Why is % allocated always at 99.96%?

Hello,

In my Compellent Enterprise Manager application in this view:

Dell Compellent > Storage Centers > Storage Center 14xxx > Disks > Assigned

• I can see 3 tiers of disk types (7K with 20TB, 10K with 12TB, 15K 6TB)
• My 7K, and 15K disks are 99.9% allocated (red warning) with:
  • 2GB free on 7K disks, and
  • 3GB free on 15K disks
    
    
• Can someone help explain why this may be?
  Overall we have about 50% free space (20TB).  The system is about 7 years old and we have a lot of old data so I can understand that a lot of the data has been migrated to tier 3, but that doesn't explain why the 15K disks are fully allocated.  Don't understand that, sorry.
• (Tier 2 is only 70% allocated and is green in the view)

In this view:

Dell Compellent > Storage Centers > Storage Center 14xxx > Storage Types

• Tier 1
  • Raid 10 - standard.  3.74 TB allocated, 14% full (green)
  • Raid 5-9 - standard.  2.53 TB allocated, 46% full (green)
• Tier 3
  • Raid 10-DM - standard 1.8TB allocated, 0% full (green)
  • Raid 6-10 - standard 18 TB allocated, 93 % full (red)

A fairly simple setup, a single Compellent, 2 controllers, 3 enclosures.  All volumes set to use the Dell Recommended profile and replays that expire after 50 hours (we've never had to use a replay and don't rely on it for a backup as we have other things we depend on it for).

Model: SC040
Version: 6.3.10