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Thin provisioning and virtual disks
53 Dell EMC SC Series: Best Practices with VMware vSphere | 2060-M-BP-V
those files. Although Windows reports only 5 GB in-use, thin provisioning has assigned those blocks to that
volume, so the array will still report 15 GB of data used. When Windows deletes a file, it merely removes the
entry in the file allocation table, and there are no onboard mechanisms for the SC Series to determine if an
allocated block is still in use by the operating system. However, the DSM server agent contains the necessary
functionality to recover this free space from machines running earlier versions of Windows that do not
possess SCSI UNMAP capabilities. It compares the Windows file allocation table to the list of blocks allocated
to the volume. Once finished, it then returns those free blocks into the storage pool to be used elsewhere in
the system. Blocks kept as part of a snapshot (replay) cannot be freed until that snapshot is expired.
The free space recovery functionality can only be used in Windows virtual machines under the following
circumstances:
• The virtual disk needs to be mapped as an RDM set to physical compatibility mode (pRDM). The free
space recovery agent can then perform a SCSI query of LBAs in use and correlate them to the blocks
allocated on the array that can be freed. The disk must be an NTFS basic disk (either MBR or GPT).
• The virtual disk cannot be a VMDK or an RDM set to virtual compatibility mode (vRDM).
- VMware does not provide APIs for the free space recovery agent to correlate the virtual LBAs to
the actual physical LBAs needed to perform the space recovery, since UNMAP handles this
operation.
- If a virtual machine has a drive C (VMDK) and a drive D (RDMP), Windows free space recovery
will only be able to reclaim space for the drive D.
- The restriction against using vRDMs for space recovery also implies that these disks cannot
participate in ESXi host snapshots. Software that uses VMware snapshots is needed, an
alternative method of backing up the pRDMs is needed. For example, the Dell Storage
PowerShell SDK installation provides an example PowerShell script. This script can be used to
back up physical mode RDMs as part of the pre-execution steps of the backup job.
• The free space recovery agent also works with volumes mapped directly to the virtual machine
through the Microsoft software iSCSI initiator. Volumes mapped to the virtual machine through the
Microsoft iSCSI initiator interact with the SAN directly, and therefore, space recovery works as
intended.
For more information about Windows free space recovery and compatible versions of Windows, consult the
Dell Storage Manager Administrator’s Guide on Dell.com/support.
12.6 Affinity Manager 2.0
With the release of vSphere 7.0, VMware has improved the first-write process when using thin provisioned
VMDKs. While VMware still recommends using Eager Zeroed Thick (EZT) virtual disks for maximum
performance, this new method should decrease the on-first-write performance overhead previously
experienced with thin VMDKs. For an overview of the new feature, see the VMware vSphere 7 Core Storage
whitepaper.