The disk space reclamation feature is for longer-lived instant clones.
Instant clones can be either short-lived or longer-lived. Short-lived instant clones are clones created with floating assignment or dedicated assignment with the Refresh on Logoff set to Always. Short-lived instant clones are deleted and then recreated whenever a user logs out. Because of the frequent refresh of short-lived instant clones, the clones’ OS disks do not get much chance to grow unlike the OS disks for longer-lived instant clones, so it is beneficial to allow ESXi hosts to reclaim unused disk space, thereby reducing the total storage space required.
Horizon creates all instant clone virtual machines in an efficient disk format. As users interact with instant clone desktops, the clones' OS disks grow and can eventually use almost as much disk space as full-clone desktops. Disk space reclamation reduces the size of the OS disks. Space can be reclaimed while the virtual machines are powered on and users are interacting with their remote desktops. With disk space reclamation, Horizon can maintain instant clones at close to the reduced size they start out with when they are first provisioned.
Space reclamation works differently depending on whether you are using vSAN or non-vSAN datastore, and for the specific version of vSphere/vSAN you are running:
- On vSAN storage, there is no support for space reclamation prior to vSphere/vSAN 6.7U1.
- Starting vSphere/vSAN 6.7U1, space reclamation is supported via vCenter TRIM and UNMAP feature on vSAN datastores
- On non-vSAN storage, prior to vSphere 6.7, Horizon implements a periodic space reclamation operations.
- On non-vSAN storage, from vSphere version 6.7 and later, VMFS-6 supports the Automatic UNMAP feature, which reclaims dead blocks automatically and asynchronously if it is not disabled by the vSphere or vCenter Server administrator. Therefore, you no longer need to enable the manual process on Horizon.
Disk space reclamation is not applicable for full clones. For more details, see the Horizon Installation document.