Directly Attached Storage
in Your InfrastructureServer-to-storage access, or directly attached storage, has been in use in much of
the history of computing, and still exists in over 90 percent of implementations
today. An example of server-to-storage access, as shown in Figure 5.1, could be a
workstation that has an internal hard drive, or a networked server that has an
external disk array directly attached to it.
In these network implementations, storage devices are directly connected to a
server using either interfaces and/or bus architecture such as EIDE or SCSI. In
more recent implementations, it is common to find newer devices that use fiber
channel to directly attach to a server. Regardless of the method used to connect
these devices, they are all the same in architecture; a server or host is directly
connected to a storage device using a storage bus.
This is not a very flexible model with which to work. Given that some hosts
may require more storage space than others may, it is very difficult to move
capacity from one server to another.To do so, you would actually need to remove
hard drives from one storage array or device and install them in another device
when that device needs more space. Even with this solution, you may run out of
physical space in a storage array, and need to attach an additional array of disks.
All of this “upgrade” would require the reconfiguration of the storage device
and host systems, and would obviously become quite cumbersome and time consuming.
In addition to these drawbacks, performance is limited completely by the
directly attached server’s abilities and the central processing unit (CPU).
For instance, if a server is too busy doing calculations for other applications, it
will have to wait or free up valuable CPU clock cycles in order to read and write
from the storage device.This will impair its application and input/output (I/O)
performance significantly.This may be acceptable for someone’s personal computer,
but in a mission-critical, performance-impacted business environment, it
can prove to be a serious problem with severe consequences and limited options.
Network Attached Storage Solutions
Network attached storage (NAS) is one of the latest solutions to hit the streets.
When you hear someone talking about SAN, you usually hear “NAS” in the
same sentence.While they both provide methods for accessing data, and resolve
many file access issues when compared to traditional methods such as directly
attached storage, in practice they differ significantly.
A NAS is a device that provides server-to-server storage.What does this mean?
The answer is simple: It means that NAS is basically a massive array of disk storage
connected to a server that has been attached to a local area network (LAN) as
depicted in Figure 5.2. In fact, it is very simple, and means exactly what it states.
As an example, imagine a host accessing data on a NAS server.The actual
data is transmitted between these devices over their LAN interfaces, such as Fast
Ethernet, using a communications protocol such as Internet Protocol (IP) or
Internet Packet eXchange (IPX).
With the existing network infrastructure, the communications protocol might
also allow data to be transmitted between hosts that are extremely far apart. For
instance, a personal computer might access data on a file server that is thousands
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of miles away by using the existing network infrastructure of the Internet, or a
customer computer might mount a drive on a remote server over a private wide
area network (WAN) connection such as a T1. In both of these cases, the server
being accessed is, for all intents and purposes, acting as NAS.
This can provide a great solution for applications, and will more than likely
be the method most of your customers will use to connect to data that resides on
your systems. It offers quite a lot of flexibility and requires very few upgrades to
your network infrastructure.We already discussed the best benefit of this type of
architecture, but it bears repeating it here: you can use your existing network
infrastructure for accessing data that resides on NAS servers.
There can be some serious drawbacks that are inherent to this solution,
though. Probably the most important is the impact that such an architecture will
have on your LAN and WAN.When we talk about sharing data, we might mean
terabytes of data. Using a NAS device can easily bottleneck your network and
seriously impact some of the other applications within your network.
I do not want to scare you away from this architecture, because it is still a
very viable and robust solution. In fact, when connecting hosts or servers to data
over very long distances, it is still a very good solution, and sometimes the only
option available. Many of your customers will more than likely already have an
existing connection into your network, so it becomes easy to add services with
very little impact on your other clients. Some methods can be used to help eliminate
the impact that a cluster of SAN devices might impose on your network.
Quality of Service
You can combat network performance problems by designing Quality of Service
(QoS) into your network. In fact, we recommend using QoS throughout your
network, even if you decide not to use NAS.QoS has the ability to delegate priority
to the packets traversing your network, forcing data with a lower priority to
be queued in times of heavy use, and allowing for data with a higher priority to
still be transmitted.
A well-designed and implemented QoS schema can definitely help eliminate
the impact that large volumes of data may have on other time-sensitive data, but
it could still expose your network to a level of latency that is capable of growing
exponentially.This is especially true if you do not plan correctly.When designing
QoS in your network, it is very important to look at all the data traversing your
network, and carefully weigh the advantages and disadvantages of using a particular
QoS strategy and its effect on types of data and the network as a whole.
Location of NAS in Your Network
When designing NAS in your network, probably the most effective solution
for latency and saturation issues is the location of your NAS servers in relation to
the hosts and systems that access their data.The placement of NAS devices
becomes extremely important, and performance can vary significantly depending
on your design.
