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| What
is SCSI? |
| SCSI stands for Small Computer System Interface.
It's a standard for connecting peripherals to your computer via a standard
hardware interface, which uses standard SCSI commands. The SCSI standard
can be divided into SCSI (SCSI1) and SCSI2 (SCSI wide and SCSI wide and
fast) and now SCSI-3 which is made up of at least 14 separate standards
documents.
SCSI2 is the most popular version of the SCSI command specification and
allows for scanners, hard disk drives, CD-ROM players, tapes [and many
other devices]. SCSI-3 resolves many long time "gray areas" and
adds much new functionality and performance improvements. It also adds new
types of SCSI busses like fibre channel which uses a 4 pin copper
connection or a pair of glass fibre optic cables instead of the familiar
ribbon cable connection.
In order to put together a PC with SCSI I/O you'll need:
- A SCSI host adapter (also called a SCSI controller by sales types)
- A SCSI cable (either WIDE, 68 pin, or narrow, 50 pin, external or
internal)
- A SCSI device of some sort (disk, tape, CD-ROM, CD-RW, DVD-ROM,
scanner)
The SCSI device will most likely have a built in terminator that can be
enabled or disabled. If it doesn't you'll also need a terminator (active
terminator perferably). You'll find that there is quite a variety of SCSI
cables out there. This is due to the fact that SCSI is so flexible. You
are not limited to one SCSI device of course, that's just a minimum.
- Host adapter
- Also called a Host Bus Adapter or HBA. The card that connects your
computer to the SCSI-bus. Usually called SCSI-controller by marketing
droids. An example would be a PCI SCSI host adapter like the Adaptec
2940UW.
- Terminators (passive)
- A group of resistors on the physical ends of a single ended SCSI-bus
(and only on these ends) that dampens reflected signals from the ends
of the bus. Each terminated signal is connected by:
- 220 Ohm to +5 volt (TERMPWR)
- 330 Ohm to ground.
- For NARROW SCSI the 18 signals that are terminated are:
I/O, Req, C/D, Sel, Msg, Rst, Ack, Bsy, Atn, DB(p), DB(7) ...
DB(0).
For WIDE SCSI there are 9 more signals; DB(p1), DB(8) ...
DB(15)
- Terminators (active)
- Rather than passive terminators that use TERMPWR which may not be
exactly +5v, active terminators use a voltage regulator. Basically it
is a set of 110 Ohm resistors from each signal to a 2.8 Volt regulated
Voltage source.
- Single ended
- "Normal" electrical signals. Uses open collector drivers
to drive the SCSI bus.
- [usually] survives wrong cable insertion.
- DIFFSENSE signal is used to detect connection of differential
devices and prevent damage.
- The max. length for SCSI-1 is a 6 meter cable with stubs of max 10cm
allowed to connect a device to the main cable. Most devices are single
ended.
-
- Differential (Now called High Voltage Differential to distinguish
it from LVD)
- Uses two wires to drive one signal.
- Max. cable length of 25 meters.
- Electrically incompatible with single ended devices!
- Much more expensive than single ended.
- Used from SCSI-1 upwards.
- Apple kludge
- The single ended 50 pins cable has been reduced to 25 pins by tying
most grounds together. DB25 connector (like a parallel port). Often
used as the external SCSI connector. Unfortunately, this abomination
is being perpetuated by being used on devices like the Zip drive!
Asynchronous SCSI
- A way of sending data over the SCSI-bus.
- The initiator sends a command or data over the bus and then waits
until it receives a reply (e.g. an ACKnowledge). All commands are sent
asynchronously over the 8 bit part of the SCSI-bus.
- Synchronous SCSI
- Rather than waiting for an ACK, devices that both support
synchronous SCSI can send multiple bytes over the bus in the following
way:
- send data1 : send data2 : ... : send data3 (max outstanding bytes)
- : wait : wait : response1 : reponse2: ...
- This improves throughput, especially if you use long cables. (The
time that a signal travels from one end of the cable to the other end
of the cable IS relevant.)
- Fast SCSI
- Fast SCSI allows faster timing on the bus. ( 10MHz instead of 5MHz )
- On a 8 bit SCSI-bus this increases the *theoretical* maximum speed
from 5MB/s to 10MB/s.
- Ultra SCSI
- Synchronous data transfer option which allows up to 20MHz data
clocking on the bus. Also called FAST20.
Ultra2 SCSI
- Synchronous data transfer option which allows up to 40MHz data
clocking on the bus. Also called FAST40.
- Use of this option also requires the use of LVD
bus drivers.
- Wide SCSI
- Uses an extra cable (or more commonly a 68 pin P cable) to send the
data 16 or 32 bits wide. This allows for double or quadruple speed
over the SCSI-bus.
- RAID [Added by Editor(GF) Corrected by Fredrik Bjork (ace@varberg.se)]
- A Redundant Array of Independent Disks is a set of disk drives
connected in such a way as to allow certain types of access
optimization, or data security. This can be accomplished in hardware
using a special dual ported SCSI adapter, or completely in software in
a special device driver.
- A RAID 0 array stripes the data across multiple drives to decrease
data latency. A RAID 1 array mirrors the data on multiple drives for
increased data integrity. A RAID 5 array uses extra drives in a
distributed manner to store parity information that can be used to
apply data correction and recover any data in the event of any
individual disk failure. This provides high reliability.
- The following was submitted by RAIDER@ultrafast.net:
- The minimum number of drives required for each RAID level is:
- RAID 0 - TWO (2) drives
- RAID 1 - TWO (2) drives
- RAID 0+1 - FOUR (4) drives
- RAID 3 - THREE (3) drives
- RAID 4 - THREE (3) drives
- RAID 5 - THREE (3) drives
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| Do
you need SCSI? |
| Nowadays, the performance difference between a
single SCSI hard disk and a single IDE hard disk is becoming pretty small.
