Virtualization on the server level can give administrators an exponential increase in hardware agility, flexibility and efficiency. To achieve this successfully, the surrounding architecture needs to share all of these traits. Knowledge Center contributor David Hubbard explains how sophisticated, virtualized, IP-based storage is the best way to achieve this.
Virtualization
is billed as the silver bullet for a host of business IT challenges. It
seems to offer all the answers, from increased application availability
and simplified disaster recovery, to reduced infrastructure and
cost. It also seems to offer simplified IT administration and even the
solution for greener computing.
However, in order to get the most out of server virtualization, it
is critical that other elements of infrastructure complement the
environment--especially storage. Otherwise, a lot can go wrong.
Applications can unexpectedly slow to a crawl. What is billed as a
cost-lowering computing alternative can require significant investment
to achieve full functionality. And using virtualization to improve
application and server uptime can suddenly reveal painful weaknesses in
other areas of your IT infrastructure. Lets look at two of the most
common pitfalls when it comes to virtualization.
Pitfall #1: Choosing the incorrect storage platform
One of the major benefits of server virtualization is the ability to
move live guest applications between hypervisors on different machines.
Whether this is done for scheduling, load balancing or disaster
recovery, hardware independence is usually one of the major drivers
behind any virtualization implementation. However, if your storage is
tied to specific server hardware, moving applications becomes a little
more complex, if not pointless.
Network-attached storage (NAS) is often used as a way to simplify
storage provisioning for virtualized servers. NAS volumes are quite
simple to implement and grow without getting the hypervisor
involved. Unfortunately, the use of NAS has its own weak
performance issues, and many applications (such as Microsoft Exchange)
do not work well at all with NAS. For these reasons, most
virtualization vendors will recommend Storage Area Network (SAN)
storage for anyone looking for more effective application performance.
Fibre Channel SANs
With Fibre Channel (FC) SANs, users not only need to justify the
increased cost of FC storage, switching and administration, but they
also need to invest in costly host bus adapters (HBAs) for each server
they connect to the SAN. Those companies with an existing FC SAN are
not necessarily in the clear. To capture the major benefits of server
virtualization, the complete FC infrastructure (including switches and
HBAs) needs to support NPIV (N_Port ID Virtualization)--which excludes
a large proportion of existing products.
Even with NPIV, VMware can only transfer guests between machines within
a single FC zone. This means that, while the user has achieved hardware
independence on the server side, all physical servers in a group
capable of transferring guest applications between one another are
dependent on a single FC zone (usually a single array or even one disk)
for storage. Hardware independence on the server side can result in a
dangerous multi-application hardware dependence on the storage side.
Optimal storage solutions for a virtualized environment
The Internet Small Computer System Interface (iSCSI) or IP SAN offers
the best storage solution in a virtualized server environment, not only
from the obvious cost advantage, but also in terms of the availability,
agility and scalability of the virtual architecture. An iSCSI SAN
storage system also offers significant advantages for companies using
virtualization for wide area disaster recovery. Snapshots can be used
at the storage level to duplicate data to a local or remote secondary
site.
Additionally, an iSCSI SAN storage system has a significant WAN
advantage over a FC SAN storage system. FC storage wide area
replication requires the purchase of costly Fibre Channel over IP
(FCIP) gateways. Wide area replication for iSCSI SAN storage
requires no additional systems to acquire, implement, operate and
manage. The iSCSI is a TCP/IP protocol that works natively across a
WAN. Both FC and iSCSI WAN replication are subject to throughput
droop over distance or from packet loss. This can be mitigated by
WAN or TCP/IP optimizers for iSCSI SAN storage. These same WAN or
TCP/IP optimizers have little to no effect on FCIP gateways.
Pitfall #2: The oversubscription dilemma
Even with the correct SAN solution in place, applications moved to a
virtualized environment can sometimes slow to a crawl. If the server
hardware configuration is correct, this can leave administrators
baffled as to the cause. In such cases, storage is often the culprit.
Many of the infrastructure efficiency savings of virtualization are
achieved by using hypervisors to deliberately oversubscribe physical
resources. Virtual guest applications are assigned a suboptimal share
of physical resources, on the principle that all of the applications
are, statistically, unlikely to require resources all at the same time.
Used in proportion, the principle typically stands up in practice.
However, most SANs and SAN storage already use oversubscription, and
the result of dual-layered oversubscription of physical storage
resources can be disastrous.
With the storage infrastructure genuinely over-stretched, contention
becomes a problem, bottlenecks occur and buffers overflow. To make
matters more complicated for the administrator, these contention issues
can occur at multiple levels within the storage infrastructure.
At the individual disk level, the queues for input/output (I/O)
requests simply fill up. This problem is particularly acute with slower
Serial Advanced Technology Attachment (SATA) drives where the queue
depth is typically 0-32 requests against the 256-512 request capacity
found in Serial Attached SCSI (SAS) or FC disk. This means that
companies looking to implement a virtual infrastructure--who also want
the option to use lower-cost SATA drives for tiered storage--need a SAN
solution which does not restrict their choice of disk on the backend.
At the storage logical unit number (LUN) level, the hypervisor itself
typically carves a physical storage pool, or LUN, into multiple virtual
LUNs. These are then assigned to different virtualized guest
applications. The physical LUN cannot distinguish between these guest
applications and over-contention decreases the storage performance.
Similarly, oversubscription at the hypervisor level can also cause
problems at the SAN infrastructure level with HBAs, initiators, ports
and switches. These resources are often oversubscribed by a ratio of
8:1 or more by the SAN itself. The compound effect of this dual
oversubscription can go beyond a performance drop and actually lead to
request timeouts and application crashes.
Using virtualized SAN storage to contend with over-contention
One option is to switch off the storage virtualization function within
the hypervisor and manually ascribe LUNs to each guest application.
However, this is discouraged by many vendors and leads to a loss of key
virtualized functions.
Another option is to deal with the problem from the storage side and
reduce the levels of native oversubscription within the SAN
architecture. With a physical SAN, this is complex and will
dramatically reduce the efficiency of the SAN for non-virtualized
hosts. With virtualized SAN storage, this reconfiguration is not only
far simpler, but hypervisors can often be treated differently to
physical hosts to optimize the overall SAN efficiency.
Indeed, a virtualized SAN can also be used to spread individual LUNs
across multiple storage resources to alleviate the contention issues
yet further. Virtualized SAN storage provides SAN storage performance
with NAS simplicity.
David Hubbard is President and CEO at RELDATA, Inc.
He joined the company in February 2007 as COO and was promoted to CEO
in August 2007. David has an impressive track record of executive
leadership at storage and networking vendors. His career of over 25
years includes President at Radware (IP networking solutions),
Executive VP/GM at QLogic (SAN storage solutions), Senior VP of Sales
and Marketing at Inrange/Computerm (WAN storage solutions), Senior
Director of Marketing at Computer Network Technology, and various
executive roles at Digital Equipment Corporation. He can be reached at david.hubbard@reldata.com.