Like the question says: Do rollup analyses usually respect the compression settings (compdev and excdev)?
I am asking, because for me it currently doesn't, and I just want to see if it is maybe a problem on my end.
Ok, so I figured out what the problem was:
I was trying it on historical data and recalculating. Recalculated Analyses (at least for us) basically never have compression applied (with the exception noted in this KB article (the last chunk recalculated is compressed).
We have the Buffering subsystem running on the AF Analysis Server again. Interestingly enough, the Data Archive does not register any of the values written as Out of Order. So it must be the Buffer Subsystem on the Analysis Server deciding to never apply compression on recalculated values. The live Analysis correctly compresses data.
Edit: As noted in this thread (Analysis Recalculation ignores PI Point Compression settings) this problem is created by the Buffering subsystem on the Analysis Server. Once a PI Point is used by the Buffer Subsystem, the cached snapshot value of the Buffer Subsystem is independent of the snapshot value on the Data Archive. The Buffer Subsystem on the Analysis Server never resets/deletes the cached snapshot value. Therefore, once any value past a certain point in time has been passed to the Buffer Subsystem, it is basically impossible to get compression applied again to values before that time - even if deleting the values, etc. Only by completely deleting the cache of the Buffer Subsystem, disabling the Buffer subsystem while recalculating, or creating a new PI Point it is possible to get compression to apply again.
For AF Analytics outputs will go through PI Compression if written in-order, but there is no exception processing.
I'd like to emphasize something in Dan's answer:
It does not matter if it is an Analysis Rollup or any AF Analysis or other. What matters is data being written to PI. If each new value coming in is newer than the current snapshot, then compression will be applied. Meaning, it uses the compression settings of the PI Point associated with the output attribute.
The flip side of that is compression is not used for any Analysis that is performing a re-calculation or back-filling calculation of values before the current snapshot.
Is your Analysis Service going through the PI Buffer Subsystem? I've seen odd compression behaviors when it isn't. The PI Buffer Subsystem marks data for compression and the PI Data Archive will discard the data. Without the PI Buffer Subsystem, the PI Data Archive can do some compression, but I've seen it act oddly with Asset Analytics. I was never able to systematically reproduce the behavior however or determine what the issue is. If Asset Analytics isn't going through the PI Buffer Subsystem, could you configure the it to and report back if there is a change in behavior?
Our PI System is relatively new. Everything should be set up with the Buffer Subsystem. However, my impression is that something isn't set up correctly. It seems that everything works for some time and then stops working correctly. I notice it when I suddenly can't delete the snapshot value any more.
We will open a ticket with the customer support next week to see if we can figure out what the problem is.
I'll be sure to report back.
Well, after two days with three people from OSIsoft customer support, we completely turned off the Buffer Subystem, because it still wasn't running correctly.
Now the results of the Rollup Analysis seem to be compressed (mostly) correctly.
Of course not having the Buffer Subsystem run is not really what we want, and the data access is now suddenly significantly slower than before, so there is still work left to be done ...
We'll see next week if we can finally get this working correctly (long weekend here in Germany).
Yeah, so apparently it is something on my end. As even with a completely new PI Point it does not compress.
Retrieving data ...