vendors tend to create proprietary standards in a bid to gain
turf from competitors. And a gamut of available interfaces
which evolve rapidly add to confusion among users. An overview
of standards and interfaces may diffuse your worries. by Soutiman
vendors and standards bodies battle for turf, its important
that you select the one best fit for your enterprise
standards, in an effort to 'standardize' have usually managed
to confuse the manufacturers and users of the technology.
standards and interfaces are no exception. The 'standards
folly' of the various available and emerging storage technology
have resulted in some vendors pushing proprietary standards
based gear into the market and others following whichever
standard is popular at that time while screaming "I'm
Some companies have deployed a mix of storage standards, some
follow a single standard and are not happy with it, and some
have no clue about which one to follow. In case you are any
of the above, the standards and interfaces explanations
below may help clear some of the fog surrounding standardization
EMC, HP, and other vendors formed an assemblage of storage
vendors called the Fiber Alliance who proposed to implement
SANs (Storage Area Networks) as FC (Fiber Channel)-based networks.
Elsewhere, an association called SNIA (Storage Network Industry
Association) planned a proposal of its own. The Fiber Alliance
was not interested in deviating from its stand on FC for SANs.
In course of time, SNIA emerged as a more popular body for
InfiniBand, another inter-processor communication standard
developed by companies like IBM, Sun, Compaq and Dell is also
in the market hoping to play a significant role. InfiniBand's
aim is to overcome any bottleneck in systems design by increasing
the speed with which data can move in and out of the processor.
various standards committees
drive the adoption of interface protocols to allow any peripheral
device that follows a particular standard to be used interchangeably.
Here are some of the popular ones.
(Integrated Drive Electronics), also known as ATA (Advanced
Technology Attachment) or Ultra DMA is generally the least
expensive hard drive interface. Many computer motherboards
include ATA controllers and cable connectors that typically
control the C-drive that contains the OS. ATA is a slightly
slower drive interface, but is easier to implement and is
priced low. This makes it a popular choice for
desktop PCs and low-end RAID
The original ATA interface was 16-bits wide and supported
two hard drives at a maximum transfer rate of 8.3 MBps. ATA-2
boosted maximum throughput to 16.6 MBps for a maximum of two
devices. From there, ATA designations blur as companies create
names like Fast ATA or Extended IDE (EIDE) to describe proprietary
feature additions to the ATA-2 standard. Accordingly, these
designations are more marketing terms than official standards.
ATA/100 disks which are now available, offer data transfer
rates of 100 MBps and improves reliability using the cable
plant and CRC (Cyclic Redundancy Check) introduced with Ultra
Serial ATA, a new offering has lower signaling voltages and
lesser pin count than the earlier parallel ATA. It will be
faster, more robust, and have a much smaller cable. Serial
ATA will also be completely software compatible with the earlier
(Small Computer System Interface) is widely used in mid- to
high performance servers and storage devices and offers faster
transfer rates than ATA/IDE.
SCSI generally offers faster throughput and uses less CPU
horsepower during operation. This makes it more efficient
in demanding multiple initiator applications for multi-users
and uses. This is significant because it allows the processor
to run more efficiently by making it perform more commands
at the same time. SCSI can support up to 16 devices on a single
bus and IDE offers only two.
The first SCSI standard, now known as SCSI-1, was adopted
in 1986, and was originally designed to accommodate up to
eight devices at speeds of 5 MBps. Since then, SCSI has been
refined and extended numerous times, with the introduction
of Fast SCSI (SCSI-2) at 10 MBps, Fast Wide SCSI (SCSI-2)
at 20 MBps, and Ultra SCSI
(SCSI-3 or Fast-20), which provides data transfer rates of
up to 40 MBps.
FC standard is designed to provide high-speed data transfers
between networked devices. It makes use of a circuit/packet
switched topology capable of providing multiple simultaneous
point-to-point connections between devices. It provides powerful
networking capabilities which allow switches and hubs to enable
the interconnection of systems and
storage into tightly-knit clusters.
