Community DataSource called "WinVolumeUsageConditional-" that seems to implement something along these lines. Might be worth looking at or atleast using it for inspiration. Basically it uses Complex datapoints to provide more datapoints based on conditions that you can alert on.

I personally don't like to hard code thresholds directly into the DataSource, so I would do something more like LargeDrivesFreeSpaceGB=if(gt(AvailableGB,1024),FreeSpaceGB,unkn()) and SmallDrivesFreeSpaceGB=if(le(AvailableGB,1024),FreeSpaceGB,unkn()) so you can set thresholds differently if the drive is small vs large. Note that I didn't test these and just off the top of my head.

Also keep in mind that thresholds can be real numbers, so you can set thresholds to like >=98.1531%