We are having slight performance problems with our SC4020. Performance is adequate on normal operations, but full -backups takes too much time.

We  have two tiers, SSD and 7k HDD. We are considering buying more SSD-capacity to speed things up, but I'm not sure if this is good idea because system is using only maybe 20% of SSD-capacity. 

For me, it seems that Compellent is tiering data from SSD-disk too aggressively. Is there any settings to control how much data is kept on SSD-disks?