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Great questions Scott.
1. Will PI ever archive to a registered archive labeled "Spare" and if so how would it know to use this spare?
cet> Modern PI3 systems are implemented with Auto Archive Creation enabled. When the system runs out of disk space, the spare archives come into play. The way that works is the way archive shifting, archive management worked before the feature for Auto Archive Creation was added to the PI system. Namely, the most recently created empty archive or the archive containing oldest data which is flagged shiftable becomes the destination for new data when the current primary archive becomes full. Having two or three spare archives buys you some time when the disk becomes full - you have the spare archives to fill up before the archive system starts overwriting your oldest data.
2. Are there build in disk management operations that warn of impending out of disk space conditions within PI or are you relying on IT disk management tools?
cet> PI System includes a set of interfaces we call MCN (Manufacturing Control Network) Health Monitor or IT Monitor. In this set is an interface to collect data from Windows Performance Monitor. The PI System services, subsystems, and interfaces export counters to Windows Performance Monitor. These counters (and the counters built into Windows) can be collected and written to PI points. We recommend best practice is to build from a couple dozen to a couple hundred PI points to monitor the operation of Windows server (and underlying hardware) as well as important KPIs from the PI system itself. If you have done this, you will have PI points which you can trend or monitor or use PI Notifications or PI Alarms provide alerts for things such as low disk space, high network traffic or errors, queuing or buffering problems, latency in various systems including disks, and so on.
3. If you get a low disk space notice (regardless of how) do you delete the "Spare" archive to provide sufficient space for PI to build an additional archive before the system runs out of disk space?
cet> that is a choice for the PI admin. You could let the PI system default to overwriting the oldest data - wherein the PI System manages the existing created archive files striving to keep the longest period of contiguous data online starting from the current time going backward - overwriting the oldest data when disk space is needed for new data.
Hope this helps. thanks!