Database Reference
In-Depth Information
storage management of the virtual machines you are managing.
VAAI, or Storage Acceleration, allows an ESXi host to integrate nicely with a
compatible SAN. This integration centers around the offloading of certain functions to
the storage array instead of having them being managed by the hypervisor. If you are not
familiar with VAAI features and their capabilities, we highly recommend you read up
on them and work with your storage team and storage vendor to understand which
features are available in your storage subsystem.
With VAAI, features such as Full Copy, Atomic Test Set (ATS), and Write Same / Zero
all can have an impact on the VMDK provisioning type. The one we will focus on for
this section is the Write Same / Zeroing. ESXi hosts can be configured to enable the
WRITE_SAME_SCSI
command, which allows them to zero out a large number of disk
blocks without actually sending all this information to the array. It is important to
understand the impact of this setting with the array you are using. Storage arrays will
handle the
WRITE_SAME_SCSI
command differently, so work with your SAN team
and SAN vendor to understand
how
this setting will impact the storage subsystem and
ultimately the performance of your virtual machine.
Note
For more information, read the FAQ on VAAI
(
http://kb.vmware.com/kb/1021976
) as well as the “VMware vSphere
Storage APIs—Array Integration” white paper
API-Array-Integration.pdf
).
Now, let's move on to the provisioning types of a VMDK file. The first type we will
discuss is a Thin Provisioned VMDK. A Thin Provisioned VMDK is one that does not
immediately consume space on the storage in which it resides; however, the operating
system believes it has the full amount assigned when created. The storage consumed is
equal to the amount the virtual machine is actually using. Therefore, the size of the
VMDK will start small and grow over time, up to the size configured.
Thick Provisioned Lazy Zeroed disks are VMDK files that immediately consume the
VMFS space assigned to the virtual machine. The item to pay attention to with this disk
type is that when the hypervisor needs to write IO to the underlying storage, it will send
a zero first and then the data. This is only on the first write to a block; subsequent writes
to the same block do not incur this activity. This is commonly referred to as the First
Write Penalty. For general-purpose virtual machines, this is not a big deal because it is
washed out in the cache of the arrays. However, if you have an application such as a
database that is doing a large number of writes (to a database or log file), this could
have a performance impact. If your storage array supports the Write Same / Zero
