There are three types of virtual disks can be used in a virtual machine in vSphere:
1. Thick Provision Lazy Zeroed – this is the default disk type. Space required for the virtual disk is allocated during creation. The space is zeroed on command on the first write from the virtual machine. A virtual disk that is created as 15 GB will consume 15 GB of space on the datastore, regardless of the actual disk usage in the guest operating system.
2. Thick Provision Eager Zeroed – the space required for the virtual disk is allocated during the disk creation. The space is zeroed out when the disk is created. This type of disk takes the longest to create, but there is a performance increase, since the zeroing process doesn’t happen at runtime. A virtual disk that is created as 20 GB will consume 20 GB of space on the datastore, regardless of the actual disk usage in the guest operating system. Some advanced vSphere features, such as Fault Tolerance, require that you use this disk type.
3. Thin Provision – a disk of this type uses only as much space on the datastore as needed. This means that, if you create a 10 GB virtual disk and place 3 GB of data in it, only the 3 GB of space on the datastore will be used. Note that, although the size will increase as required, it will not shrink if files within the guest operating systems are deleted. Thin provisioned disks have the lowest performance of the three disk types.
|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.|
Differences between the virtual disk types in vSphere:
|Disk type||Creation time||Block allocation||Zeroing|
|Thick Provision Lazy Zeroed||fast||fully preallocated||occurs when each block is first written to|
|Thick Provision Eager Zeroed||slow||fully preallocated||occurs when the disk is created|
|Thin Provision||fastest||on demand||occurs when block are allocated|
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