This looks like a really nice feature that would allow more efficient use of disk space but I can't find any mention of it being supported in ESXi. Is there any existing equivalent and is support for it likely to be added in the future?
The new SE sparse disk implements a space reclaim feature to reclaim blocks that were previously used but now are unused on the guest OS. These are blocks that were previously written but currently are unaddressed in a file system/database due to file deletions, temporary files, and so on.
http://www.vmware.com/files/pdf/techpaper/Whats-New-VMware-vSphere-51-Storage-Technical-Whitepaper.p...
The SEsparse virtual disk format was introduced in VMware vSphere® 5.1 for VMware® Horizon View™ environments where reclamation of storage space is critical because of the large number of tenants sharing storage. In vSphere 5.5, for VMDKs greater than 2TB in size, SEsparse becomes the default scheme for virtual disk snapshots. Various enhancements were made to SEsparse technology in the vSphere 5.5 release, which makes SEsparse perform mostly on par or better than VMFSsparse formats. SEsparse also has a significant advantage over VMFSsparse virtual disk formats by being space efficient.
http://www.vmware.com/files/pdf/techpaper/SEsparse-vsphere55-perf.pdf