 |
 |
 |
 |
| Page 1 | |
solution shouldn’t
require IT technicians to undergo costly education to implement. On the
server and enterprise storage side, a storage solution must have RAID
capability for hardware failover protection.
There are three storage standards competing for the consumer and enterprise dollar:
PATA (parallel advanced technology attachment), SATA (serial advanced technology
attachment) and SCSI (small computer standard interface). PATA is approaching
its sunset as a technology; it’s been around far longer than most computer
standards are allowed to live, and although it’s been upgraded fairly consistently
for the past decade, it’s nearing the end of its useful life. That leaves
SCSI and SATA as the storage technologies of the future, and each offers a number
of advantages and disadvantages that adopters must take into account before their
purchase.
Drive Specifications
Nearly all current internal hard drives are 3.5 inches wide, able to fit
neatly in standard PC cases. Recently, 2.5 inch hard drives have begun
to become available. The smaller drives are ideal for rack-mount servers
and other cramped conditions, and they run cooler than their larger
predecessors.
Performance-minded consumers and businesses often want the fastest technology
available. There are three key ingredients that go into a speedy hard drive:
seek time, data transfer rate, and the drive’s data buffer.
Seek time indicates how long it takes the drive to locate data that the
system requests. It often depends on how fast the drive’s platters
spin. The lower the seek time is, the better. SATA drives typically spin
at 7200 rotations per minute (RPM) and feature seek times of around 9 milliseconds
(ms), while SCSI drives spin at 10,000 to 15,000 RPM with 4.5 ms and 3.3
ms seek times.
After the data is located, it needs to be ushered from the hard drive array
into system memory. Advertised data transfer rates, in megabytes per second
(MB/s) are burst rates, indicating the fastest a drive can possibly move
data if all conditions are perfect and the data are located sequentially
on the drive. SATA’s burst rate is 150 MB/s while SCSI’s current
iteration, Ultra320, bursts at 320 MB/s.
The drive’s buffer size indicated how much data can be stored in
logic on the drive’s controller. Data stored here are able to be
moved from the drive to system memory much faster than data on the drive’s
platters can be accessed. The system will often request data that comes
right after the data currently being transferred, so the drive logic will
buffer that data before it’s requested. When the system asks for
the data that’s buffered, the request is fulfilled much faster than
if the system asks for data that hasn’t been buffered. 2 MB buffers
are the norm, with 8 MB buffers becoming quite popular.
Both SATA and SCSI technologies have the capability to exist in RAID
configurations. RAID stands for redundant array of inexpensive disks,
and uses several drives to not only increase drive performance, but
also to create redundant failover. Data are “striped” (spread
across multiple disks) so that when one drive ceases to function, the
RAID array itself still works and all of the data are available from
the rest of the drives. The malfunctioning drive can be replaced with
no effect on the data itself.
RAID is a very important technology that every business customer should
be aware of. While it’s no substitute for daily backups, it does
create a safer environment for data; given enough time, hard drives have
a 100 percent failure rate, so every business server should be fitted with
a RAID system of some sort.
Engineered as a replacement for current parallel ATA technology, serial ATA flaunts good mainstream performance, affordability, and ease of use. SATA hard drives are priced similarly to PATA devices, and SATA technology is compatible with current PATA drivers, so to customers it’s basically a drop-in replacement for PATA technology.
Unlike SCSI and PATA topology, SATA uses a point-to-point architecture. That means drives aren’t daisy-chained together; each drive has its own separate connection to the host controller. Drive cables can be as long as 1 meter, so even customers with large tower cases don’t have to worry about cable length limitations.
drives don’t have jumpers to set and no termination is required. It’s truly a plug-and-play technology: simply install the drive, run the necessary data and power cables, and it’s ready to be formatted. The connectors themselves are much smaller than the PATA connectors IT professionals are used to: SATA connectors have only seven pins, compared to PATA’s forty. SATA cables are thin and promote good airflow through the system, a big plus for IT professionals who are in a constant battle against heat. SATA was designed for internal storage, so unlike SCSI it doesn’t provide connectivity to external devices. Furthermore, the only devices currently designed for SATA consumption are hard drives; there’s currently no such thing as a SATA CD-ROM drive. SATA can exist concurrently with PATA, however, so optical drives and other EIDE devices can hang out on the legacy bus.
SATA shatters the burst rate bottleneck currently witnessed in PATA technology. The SATA iteration has a burst rate of 150 MB/s The upcoming, backward-compatible SATA II spec has a burst rate of 300 MB/s, which puts it in performance competition with current SCSI technology.
Most recent motherboards come with SATA controllers on board. IT professionals wishing to add SATA to older systems can do so by purchasing a PCI SATA adapter.
SCSI
Costly and complex, SCSI can challenge even the most patient and methodical
IT engineer. However, the payoff is unparalleled speed. Current SCSI
technology packs a lofty burst rate of 320 MB/s.
Ultra320 SCSI is actually the seventh generation of the SCSI standard.
It uses a 16-bit data conduit (or bus) and supports up to 16 devices on
its chain. SCSI isn’t limited to hard drives; many devices can be
connected to a SCSI controller, including hard drives, optical drives,
tape drives, scanners, modems, and more.
A SCSI chain can extend from inside the computer case to outside the box.
As such, it can host both internal and external equipment, and can even
connect to specialized test equipment. SCSI devices are literally chained
together: the cable extends from one device to the next until the chain
reaches the end. Each end of the SCSI chain must be terminated, and each
device in a SCSI chain has to have its own unique ID. This makes for lots
of jumper and DIP switch settings and complicates implementation.
While SCSI is backward compatible, there have been so many revisions
of the SCSI standard that there’s no guarantee that one device
will work with the others. There is a myriad of connector types, making
finding the right cable to run from one device to the next a formidable
challenge.
SCSI is expensive compared to SATA. SCSI controllers rarely come on motherboards,
so PCs require a SCSI host adapter. SCSI drives cost significantly more than
PATA and SATA drives. Even the cables don’t come cheap: SCSI cables must
be purchased separately, and they can run from twenty to more than forty dollars.
At the time of this writing, 120 gigabyte SATA drives can be obtained for just
over $120, while a 73GB SCSI Ultra320 drive goes for about $370.
SCSI is ever-evolving, and a new standard is emerging. Called serial attached
SCSI (SAS), it’s a new point-to-point technology similar to SATA. It will
actually be compatible with SATA drives, so it will reduce SCSI costs significantly.
It will offer increased performance and the ability to host dozens of drives
for massive storage arrays. 
 |
Page | 1 |
Do not duplicate or redistribute in any form.