Database Reference
In-Depth Information
We will first discuss Thick Provisioned LUNs and the three VMDK file types. If we
consider Thin Provisioned VMDK files on a Thick Provisioned LUN, then management
of the Thin Provisioning is handled by the vSphere administrator. As the virtual
machine's VMDK files grow and acquire additional space, if the LUN needs to grow,
the vSphere administrator can notify the SAN administrator to grow the LUN. Next,
Thick Provisioned Lazy / Eager Zeroed will acquire all its disk space during its
creation. This option has the least management overhead, given that the LUN was sized
properly to accommodate for overhead (vswp files, snapshot delta files, adding too
many virtual machines to a data store, and so on).
Next we will consider a Thin Provisioned LUN and the three VMDK file types.
Following the same order as the previous paragraph, let's consider a Thin Provisioned
VMDK file on a Thin Provisioned LUN. Can this be done? Sure. What are the
implications? The implications are management by both the vSphere administrator and
SAN administrator to ensure adequate space is available. Of the three possible VMDK
provisioning options on a Thin Provisioned LUN, this is the option that can run out of
space the quickest if the administrators are not watching disk utilization. Let's move on
to Thick Provisioned Lazy Zero VMDKs on Thin Provisioned LUNs: This option will
only consume SAN space that is actually used by the virtual machine. This shifts the
burden of management to the SAN administrators because they have to ensure there is
enough space available for the virtual machines. Finally, Thick Provisioned Eager
Zeroed, based on this VMDK file type's behavior, will consume all space assigned to it
when it is created, thereby nullifying any potential benefit of having a Thin Provisioned
LUN.
From a VAAI perspective, the primitive you want to pay particular attention to is the
UNMAP primitive. If you are presenting Thin Provisioned LUNs, there is a good chance
there is wasted disk space on this LUN. What do we mean by wasted disk space? Say
you delete a virtual machine: The space this virtual machine consumed is not reclaimed
and made available to the SAN, so it sits there, wasted. The UNMAP primitive's job is
to notify the storage array of blocks no longer being used. The UNMAP primitive was
introduced initially in vSphere 5.0, and vSphere 5.5 made the
UNMAP
command much
easier to use.
Tip
The
UNMAP
command must be manually initiated.
Note
For more information on using the
UNMAP
command, read