For instance, if you have a single large cluster of NAS devices in the middle
of your network, all hosts will need to traverse deep into your network in order
to access the servers and data. Consequently, you will have large amounts of data
flooding every part of your network that will more than likely create serious bottlenecks
and latency issues at every step along the way.
In contrast, if you were to use smaller clusters of SAN devices, and locate
these groupings close to the hosts that access them, the hosts will not need to traverse
your network to access the NAS servers, thereby keeping network saturation
to a minimum.
Unfortunately, there is no clear and concise way to design NAS in your network.
Your ultimate design will depend greatly on your current and future
growth patterns. As a general rule, remember that NAS devices should always be
kept as close as possible to the devices that access them. However, always keep
their purpose in mind, as well as who will be accessing the data, patterns of
usage, and the costs associated with distributing these systems.
In some cases, you may have very few clients accessing the data, or saturation
may prove to be the downfall of your network or a nonissue. However, when
comparing price versus performance issues, try to keep your projected future
growth in mind, as it can significantly alter the decision-making process.
Storage Area Networks
A storage area network (SAN) is a networked storage infrastructure that interconnects
storage devices with associated servers. It is currently the most cutting-edge
storage technology available, and provides direct and indirect connections to multiple
servers and multiple storage devices simultaneously.
With the use of technologies such as Fiber Channel, the SAN actually
extends the storage bus, thereby allowing you to place servers far away from the
storage devices that they access. In fact, the servers may be housed at locations
that are completely separate from the site housing the storage. In this situation,
we would be taking advantage of one of the greatest features that SAN technology
provides.
A SAN can be thought of as a simple network that builds off the familiar
LAN design. Instead of connecting hosts with other hosts and servers, it is
designed to connect servers and hosts with a wide range of storage devices.A
SAN uses network hardware that is very similar to what can be found in a typical
LAN, and even includes the use of hubs (very rarely), switches, and routers. In its
most basic form, it could be thought of as a LAN that is dedicated solely to
accessing and manipulating data.
The Need for SAN
There are several scenarios behind the move to storage area networks.The major
one is the need to manage the dramatically increasing volume of business data,
and to mitigate its effect on network performance.The key factors include:
E-business Securely transforms internal business processes and improves
business relationships to facilitate the buying and selling of goods, services,
and information through the Internet.
Globalization The extension of information technology (IT) systems
across international boundaries.
Zero latency The need to exchange information immediately so you
can maintain a competitive advantage.
Transformation The ability to adapt, while maintaining the ability to
immediately access and process information that drives successful business
decisions.
Distributed computing, client/server applications, and open systems give
today’s enterprises the power to fully integrate hardware and software from different
vendors to create systems tailored to their specific needs.These systems can
be fast, efficient, and capable of providing a competitive edge.
Unfortunately, many enterprises have taken a far less proactive approach with
their storage systems. Storage, unlike a Web application server or a database
system, is rarely viewed as a strategic tool for the enterprise; this view, however, is
beginning to change.
With the explosive growth of e-business, IT managers are working very hard to
keep pace with managing the significant growth of data (multiple Terabytes, if not
Exabytes, per year).They are installing high-performance storage systems to meet
the demands for smaller backup windows and greater application availability.
However, these systems are sometimes much more complex and expensive to
manage. In addition, they are often single platform, restricting access to data across
the network.To improve data access and reduce costs, IT managers are now seeking
innovative ways to simplify storage management, and SAN is a promising solution.
Benefits of SAN
SANs remove data traffic—backup processes, for example—from the production
network, giving IT managers a strategic way to improve system performance and
application availability. Storage area networks improve data access. Using Fiber
Channel connections, SANs provide the high-speed network communications
and distance needed by remote workstations and servers to easily access shared
data storage pools.
IT managers can more easily centralize management of their storage systems
and consolidate backups, increasing overall system efficiency.The increased distances
provided by Fiber Channel technology make it easier to deploy remote
disaster recovery sites. Fiber Channel and switched fabric technology can help
eliminate single points of failure on the network.
With a SAN, virtually unlimited expansion is possible with hubs (again, very
rarely) and switches. Nodes can be removed or added with minimal disruption to
the network. By implementing a SAN to support your business, you can realize:
Improved administration Consolidation and centralized management
and control can result in cost savings. By allowing for any-to-any connectivity,
advanced load-balancing systems and storage management
infrastructures, you can significantly improve resource utilization.
Improved availability With a SAN, high availability can be provided
more effectively at lower cost.
Increased business flexibility Data sharing is increased, while the
need to transform data across multiple platforms is reduced.
One of the main advantages of owning and operating a SAN is that it offers a
secondary path for file transfers, while keeping the LAN free for other types of
data communication. Figure 5.3 shows that the SAN is a separate network from
the LAN, and truly provides a secondary path for file transfers.
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