Many technologies of SCSI hard disks have been adopted by IDE disks. So,
the question is, do you need SCSI which is in general expensive than IDE
solution? To answer the question, you will need to know the advantages of
SCSI over IDE
The first advantage of SCSI is low CPU usage. A SCSI host
adaptor has its onboard processor taking cares of computation and disk
operations. The usage of Central Processor Unit (CPU) is very often less
than 5%. On the other side, IDE disk sub-system relies on CPU. Intensive
disk operations may fully occupy CPU and render the system unstable and
not available for other tasks.
The second advantage of SCSI is the number of drives that can be
connected to a SCSI channel. As we know, an IDE channel can have at
most 2 drives (one master, one slave). A computer usually has two IDE
channels on motherboard and therefore can only have 4 IDE drives in total.
Unlike IDE, a SCSI channel usually can connect up to 14 drives. A
4-channel SCSI host adaptor can connect up to 64 SCSI drives. The bigger
capacity determines SCSI is still the first choice for a high-end
workstation or server.
The third advantage of SCSI is that a SCSI bus can read and write
multiple SCSI drives simultaneously. This is a big advantage for a
system with many drives. On the other side, IDE bus can read or write only
one drive at a time.
Of course, the latest SCSI drives are still faster than the latest IDE
drives in terms of either access time or data transfer rate. Especially
for those 15K rpm SCSI drives, they are the first choice for servers
demanding the most.
|
| Do
you need SCSI RAID? |
| SCSI RAID is
expensive. You will need a SCSI RAID controller, at least two SCSI drives,
and sometimes a SCSI case. Identical drives are preferred for RAID. A SCSI
RAID controller not only has its onboard processor, but also has its own
memory sub-system. SCSI RAID greatly improves disk sub-system performance
by reading data from and writing data to multiple SCSI drives
simultaneously. Another important contribution of SCSI RAID is data
redundancy. Through either mirroring or parity calculation, your data is
stored redundantly. In cases of disk failure, your data can be recovered.
Everyone knows how important it is.
Well, if data redundancy is of no importance to you and you don't care
about data transfer rate that much, the only thing you care about is
access time, SCSI RAID won't help. RAID simply will not improve access
time.
|
| How
to troubleshoot problems? |
| Different SCSI
controllers may have their different troubleshooting tips. Following is
just a few basic tips on troubleshooting your SCSI or SCSI RAID
controller.
1. test the
controller in a different computer with a different motherboard
2. try
different PCI slots
3. check
SCSI ID setting for every hard drive
4. If you
use 80-pin LVD drives with converters, make sure the converters are LVD
too.
5. check
termination of each SCSI bus (channel)
6. disable
any onboard SCSI devices
7. remove
any other SCSI controllers in the same computer
8. use
different cables and hard drives
9.
DPT RAID controllers require at least 2 hard drives to function
10. choose
"8 GB drive geometry" in Mylex eXtremeRAID 1100's bios options.
11. SCSI
disk array requires a more powerful power supply than non-RAID disks. If
your SCSI RAID controller frequently detects "dead" or
"off-line" drives which are not really dead, then probably you
need to get a better power supply.
12. SCSI ID
for most SCSI controllers is by default 7. Don't change it unless if
necessary.
13. If you
got a black screen after you put a SCSI RAID controller in your computer,
the memory on the controller is probably dead.
14.
Sometimes you have to enter a SCSI controller's firmware and then you will
be able to flash (or update) its firmware, like DPT controllers. If its
firmware is corrupt, you can not flash it because you have no way to enter
the corrupt firmware. In this situation, you may get an identical
controller with a working firmware and use it as a master firmware. It
works on some SCSI (or RAID) controllers.
15.
Flashing bios (or firmware) very often helps you solve compatibility or
corrupt bios (firmware) problems. But you must know what you are doing and
don't make mistakes.
16. Some
SCSI RAID controllers can not do background initialization. So, when you
create a RAID 5 array, the initialization process could take a long long
time before you can do anything else.
17. If you
are running on a P4 motherboard with a Phoenix BIOS and you can not enter
the SCSI controller's bios during booting, the following may apply. The manufacturer
of your motherboard has disabled the bios call that allows the motherboard
to release control to a pci card. This is INT19. You can look
to the motherboard manufacturer for a way around this bios call being
diasbled. Your bios settings may have a way to enable INT19, or
there may be a bios upgrade from the motherboard manufacturer to correct
this issue.
18. In case you can not enter AMI MegaRAID
controller's bios, you can use MEGACONF.EXE. This utility cab be
downloaded at this link http://support.lsilogic.com/support/download.cfm.
You will need to boot to dos (you do not have to have the DOS driver
loaded), and then run megaconf. Megaconf looks like CTRL-M, and you
will be able to configure your array.
19. Similar to 18, if you can not enter Mylex RAID
controller's bios, you can use EZAssist which can be downloaded at http://www.mylex.com/support/productgd/index.html.
You will need to boot to dos and run EZAssist. You will be able to
configure array.
20. Not
every SCSI RAID controllers can do RAID 5 well. Some controllers may hang
when create a RAID 5 array. Be sure you choose a good controller if RAID 5
is your purpose. Mylex eXtremeRAID controllers and IBM ServeRAID-4
controllers are very good on RAID 5.
21. Good luck.
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The
following links will answer most of your questions about SCSI.
SCSI Trade Association
The PC
Guide - SCSI
Gary
Field's SCSI FAQ
The
Official SCSI Cheat Sheet - By Scott Makarchuk
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