What is an Event Queue used for? It's indicated that this is a memory-mapped file. Perhaps the PI interfaces are writing here and the archive is reading but what are the events which are queued?
I would recommend reading through the cause section of KB01729 for a bit of background on the Data Archive event queue (EVQ). Essentially, the event queue is a memory mapped queue file (MMQ) which acts as a bridge between the PI Snapshot Subsystem and the Archive Write Cache. The event queue contains PI events which have passed through snapshot and compression testing and are destined to be archived within the PI Archive files.
As for being memory mapped, in simpler terms, an MMQ file is a file which exists in memory and on disk, and the contents of the file in memory are periodically flushed to disk. This allows resources to access the file much quicker (it exists in memory) with the added persistence of data across graceful shutdowns. This allows for snapshot and archive subsystems to read/write to the EVQ as quickly as possible, while still persisting events across possible restarts/shutdowns of the PI Data Archive.
Is there a particular reason you are interested in the event queue? Are you seeing questionable behavior on your system?
Hope this helps,
Adam, thank you for the quick reply. It's clear for me now, the picture in the KB documents and your explanation was helpful. The tuning parameter "snapshot_eventqueuepath" was mentioned in the course exercise and I was wondering what the event queue is used for.
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