Information Technology Reference
In-Depth Information
Virtual disks come in three formats:
Thin-Provisioned Disk
In this format, the size of the VDMK i le on the datastore is only as
much as is used (or was at some point used) within the VM itself. The top of Figure 6.53
illustrates this concept. For example, if you create a 500 GB virtual disk and place 100 GB of
data in it, the VMDK i le will be 100 GB in size. As I/O occurs in the guest, the VMkernel zeroes
out the space needed right before the guest I/O is committed and grows the VMDK i le
similarly. Sometimes, this is referred to as a
sparse
i le
. Note that space deleted from the guest
OS's i le system won't necessarily be released from the VMDK; if you added 50 GB of data but
then turned around and deleted 50 GB of data, the space wouldn't necessarily be released to
the hypervisor so that the VMDK can shrink in size. (Some guest OSes support the necessary
T10 SCSI commands to address this situation.)
Figure 6.53
A thin-provisioned
virtual disk uses
only as much as
the guest OS in the
VM uses. A fl at disk
doesn't pre-zero
unused space, so
an array with thin
provisioning would
show only 100 GB
used. A thickly
provisioned (eager
zeroed) virtual
disk consumes
500 GB immedi-
ately because it is
pre-zeroed.
100 GB VMDK file size
500 GB virtual disk
100 GB
actual usage
(footprint)
500 GB VMDK file size
400 GB
stranded (but not
pre-zeroed) storage
500 GB virtual disk
100 GB
application usage
500 GB VMDK file size
400 GB
stranded (and
pre-zeroed)
500 GB virtual disk
100 GB
application usage
Thick Provisioned Lazy Zeroed
In this format (sometimes referred to as a l at disk), the
size of the VDMK i le on the datastore is the size of the virtual disk that you create, but
within the i le, it is not pre-zeroed at the time of initial creation. For example, if you create
a 500 GB virtual disk and place 100 GB of data in it, the VMDK will appear to be 500 GB at
the datastore i le system, but it contains only 100 GB of data on disk. This is shown in center
of Figure 6.53. As I/O occurs in the guest, the VMkernel zeroes out the space needed right
before the guest I/O is committed, but the VDMK i le size does not grow (since it was already
500 GB).