These clusters can provide high levels of perfor-mance for
file service, database management, and general purpose computing.
FC is able to span distances up to 10 Km between nodes and
allows very high speed movement of data between systems (up
to 4 GBps) that are greatly separated from one another.
It can be deployed in point-to-point, switched topologies,
or arbitrated loops (FC-AL). FC nodes log in with each other
and the switch to exchange operating information on attributes
Point-to-point is the simplest topology connecting two FC
devices that communicate at full bandwidth. A switch fabric
is a very flexible topology which enables all servers and
storage devices to communicate with each other. It also provides
a failover architecture in the event a server or disk array
ceases to operate.
(Fiber Channel-Arbitrated Loop) is an enhancement to the FC
standard that supports copper media and loops containing up
to 126 devices, or nodes. Like SSA, FC-AL loops are hot-pluggable
and tolerant of failures.
The FC-AL interface is robust enough to allow multiple devices
to be removed from the loop at one time with no interruption
in data transfer. In addition, the interface attaches sophisticated
detecting codes to each packet of user data. These codes are
checked at the receiver's end, which requests a re-send if
there is any discrepancy.
storage refers to a group of technologies that allow block-level
storage data to be transmitted over an IP-based network. The
IP storage protocols provide the means to encapsulate these
block-level requests for transmission over the IP network
using TCP (Transmission Control Protocol). This allows the
direct block-level requests used by SANs to take place over
an IP-based network.
The IETF (Internet Engineering Task Force) is currently working
three IP storage encapsu-lation
protocols. They are iSCSI (Internet SCSI), FC/IP (FC Over
TCP/IP), and iFCP (Internet FC
iSCSI is an emerging standard that defines the encapsulation
of SCSI packets in TCP which are then routed using IP. This
development allows block-level storage data to be transported
over widely used IP networks. It enables data access from
anywhere and effectively eliminates the physical boundaries
of the storage network.
iSCSI enables block level data to be accessed over a standard
Ethernet/IP network whether it resides on a direct attached
SCSI-based device or a FC SAN. With iSCSI, enterprises and
SSPs (Storage Service Providers) can build global storage
networks and manage them from a central location using existing
IP network infrastructures.
FC/IP transports FC frames over an IP infrastructure. It provides
mechanisms to allow islands of FC SANs to be interconnected
over IP-based networks to form a single unified FC SAN fabric.
The extended FC SAN fabric continues to use standard FC addressing.
Essentially, IP tunnels are set up between FC/IP end points.
Once these tunnels are in place, FC devices view these extended
links as standard FC links and use FC addressing.
iFCP encapsulates FC frames to be sent over the IP infrastructure
just like FC/IP. Because of this, the IETF chose to specify
a common FC encapsulation format. The main difference between
the two protocols lies in their addressing schemes. The FC/IP
protocol establishes point-to-point tunnels that can be used
to connect two FC SANs with Ethernet and create a single,
(Serial Storage Architecture) is a high-speed serial interface
designed to connect data storage devices and other networked
devices. SSA was developed and
promoted as an industry standard by IBM.
Although the basic transfer rate through a SSA port is only
20 MBps, SSA is dual-ported and full-duplex to allow a maximum
aggregate transfer speed of 80 MBps. SSA connections are carried
over thin, shielded, four-wire (two differential pairs) cables.
These are less expensive and more flexible than typical 50-
and 68-conductor SCSI cables.
SSA networks can be constructed using string, loop, or switched
topologies. SSA loops can contain up
to 126 devices. Devices are hot-pluggable,
and some measure of
fault tolerance exists due to the
redundant connectivity inherent
in a loop topology. The devices can also be separated by much
larger distances than possible on a SCSI bus.
While vendors and standards bodies battle for turf, its important
that you select the one best fit for your enterprise and make
the vendor promise to support the technology in future.
Das Gupta can be reached at firstname.lastname@example.org