I've recently just added an osd to my cluster and it started backfilling. The new osd disk max space is added to the cluster max size as expected. But there is a strange behaviour i can't understand. While backfilling is going on my max available space keeps decreasing. Is this expected behaviour? Will there be a available space increase after backfilling is done?
Yes, this is expected. I don't know the exact mechanism requiring this but during recovery/backfill the PGs that have to be remapped take additional space. After the remapping is finished you should see the expected results.