I have problems with LUN compression on VNX 5300. System: VNX5300 (Block) 05.32.000.5.206.

I have Storage Pool for backup data. 10x 3TB NL SAS disks and 5x 900GB SAS disks (RAID 6 and RAID5 respectively). Total 25271GB in pool. Now the long story:

There was two Thick LUNs configured 16TB and 6TB, Leaving like 9% free in pool for Tiering to operate normally.

Then we started to get low on free space on LUNs, so I started to look into compression. Created new Thin 1TB LUN, moved there some typical backup data. Got some nice savings percentage. Then I removed this LUN and enabled compression on 6TB LUN. After few weeks storage reported like 2TB saving. But I can't expand this LUN beyond 6TB, Unisphere returns error that says not enough free space.

After a month I enabled compression on 16TB LUN.

After a week or two, I had to move around big amount of data and did need some additional space. I tried to expand 6TB LUN (Hey, i'm compressing this data for long time, storage report savings, can I get some space now?). I was able to expand it to 6500GB, just 350GB. So there is like 6700GB free in Pool, but I cant expand 6TB LUN further. WHY?

I moved some new data to 6.3TB LUN, reorganized some on 16TB. And then 16TB LUN went offline. With no alerts or warning, and 6678GB free left in pool. On compression tab notice that compression is paused because it cannot allocate free blocks. Unisphere shows that LUN is Ready, but it is offline in Microsoft Cluster and I cannot bring it online. So I moved all data from 6TB to other LUNs on performance Pool. Deleted 6TB LUN and 16TB LUN is now online.

Now I'm going to turn OFF this *** compression, which gave me nothing, just my backup VMs offline for 30 hours.

Pool is 25271GB, free are 10889GB. When i untick compression checkbox in LUN properties, I got this:

"Following error occurred while setting LUN Parameters:

There is not enough free space in the specified Storage Pool (ID: 0) to complete the operation. Please add available storage to the Pool and retry the operation. (0x71658314)"

If try to create new Thick volume (just get back my 6TB LUN), I can't. Max size it allows is under 4TB. Storage triest to leave unused 6678GB, same number I saw at the moment when LUN went offline.

Now I want someone explains me what is happening? Why i can't turn off compression, if my pool is only 56% full? Why I was not able to reuse space freed up by compression? And why LUN went offline with 6678GB free in Pool?