Understanding Storage Terminology
DAS, SAN, NAS, NAS Gateways, Fibre Channel,
DAS = Direct Attached Storage
Direct Attached Storage (DAS), usually disk or tape, is directly
attached by a cable to the computer processor. The hard disk
drive inside a PC or a tape drive attached to a single server
are simple types of DAS. I/O requests, also called protocols
or commands, access devices directly. This is the simplest
form of storage to deploy, but severely lacks in terms of
reach, scalability, and performance in a networked environment.
SAN = Storage Area Network
In a Storage Area Network (SAN), storage devices reside on
a dedicated network separate from the LAN. Like DAS, I/O requests
access devices directly. Today, most SANs use Fibre Channel
media, providing an any-to-any connection for processors and
storage on that network. Ethernet media using an I/O protocol
called iSCSI is emerging in 2001.
SANs can provide better performance and more cost effective
storage management for companies with certain data access
requirements. A SAN provides a separate network within an
enterprise so that the main communications LAN(s) are not
used to pass data between devices within the SAN. This translates
into better performance.
An example of a typical SAN is the notion of a "fibre
cloud" whereby storage devices (disk drives, tape drives,
autoloaders, and libraries) and servers can pass data back
and forth seamlessly. Furthermore, the benefit of SAN is a
more efficient and thereby economical use of storage resources
by the various application servers. Storage consolidation
to a subsystem architecture (like ESS) within a SAN can save
an organization significant dollars as compared to adding
storage to separate application servers.
NAS = Network Attached Storage
With Networked Attached Storage (NAS), a storage device called
an appliance, which is usually an integrated processor plus
disk storage, is attached to a TCP/I P-based network (LAN
or WAN), and accessed using specialized file access/file sharing
protocols. File requests received by a NAS are translated
by the internal processor to device requests. With NAS appliances,
organizations do not need to invest in a SAN, but they also
do not realize the benefits of a SAN. However, NAS has benefits
of its own, usually in terms of lower costs and faster, simpler
A NAS gateway is a way to incorporate DAS with NAS, or SAN
and NAS. A NAS gateway is a NAS device without integrated
storage (i.e. just the NAS processor). Instead, the NAS device
connects externally to storage by direct attachment or by
a SAN. For companies that have departmental requirements that
would make it attractive to tap into a SAN but cannot cost
justify the acquisition costs to acquire and install fibre
adapters to networked servers, NAS gateways allow local or
remote attachment of desktop workstations and small servers
so as to have a cost effective access to a SAN.
iSCSI is an I/O protocol whereby storage is attached to a
TCP/IP-based network, and is accessed by block I/O SCSI commands.
iSCSI could be direct attached (DAS) or network attached via
a SAN. Key point: DAS and SAN are approaches to connectivity,
while NAS is really a type of device whereby either DAS or
SAN is deployed. iSCSI is a protocol, and in a SAN is an alternative
to Fibre Channel.
SANergy is software from IBM and Tivoli that provides NAS-like
file sharing, with data sent over the SAN rather than the
LAN for improved performance. (IBM NAS gateways also include
SANergy function.) Some in the industry are calling SANergy
and similar facilities SAFS - SAN Attached File Systems. SANergy
has attributes of NAS and SAN, with additional flexibility.
SANergy supports the NFS and CIFS protocols, but allows the
installation to use virtually any disk storage they want (Fibre
Channel, iSCSI, parallel SCSI, and SSA storage will all work.).
Which Approach Do I Choose?
Each type of approach has its strengths and shortcomings,
and which one is right for a given customer is based upon
a variety of factors. But why are there so many forms of storage
networking? For one, new technologies emerge and evolve, but
customers cannot justify replacing their investments in previous
technologies overnight. Furthermore, no single storage networking
approach solves all problems or optimizes all variables. There
are trade-offs in cost, ease of management, performance, distance,
maturity, and other variables. For the foreseeable future,
multiple storage network alternatives will coexist, often
within the same organization.
The benefits of the major types of processor-to-storage connectivity
can be briefly summarized as follows:
- DAS is optimized for single,
isolated processors and low initial cost
- SAN is optimized for performance and scalability.
Some of the major potential benefits include support for
high-speed Fibre Channel media which is optimized for storage
traffic, managing multiple disk and tape devices as a shared
pool with a single point of control, specialized backup
facilities that can reduce server and LAN utilization, and
wide industry support.
- NAS is optimized for ease-of-management
and file sharing using lower-cost Ethernet-based networks.
Installation is relatively quick,and storage capacity is
automatically assigned to users on demand.
- NAS gateways are optimized to provide NAS
benefits with more flexibility in selecting the disk storage
than offered by a conventional NAS device. Gateways can
also protect and enhance the value of installed disk systems.
- iSCSI, because it is still early in its
product evolution cycle, is likely best suited for situations
of relatively low I/O activity. In general, an iSCSI SAN
likely has a lower cost than a Fibre Channel SAN and can
be built more quickly and with fewer new skills than a Fibre
Channel SAN. An iSCSI disk device, all else equal, may be
lower cost than a NAS appliance since the iSCSI device does
not need to support file systems, file sharing protocols,
and other facilities often integrated into NAS products.
- SANergy is optimized for data sharing (like
a NAS), but at SAN speeds. Tivoli SANergy is disk vendor-independent,
and can be added to an existing SAN to enhance its value.