Compression doesn not work when i backfill from Analyses!

This is as expected. Backfilling is out-of-order data and is therefore not compressed.

See some explanation here: https://customers.osisoft.com/s/knowledgearticle?knowledgeArticleUrl=000025307

Unfortunately there is very little you can do about that. What happens is that the buffer subsystem on the analysis server interferes with applying compression. Easiest workaround is to disable the buffer subsystem on the analysis server, delete all the relevant data and then recalculate. Disabling the buffer subsystem will however severely degrade the performance of the analysis server while buffering is disable. Otherwise there are a lot of hoops you have to jump through to make it work (once).

See my question about that here, for those workarounds and also for an explanation for what is happening:

Analysis Recalculation ignores PI Point Compression settings

My suggestion for your specific workcase is to check your output in the analysis yourself and only output if necessary.

One easy way of doing that is to use HasValueChanged and NoOutput in order to only write a value if it is changed.

While using HasValueChanged() and NotOutput() will work well for Step=On tags, I would not recommend it for Step=Off tags (i.e. continuously changing numeric values, probably the vast majority of your tags).

The normal compression algorithm will store the previous value when it detects a change in direction, so it records the start and end of each 'straight' line, but an analysis can only store the current value when it detects a change in direction, not the previous value. This will change the profile of the trend.

We really need out-of-order event processing to honour the compression settings. I believe Aveva may be working on this and I for one cannot wait!