To understand how and when to use technologies such
as NAS or SAN, you need to understand what they are and what they offer.
The technologies differ in how they are used and what advantages they
provide. Many administrators assume that they need a SAN when often a
NAS can suffice. Because information technology (IT) budgets are far
from limitless, it is to your advantage to know that you aren’t
overbuying for your solution. By the same token, it is often less
expensive to buy your solution all at one time rather than trying to
expand it later.
What Is a SAN?
A SAN is a high-speed,
special-purpose network or subnetwork that connects various data storage
devices with associated data servers on behalf of a larger network of
users. Typically, a SAN is part of an overall network of computing
resources for an enterprise. A SAN is usually located in relative
proximity to other computing resources, such as databases and file
servers, but might also extend to remote locations for backup and
archival storage. These remote locations traditionally connect via wide
area network (WAN) carrier technologies, such as asynchronous transfer
mode (ATM) or Synchronous Optical Networks (SONETs).
It is important to
understand that the SAN is more than just the chassis that contains the
disks. It includes the redundant array of inexpensive or independent
disks or drives (RAID) controllers for the disks, the Fibre Channel
switching fabric, and the host bus adapters (HBAs) that reside in the
data servers. SANs are traditionally connected to hosts via Fibre
Channel and talk via Fibre Channel Protocol. Although it can be fairly
easy to support dual-arbitrated fiber loops in a corporate environment,
keep in mind that one of the primary benefits of SAN is the capability
to do block-level mirroring to another SAN. If this SAN is located
remotely, up to 1,000km away with current fiber technology, a company
needs to have fiber between the two locations. A fiber connection across
those kinds of distances can be quite expensive.
SAN
technologies excel in the area of disk performance. Fibre Channel
networks regularly push 4–8Gb/sec of throughput. Although SCSI
technologies can move data at up to 320Mb/sec and can be bonded together
for higher throughput, they are limited to less than 25 feet of
distance. SAN, not unlike SCSI, is seen by the host system as raw disk
space. This is also referred to as a block-level technology. In the
past, database applications required block-level access to the disk and
the “near 0 latency” offered by SAN.
Tip
Although most SAN
manufacturers refer to the performance of their products as having zero
latency, it is important not to misinterpret this. Zero latency refers
to the fact that Fibre Channel has extremely low overhead and doesn’t
add additional latency. The laws of physics, on the other hand, are
still in effect. A 1,000-km fiber run between remote locations still
takes 7 milliseconds round trip.
What Is NAS?
NAS is a hard disk storage
technology that uses an Ethernet connection rather than being attached
directly to the host computer that serves applications or data to a
network’s users. By removing storage access and its management from the
host server, both application programming and files can be served faster
because they do not compete for the same processor time. The NAS device
is attached to a local area network (LAN) via Ethernet and given an IP
address. File requests are mapped by the host server to the NAS device.
NAS consists of
hard disk storage, including multidisk RAID systems and software for
configuring and mapping file locations to the network-attached device.
NAS software can usually handle a number of network protocols, including
Microsoft’s Internetwork Packet Exchange, Common Internet File System,
and NetBEUI; Novell NetWare Internetwork Packet Exchange; and Sun
Microsystems Network File System. Configuration, including the setting
of user access priorities, is usually possible using a web browser
though many NAS offerings require command-line configuration. Most NAS
manufacturers include specialized software to allow specific
applications such as Structured Query Language (SQL) or Exchange Server
to take advantage of special functions provided by the NAS. These
functions include things such as mirroring, failover, automated
recovery, and snapshotting.
NAS has the advantage
of using existing Ethernet technologies that are much less expensive
than fiber technologies. With the availability of 10Gb Ethernet, NAS can
compete with Fibre Channel–based technologies even with the added
overhead of Ethernet over Fibre Channel. In most scenarios, Gigabit
Ethernet is sufficient for Exchange 2010 servers, especially if multiple
connections are employed.
Depending on the vendor
you work with, you might hear the terms SAN and NAS used somewhat
interchangeably when referring to the capability to support iSCSI.
Network card vendors have even gone to the point of referring to TCP
offloaded NICs as iSCSI HBAs.
