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The default option when creating, cloning, or converting virtual disks in the vSphere Client is called Thick Provision Lazy Zeroed. The Thick Provision Lazy Zeroed selection is commonly known as the zeroedthick format. In this allocation scheme, the storage that is required for the virtual disks is reserved in the datastore but the VMware kernel does not initialize all the blocks. The guest operating system initializes the blocks as write activities to previously uninitialized blocks are performed. If the VMFS attempts to read blocks of data that it has not previously written to, it returns zeros to the guest operating system. This case is true even in cases where information from a previous allocation (data “deleted” on the host, but not deallocated on the thin pool) is available. The VMFS does not present stale data to the guest operating system when the virtual disk is created using the zeroedthick format.
Since the VMFS volume reports the virtual disk as fully allocated, the risk of oversubscribing is removed. This condition is because the oversubscription does not occur on both the VMware layer and the array layer. The virtual disks do not require more space on the VMFS volume as their reserved size is static with this allocation mechanism. More space is needed only if additional virtual disks are added. For example, in Figure 94, a single VM resides on the 500 GB datastore iSCSI_1. The VM has a single 90 GB virtual disk that uses the zeroedthick allocation method. The datastore browser reports the virtual disk as consuming the full 90 GB.
However, since the VMware kernel does not initialize unused blocks, the full 90 GB is not consumed on the thin device backing the datastore. In the example, the virtual disk resides on the thin device 0003A. No space is consumed, as seen in Figure 95.
If the VMware administrator does not have access to Unisphere, to view the relationship between the back-end device and datastore, VSI is recommended. The highlighted area in Figure 96 is pulled directly from the array and shows the current allocation, again zero.