The Analysis server uses the PI Buffer subsystem in order to apply Exception and Compression testing on the results of Analysis. However, when deleting data (either through Recalculation or deleting data via Powershell or PI SMT, or whatever) the PI Buffer subsystem on the Analysis server never resets the stored snapshot value/timestamp.
The consequence of this is that if the same Analysis is recalculated multiple times (for example after corrections, optimization, etc), compression is only ever applied the very first time an Analysis is recalculated. After that, compression is never applied up to the timestamp that the recalculation was run up to, as the PI Buffer subsystem never resets the actual current snapshot value/time.
In the referenced thread, the only workaround is to completely stop the PI Buffering Subsystem on the Analysis Server, delete the cache, release ownership of the PI Points on the Data Archive, restart the Buffering Subsystem, and then hope that everything works.
It would be way easier to be able to tell the Buffering Subsystem to get the current Snapshot from the Data Archive.
Reference in this thread on PI Square:
https://pisquare.osisoft.com/thread/43696-analysis-recalculation-ignores-pi-point-compression-settings#comment-146126
Since backfilling occurs in chunks and the snapshot only gets deleted during the last chunk, this would still only apply compression to that last chunk of recalculated data (and I believe this may already work to compress that last chunk).
Two solutions have been suggested for this:
delete all the data for the recalculation period, including the snapshot if the end-time is '*'
apply the compression settings to all OOO events, whether they come from analyses or some other source. If this is considered a big change to the buffer subsystem then maybe add it as a configuration option that customers could enable if they want.