DELL EMC Unity : VMware ESXi Datastore more space utilized though Unity has "Data reduction" enabled.


   Article Number:     534493                                   Article Version: 2     Article Type:    Break Fix 




Dell EMC Unity Family





VMware ESXi datastore shows higher used space whereas on "Unity" side "Data reduction" is enabled.    
    Customer might have queries like below:   
    1. Why is the space utilization of the Datastore is different on Unity and VMware side.   
    2. Is compression working properly, if so, then why don't they see any savings on the Host side   
    3. What is the use of this "Data reduction" feature if host does not see any saving on his end.   






 VCenter will not be aware of the "Data savings " happening on Unity as it's a feature on Unity.   

  •         A VMFS datastore named  "LI_UNITY400" is created on Unity with "Data reduction":     
              Total size: 2 TB   
              Allocation size : 570 GB   
              Current savings : 700 GB   
  •         On the VMware end the Datastore "LI_UNITY400" shows the size used space of  1.27 TB which is way more than 570GB Allocation size on Unity.     
  •         If you add up the "Allocated size" and "Current savings"  on Unity (570+700 GB= 1.27TB) , it gives you the same "used space" as on ESXI host because host is not aware of the "Compression feature" .     
    This is because on Unity If the data is compressed, it must first be uncompressed before the data is sent to the host.  If the compressed data already resides in System Cache, the data is uncompressed to a temporary location, the data is sent to the host, and the temporary location is released. If the compressed data being requested resides on disk, the data is first read into System Cache, uncompressed to a temporary location, and the host is sent the data. Data is never uncompressed on disk due to a read operation, as this would reduce the amount of savings on the storage resource.   
    Please go through the below document for Compression for more detailed explanation:   






"Data reduction" is enabled while creating a "Datastore" on Unity                                                           






The array is working as per the design and needs no changes.                                                           






NOTE:  In Dell EMC Unity OE version 4.3 and later, Dell EMC Unity Data Reduction replaces Dell EMC Unity Compression. Data reduction includes compression and deduplication logic within the space savings algorithm.