> 1. Some hard disk manufactures are using same disk drive (physically) to
> make SCSI and ATA. They only change the PCB board.
Absolutely true; see Seagate Medalist drives.
> 2. SCSI-1 and SCSI-2 have 8 bit data bus, while ATA have 16 bit data bus.
Not necessarily. SCSI-2 provides for a 16 bit (Wide) and a 32 bit (rarely
used) data bus, and both busses operate at up to 10MHz synchronously (Fast
SCSI). Narrow Fast SCSI easily pushes 10MB/s over the bus, and Wide Fast
SCSI pushes 20MB/s. Double those values for Ultra SCSI. Compare that to ATA
which maxes out at 16.6MB/s (not necessarily sustainable), both PIO and DMA.
UDMA maxes out at 33MB/s burst. Slower than widely available Ultra Wide
SCSI.
> 3. ATA got disk cache built in as well.
Often the difference between the ATA and SCSI version of a drive (same HDA)
is the amount of buffer on the drive. The buffering can surely be used more
effectively by SCSI as ATA does not provide for disconnects, and hence has
much reduced (eliminated?) concurrency.
> 4. Some low end of SCSI host card is only 8 Bit card.
And I can pick up those HBAs for about US$10.00, new. Which, mind you, is
similar to the price of a similar ATA adapter. You'd also be hard-pressed to
find a high-end ATA controller that supports caching and RAID with up to 15
drives for any price. I can pick one up for approximately US$400.00.
Overall, the value of a SCSI storage solution in a PC is very high. A Ultra
Wide-PCI HBA that outperforms any UDMA interface can be had for approximately
US$90.00. Low-end SCSI drives now carry very little premium over their ATA
counterparts, sometimes only US$50-75. There is absoultely no comparison
between a high-end SCSI drive meant for servers and workstations to the large
but not-necessarily-speedy UDMA drives. Compare a 10.1GB IBM UltraDMA/33
drive, which can be had for approximately US$300.00, with a 9.1GB Seagate
Cheetah U2W drive which can be had for approximately US$700.00; the Cheetah
has twice the buffer, the IBM rotates at 7,200 RPM versus the Cheetah's
10,000 RPM, and the seek time on the Cheetah is a mere 7.5ms, versus 9.5 for
the IBM drive. And additionally, compare UDMA's 33MB/s burst transfer rate
with 80MB/s sustainable transfer rate with Wide Ultra2 SCSI. What you don't
see in the specifications is how the drives perform with many outstanding
requests, and long-term reliability. I administer and sell systems operating
with both drives, but I do believe you can guess which drive performs better
in the real world, and which is more reliable. And that's what you pay for.
In summary:
If you're using Win95/98 only, get UDMA.
If you're using WinNT, stop ;)
If you're using a multiuser/multitasking operating system, but you rarely
perform more than one task at a time, keep your system load low, or have few
users, get UDMA if and only if you cannot afford the SCSI premium.
If you load your system down, run multiple tasks that are even moderately
disk-intensive, or have multiple users (NFS, locally, etc.), get SCSI. Don't
even consider UDMA unless you're planning on using RAID-0, with each drive on
a discrete bus. I've seen the best UDMA drives in multiuser systems, and it
is not a pretty sight.
If you have or need many storage devices, get SCSI.
ATA/UDMA's main advantage is cost. The drives are cheap, and you likely
already have a controller in your system already. However you are limited to
2 drives per interface, and only one drive can be used at once per IF. The
interface itself is slower, but this normally does not matter as few disks
can even approach the transfer rate of the bus; it only matters with
transfers previously buffered.
SCSI's main advantages are scalability, flexibility and reliability; you can
get better SCSI drives that are not available in ATA versions, the bus itself
is (or can be) faster, and you can attach more drives to the bus. Many
(most) SCSI devices provide for disconnecting themselves from the bus,
allowing other SCSI transfers to take place while they're retrieving the
data. You can attach up to 7 devices to a narrow SCSI bus, and 15 to a Wide
SCSI bus. You can attach even more with SCSI-SCSI controllers (for example,
30 device RAID towers that appear as one device on the bus).
At the same price and quality levels, they're quite competitive. SCSI just
continues scaling where ATA levels off. And rightly so. ATA pales in
comparison to SCSI, just so long as you're disregarding price. In the
low-end, ATA is the better value.
--
Robert Minichino
Chief Engineer
Denarius Enterprises, Inc.
http://www.denarius.com/
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