01-03-2013, 11:21 AM
Storage Area Network Architectures
Storage Area Network.pdf (Size: 388.19 KB / Downloads: 660)
Abstract
This PMC-Sierra white paper provides an architectural overview of storage technology, SAN
switches and Storage Elements. The systems discussed are partitioned into the functional building
blocks needed to address individual functional requirements of the systems. Finally the paper
explores the technology trends in the SAN industry and their evolution with the changing trend in
IC technology and SAN protocols.
Introduction
This white paper provides an overview of key technologies that have evolved around data storage
and storage networking. The paper focuses on analyzing the system architectures of the different
building blocks of storage networks.
In recent years, enterprise data storage has seen explosive growth in demand from users. This
growth is driven by increasingly more sophisticated applications that generate more rich and
numerous quantities of content data, and an increasingly larger number of users/consumers of this
rich content data. The rapid advancement of networking technology both in the LAN and the
WAN has enable new applications that generate large demands on data storage. The rapid growth
of information content is fueled by a profound change in the underlying infrastructure of
computer networks that facilitates acquisition, processing, exchange and delivery of information
among the processing units and the users.
Storage Models
The following discussion examines three categories of data storage technologies including Direct
Attached Storage (DAS), NAS (Network Attached Storage), and SAN (Storage Area Networks).
Out of this comparison will appear the benefit of sharing storage resources over the network, and
how different schemes can be used to accomplish the task of sharing storage resources.
Direct Attached Storage (DAS)
Direct attached storage is the simplest and most commonly used storage model found in most
standalone PCs, workstations and servers. A typical DAS configuration consists of a computer
that is directly connected to one or several hard disk drives (HDDs) or disk arrays. Standard buses
are used between the HDDs and the computers, such as SCSI, ATA, Serial-ATA (SATA), or Fibre
Channel (FC). Some of the bus cabling definitions allow for multiple HDDs to be daisy chained
together on each host bus adapter (HBA), host channel adapter, or integrated interface controller
on the host computer.
Network Attached Storage (NAS)
After seeing the consequences of binding storage to individual computers in the DAS model, the
benefits of sharing storage resources over the network become obvious. NAS and SAN are two
ways of sharing storage over the network. NAS is generally referred to as storage that is directly
attached to a computer network (LAN) through network file system protocols such as NFS and
CIFS.
The difference between NAS and SAN is that NAS does “file-level I/O” while SAN does “blocklevel
I/O” over the network. For practical reasons, the distinction between block level access and
file level access is of little importance and can be easily dismissed as implementation details.
Storage Area Network (SAN)
SAN provides block-orient I/O between the computer systems and the target disk systems. The
SAN may use Fibre Channel or Ethernet (iSCSI) to provide connectivity between hosts and
storage. In either case, the storage is physically decoupled from the hosts. The storage devices
and the hosts now become peers attached to a common SAN fabric that provides high bandwidth,
longer reach distance, the ability to share resources, enhanced availability, and other benefits of
consolidated storage.
Figure 5 gives an example of a typical SAN network. The SAN is often built on a dedicated
network fabric that is separated from the LAN network to ensure the latency-sensitive block I/O
SAN traffic does not interfere with the traffic on the LAN network. This examples shows an
dedicated SAN network connecting multiple application servers, database servers, NAS filers on
one side, and a number of disk systems and tape drive system on the other. The servers and the
storage devices are connected together by the SAN as peers. The SAN fabric ensures a highly
reliable, low latency delivery of traffic among the peers.
Storage Network Elements
A storage network generally consists of a variety of hardware and software elements. As the
scope of this paper is focus on hardware architectures, the software elements that mostly involved
with management functions are not discussed in detail. The hardware elements can be divided
into three categories: Host systems, storage systems, and switches and bridges that provide the
connectivity.
The following sections discuss these major building blocks of storage networks and highlight the
hardware architectures of these building blocks.
Host Systems
The Host Systems of storage networking is any computer device that is attached to the storage
network to access the storage resources over the network. Typical host end systems include
personal computers, workstations, a variety of servers, or other network appliances that have
access to the storage resources.
Storage Systems
The basic function of storage systems is to provide storage resources to the network for primary
data store, mirror data store, or backup data store. A wide variety of storage systems are available
in the marketplace and serve different purposes with varied performance and features. The
storage devices include RAID disk arrays, Just a Bunch Of Disks (JBODs), tape systems,
Network Attached Storage Systems, optical storage systems etc. The type of interfaces provided
on these devices include SCSI, Fibre Channel, Ethernet.
In order to understand the storage systems, it is important to understand the disk drives, the most
common building block of system systems, and the disk drive interface technologies used. The
next few sections will discuss the key characteristics of disk drives and internal architecture of
various storage systems.
Storage Systems – Various Front-end Architectures
The previous discussion focussed on the various disk drive technologies, JBOD technologies, and
RAID technologies. These technologies are the common building block of the back-end storage
arrays that provides the storage resources to various storage systems. The back-end storage
resources are presented to the outside world via the front-end interface of the storage systems. To
the user, what differentiate the various storage systems are the front-end interfaces and the
protocol processing associated with the front-end network interfaces.
Storage Switches
Storage switches are basic building blocks to storage network topology. Each switch consists of
several ports interconnected by a switch construct. The basic function of a storage switch is to
forward frames among the ports according to the protocol supported by the storage network. In a
Fibre Channel Network, the switch obeys the rules specified by the FC-SW standard to handle the
frames. In an iSCSI storage network, the switching is typically done at the Ethernet layer
according to the Ethernet bridging standards, such as 802.1D or the IP layer. Although the
specific protocols are different, the various types of storage switches share the common basic
structure. However, because of the difference in network protocols, different types of switches are
still required: Fibre Channel switches for FC SANs, the Ethernet switches or IP routers for iSCSI
network. New multiservice switches are being developed to help the convergence of
heterogeneous storage networks running multiple network protocols