Information Technology Reference
In-Depth Information
Thick Provisioned Eager Zeroed
Thick provisioned eager zeroed virtual disks, also referred
to as eagerly zeroed disks or eagerzeroedthick disks, are truly thick. In this format, the size of
the VDMK i le on the datastore is the size of the virtual disk that you create, and within the i le,
it is pre-zeroed, as illustrated at the bottom of Figure 6.53. 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, and it contains 100 GB of data and 400 GB of zeros on disk. As I/O occurs in the
guest, the VMkernel does not need to zero the blocks prior to the I/O occurring. This results
in slightly improved I/O latency and fewer backend storage I/O operations during initial I/O
operations to new allocations in the guest OS, but it results in signii cantly more backend stor-
age I/O operation up front during the creation of the VM. If the array supports VAAI, vSphere
can ofl oad the up-front task of zeroing all the blocks and reduce the initial I/O and time
requirements.
This third type of virtual disk occupies more space than the i rst two, but it is required if you
are going to use vSphere FT. (If they are thin-provisioned or l at virtual disks, conversion
occurs automatically when the vSphere FT feature is enabled.)
As you'll see in Chapter 12 when we discuss Storage vMotion, you can convert between these
virtual disk types using Storage vMotion.
Aligning Virtual Disks
Do you need to align the virtual disks? h e answer is it depends on the guest operating system.
Although not absolutely mandatory, it's recommended that you follow VMware's recommended
best practices for aligning the volumes of guest OSes—and do so across all vendor platforms and
all storage types. h ese are the same as the very mature standard techniques for aligning the parti-
tions in standard physical confi gurations from most storage vendors.
Why do this? Aligning a partition aligns the I/O along the underlying RAID stripes of the array,
which is particularly important in Windows environments (Windows Server from 2008 onward
automatically aligns partitions). h is alignment step minimizes the extra I/Os by aligning the I/Os
with the array RAID stripe boundaries. Extra I/O work is generated when the I/Os cross the stripe
boundary with all RAID schemes as opposed to a full stripe write. Aligning the partition provides
a more e cient use of what is usually the most constrained storage array resource—IOPS. If you
align a template and then deploy from a template, you maintain the correct alignment.
Why is it important to do this across vendors and across protocols? Changing the alignment of
the guest OS partition is a di cult operation once data has been put in the partition—so it is best
done up front when creating a VM or when creating a template.
Some of these types of virtual disks are supported in certain environments and others are
not. VMFS datastores support all three types of virtual disks (thin, l at, and thick), but NFS
datastores support only thin unless the NFS server supports the VAAIv2 NAS extensions and
vSphere has been coni gured with the vendor-supplied plug-in. Figure 6.54 shows the screen for
creating a new virtual disk for a VM (a procedure we'll describe in full detail in Chapter 9) on a
