When you take a snapshot, the state of the virtual disk is preserved, which prevents the guest operating system from writing to it. A delta or child disk is created. The delta represents the difference between the current state of the VM disk and the state that existed when you took the previous snapshot. On the VMFS datastore, the delta disk is a sparse disk.

Sparse disks use the copy-on-write mechanism, in which the virtual disk contains no data, until the data is copied there by a write operation. This optimization saves storage space.

Depending on the type of your datastore, delta disks use different sparse formats.

VMFSsparse

VMFS5 uses the VMFSsparse format for virtual disks smaller than 2 TB.

VMFSsparse is implemented on top of VMFS. The VMFSsparse layer processes I/Os issued to a snapshot VM. Technically, VMFSsparse is a redo-log that starts empty, immediately after a VM snapshot is taken. The redo-log expands to the size of its base vmdk, when the entire vmdk is rewritten with new data after the VM snapshotting. This redo-log is a file in the VMFS datastore. Upon snapshot creation, the base vmdk attached to the VM is changed to the newly created sparse vmdk.

SEsparse

SEsparse is a default format for all delta disks on the VMFS6 datastores. On VMFS5, SEsparse is used for virtual disks of the size 2 TB and larger.

SEsparse is a format similar to VMFSsparse with some enhancements. This format is space efficient and supports the space reclamation technique. With space reclamation, blocks that the guest OS deletes are marked. The system sends commands to the SEsparse layer in the hypervisor to unmap those blocks. The unmapping helps to reclaim space allocated by SEsparse once the guest operating system has deleted that data. For more information about space reclamation, see Storage Space Reclamation.

Snapshot Migration

You can migrate VMs with snapshots across different datastores. The following considerations apply:

  • If you migrate a VM with the VMFSsparse snapshot to VMFS6, the snapshot format changes to SEsparse.

  • When a VM with a vmdk of the size smaller than 2 TB is migrated to VMFS5, the snapshot format changes to VMFSsparse.

  • You cannot mix VMFSsparse redo-logs with SEsparse redo-logs in the same hierarchy.