Hi,
Thin: Allocate and zero on first write
Thick Lazy: Allocate in advance and zero on first write
Thick Eager: Allocate and zero in advance
Thin – These virtual disks do not reserve space on the VMFS filesystem, nor do they reserve space on the back-end storage. They only consume blocks when data is written to disk from within the VM/Guest OS. The amount of actual space consumed by the VMDK starts out small, but grows in size as the Guest OS commits more I/O to disk, up to a maximum size set at VMDK creation time. The Guest OS believes that it has the maximum disk size available to it as storage space from the start.
Thick (aka LazyZeroedThick) – These disks reserve space on the VMFS filesystem but there is an interesting caveat. Although they are called thick disks, they behave similar to thinly provisioned disks. Disk blocks are only used on the back-end (array) when they get written to inside in the VM/Guest OS. Again, the Guest OS inside this VM thinks it has this maximum size from the start.
| NAME | DESCRIPTION |
| Eager Zeroed Thick | An eager zeroed thick disk has all space allocated and wiped clean of any previous contents on the physical media at creation time. Such disks may take longer time during creation compared to other disk formats. The entire disk space is reserved and unavailable for use by other virtual machines. |
| Thick or Lazy Zeroed Thick | A thick disk has all space allocated at creation time. This space may contain stale data on the physical media. Before writing to a new block a zero has to be written, increasing the IOPS on new blocks compare to Eager disks. The entire disk space is reserved and unavailable for use by other virtual machines. |
| Thin | Space required for thin-provisioned virtual disk is allocated and zeroed on demand as the space is used. Unused space is available for use by other virtual machines. |
