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Like zeroedthick, the thin allocation mechanism is also virtual provisioning-friendly. However, as explained in this section, it should be used with caution together with array virtual provisioning. Thin virtual disks increase the efficiency of storage utilization for virtualization environments by using only the amount of underlying storage resources needed for that virtual disk, exactly like zeroedthick. But unlike zeroedthick, thin devices do not reserve space on the VMFS volume which allows more virtual disks per VMFS. Upon the initial provisioning of the virtual disk, the disk is provided with an allocation equal to one block size worth of storage from the datastore. As that space is filled, additional chunks of storage in multiples of the VMFS block size (1 MB) are allocated for the virtual disk. That way the underlying storage demand grows as its size increases.
Using the same-sized VM as in Figure 94, a single 90 GB virtual disk is created that uses the thin allocation method rather than zeroedthick. Note now in Figure 97 that VMDK reports as zero.
As is the case with the zeroedthick allocation format, the VMware kernel does not initialize unused blocks for thin virtual disks. The full 90 GB is not reserved on the VMFS or consumed on the thin device on the array. The virtual disk that is presented, as shown in Figure 97, resides on thin device 0003A. It consumes no space, as shown in Figure 98.