Business Continuity
NFS offers virtualization benefits
New appliances provide an alternative to
typical Fibre Channel and iSCSI approaches.
by Gary Orenstein
Running virtual machines on network file
systems (NFS) provides a number of
architectural advantages, starting with the
fact that virtual machines use files to
store their image information. Since
network-attached storage (NAS) systems are
built from the ground up for file
management, the administrative time and
resources to oversee a large number of
virtual machine files is less than what is
required to manually assign and place those
files on individual logical unit numbers
(LUNs).
New offerings, such as scalable caching
appliances, now allow customers to extract
more performance out of their existing NAS
and NFS infrastructure. In fact, there are
environments where NFS can perform as fast,
if not faster, than other storage solutions
based on Fibre Channel or iSCSI. By
deploying NAS and NFS storage solutions,
administrators can forgo many of the
complexities associated with maintaining
separate Fibre Channel adapters, switches
and storage systems.
The first aspect of improving performance with NFS is the ability to remove the need for a host-based file system.
The necessity of Fibre Channel adapters
is also one of the biggest problems in
virtual environments, since rack-mounted
servers often have a limited number of slots
available for adapter cards. An Ethernet and
IP-based solution avoids excessive adapters
and can make effective use of embedded
Gigabit Ethernet ports on the mother- board.
When provisioning virtual disks with NFS,
new data stores can be added quickly by
mounting a file system. There is no need to
create or provision LUNs, as is the case
with Fibre Channel or iSCSI data stores. In
addition, some NFS systems allow for
expanding and decreasing NFS volumes on the
fly, and incorporate thin provisioning
features by default, saving disk space.
Users also can provision new virtual disks
from an NFS snapshot, allowing rapid
deployment of new virtual disks from a
copy/snapshot of an existing virtual disk.
Managing virtual disks is typically
easier with the file-management tools of NAS
systems compared to manual handling of files
within a LUN. Managing data stores between
virtual servers is also simpler with NFS.
With Fibre Channel or iSCSI, sharing data
stores requires manually matching LUN IDs.
Protecting and recovering virtual disks
can become a significant task for
administrators. With a limited number of
virtual machines, managing snapshots with
host-based tools is practical and
reasonable, but as the number of virtual
machines increases, the need for more
powerful tools escalates. Typically, the
most powerful snapshot tools are based at
the storage subsystem because the snapshot
processing is close to the data and does not
require traversing the network.
Because NFS systems are designed
specifically for file-centric environments,
they can be useful for file-level
snapshot-and-restore capabilities. For
example, they typically include the option
to handle single file restore compared to
volume-level restore capabilities of
block-based approaches. The latter requires
significantly more overhead in terms of time
and storage resources for file-level
recovery.
The first aspect of improving performance
with NFS is the ability to remove the need
for a host-based file system. Removing the
host-based file system container and
offloading that to the storage system clears
the bottlenecks of a host-based file system.
This sets the stage for separating functions
between the host and storage system, such as
snapshots.
Gary Orenstein
Another critical aspect of performance
with virtualization is the maximum number of
outstanding I/Os allowed for LUNs. With
block-based approaches, this maximum number
can artificially cap performance, because it
must be shared across all of the virtual
machines using the LUN. While the maximum
LUN queue depth varies by host bus adapter
and manufacturer, enabling the maximum
number of outstanding I/Os (e.g., 256) can
lead to considerable latency.
With NFS, there is no such I/O
limitation. Consequently, increasing the
bandwidth and connectivity between the
virtual machines and the data store will
continue to improve I/O performance.
Gary Orenstein is the vice president
of marketing at Gear6. He is also the author
of "IP Storage Networking: Straight to the
Core," a book that outlines the business
value of enterprise storage technology.
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