vSphere Storage - ESXi 5.1 - Documentation - VMware

Not all applications need to be on the highest-performance, most-available storage—at least not throughout
their entire life cycle.
NOTE If you need some of the functionality of the high tier, such as snapshots, but do not want to pay for it,
you might be able to achieve some of the high-performance characteristics in software. For example, you can
create snapshots in software.
When you decide where to place a virtual machine, ask yourself these questions:
n How critical is the virtual machine?
n What are its performance and availability requirements?
n What are its PiT restoration requirements?
n What are its backup requirements?
n What are its replication requirements?
A virtual machine might change tiers throughout its life cycle because of changes in criticality or changes in
technology that push higher-tier features to a lower tier. Criticality is relative and might change for a variety
of reasons, including changes in the organization, operational processes, regulatory requirements, disaster
planning, and so on.
Layered Applications
SAN administrators customarily use specialized array-based software for backup, disaster recovery, data
mining, forensics, and configuration testing.
Storage providers typically supply two types of advanced services for their LUNs: snapshotting and
replication.
n Snapshotting creates space with efficient copies of LUNs that share common blocks of data. In general,
snapshotting is used locally on the same storage systems as the primary LUN for quick backups,
application testing, forensics, or data mining.
n Replication creates full copies of LUNs. Replicas are usually made to separate storage systems, possibly
separate sites to protect against major outages that incapacitate or destroy an entire array or site.
When you use an ESXi system in conjunction with a SAN, you must decide whether array-based or host-based
tools are more suitable for your particular situation.
Array-Based (Third-Party) Solution
When you use an ESXi system in conjunction with a SAN, you must decide whether array-based tools are more
suitable for your particular situation.
When you consider an array-based solution, keep in mind the following points:
Chapter 2 Overview of Using ESXi with a SAN
n Array-based solutions usually result in more comprehensive statistics. With RDMs, data always takes the
same path, which results in easier performance management.
n Security is more transparent to the storage administrator when you use an RDM and an array-based
solution because with RDMs, virtual machines more closely resemble physical machines.
n If you use an array-based solution, physical compatibility RDMs are often used for the storage of virtual
machines. If you do not intend to use RDMs, check the storage vendor documentation to see if operations
on LUNs with VMFS volumes are supported. If you use array operations on VMFS LUNs, carefully read
the section on resignaturing.
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