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For years the
parallel interface
has been widely used in
storage systems. The need for increased
bandwidth and flexibility in storage systems made the
SCSI and
ATA
standards an inefficient option. A parallel interface is a channel
capable of transferring date in parallel mode — that is transmitting
multiple bits simultaneously. Almost all personal computers come
with at least one parallel interface. Common parallel interfaces
include SCSI and ATA.
SCSI
(sku4zē) Short for small computer system interface, a parallel
interface standard used by Apple Macintosh computers, PCs and
many UNIX systems for attaching peripheral devices to computers.
Nearly all Apple Macintosh computers, excluding only the
earliest Macs and the recent iMac, come with a SCSI port for
attaching devices such as disk drives and printers. SCSI
interfaces provide for data transmission rates (up to 80
megabytes per second). In addition, you can attach multiple
devices to a single SCSI port, so that SCSI is really an I/O bus
rather than simply an interface.
ATA
(Also known as IDE) is a disk drive implementation that
integrates the controller on the disk drive itself. ATA is used
to connect hard disk drives, CD-ROM drives and similar
peripherals and supports 8/16-bit interface that transfer up to
8.3MB/s for ATA-2 and up to 100MB/s (ATA-6).
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Key terms to
Understanding Parallel and Serial Interfaces:
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So, what do parallel interfaces have to do
with SAS (Serial Attached SCSI) and SATA
(Serial ATA)? A lot, actually. It is the architectural limitations of the
parallel interfaces that serial technologies like SAS and SATA address. In
contrast to multiple parallel data stream, data is transmitted serially,
that is in a single steam, by wrapping multiple bits into packets and it is
able to move that single stream faster than parallel technology.
Serial Attached SCSI (SAS)
Abbreviated as SAS, Serial Attached SCSI, an evolution of parallel SCSI
into a point-to-point serial peripheral interface in which controllers
are linked directly to disk drives. SAS is a performance improvement
over traditional SCSI because SAS enables multiple devices (up to 128)
of different sizes and types to be connected simultaneously with thinner
and longer cables; its full-duplex signal transmission supports 3.0Gb/s.
In addition, SAS drives can be hot-plugged.
Serial ATA (SATA)
Often abbreviated as SATA, Serial ATA is an evolution of the Parallel
ATA physical storage interface. Serial ATA is a serial link — a single
cable with a minimum of four wires creates a point-to-point connection
between devices. Transfer rates for Serial ATA begin at 150MB/s.
Starting with SATA, it extends the
capabilities of ATA and offers transfer rates starting at 150MB/s
and, after years of development, has moved to the mainstream of disk
interfaces. The successor the SCSI interface is SAS at speeds of up to 3Gb/s. Additionally, it also addresses parallel interface issues such as drive
addressability and limitations on the number of device per port connection.
SAS devices can communicate with both SATA
and SCSI devices (the backplanes of SAS devices are identical to SATA
devices). A key difference between SCSI and SAS devices is the addition in
SAS devices of two data ports, each of which resides in a different SAS
domain. This enables complete failover redundancy. If one path fails,
there is still communication along a separate and independent path.
Cables & Connectors
Another big advantage of SATA over ATA is the cabling and connectors. The
serial interface reduces the amount of wires needed to transmit data, making
for much smaller cable size and making it easier to route and install SATA
devices. The IDE cables used in parallel ATA systems are bulkier than Serial
ATA cables and can only extend to 40cm long, while Serial ATA cables can
extend up to one meter. In addition to the cabling, a new design of connectors is also used
that reduces the amount of crosstalk between the wires, and the connector
design also provides easier routing and better air flow.
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StarTech's 26-inch
Internal SCSI Cable |
StarTech's
External SCSI4 Cable |
StarTech's Serial
ATA Drive Connection Cable |
StarTech's LP4 to
2 SATA Internal Power Splitter Cable |
Examples of SCSI and Serial
ATA cables from StarTech.com
The Benefits of SAS & SATA in Storage
Serial interfaces offer an improvement over older parallel SCSI (with a serial version)
in storage applications and environments.
These benefits include better performance, better scalability, and also better
reliability as the parallel interfaces are at their limits of speed with
reliable data transfers. SAS and SATA drives can also operate in the same
environment while SCSI and ATA cannot. For example, using faster SAS drives
for primary storage and offloading older data to cheaper SATA disks in the
same subsystem, something that could not be achieved with SCSI and ATA.
