I really need help creating a calculation for the rate of change for free megabytes (in MB/day) for my windows machine. Could you please help me?
It seems to me the easiest way to do this would be an AF analysis that's something to the effect of:
TagVal('FreeMB','t') - TagVal('FreeMB','y')
Where "FreeMB" is an attribute linked tot he tag for the machine you're looking to monitor the amount of disk space on. TagVal is a function which will retrieve the archive value for a point/attribute at a given time, and in my equation above I've referenced yesterday at midnight to today at midnight as the time frame (there's also an argument to be made for doing from * to *-24h to account for DST, but I'll leave that decision up to you.)
Thank you for your help Robert
To add on Robert's reply, you can use the PI Interface for Performance Monitor to collect available disk space into a PI Point. The "Logical Disk" Windows performance counter object offers a "Free Megabytes" counter as total or per logical drive.
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