Server+ Certification Exam Notes
Click the hyperlink to view commonly used acronyms in
Click the hyperlink to view commonly used acronyms in Server certification.
2. SCSI bus width:
For a 16-bit Wide
SCSI, there are 16 possible SCSI Ids, 0-15.
SCSI 3 ultra-wide - 16 devices, 0=bootable drive, 15=controller
A standard 8-bit SCSI
can support only 8 devices (including SCSI card), the
Ids allows are 0-7.
SCSI ID - 0=bootable drive, 7=controller, 1-6=any other devices
3. If the termination
is not done, a SCSI devices on the bus will not function
properly. This is due to reflection of the signals at
the end of the bus. To prevent this, both ends of the
SCSI bus need to be terminated. If one end of the SCSI
bus is terminated, you may find intermittent problems.
Never terminate the bus at a device connected in
4. LUN stands for
Logical Unit Number. LUN is used to identify a logical
device that is an independent functional part of a SCSI
device. SCSI-2 specification allows 8 logical units for
each SCSI device address. The logical unit addresses are
numbered 0 to 7.
5. SCSI Types:
6. Important Disk RAID levels:
RAID 0: Disk Striping
without parity. Minimum 2 disks are required. RAID 0
doesn't provide any fault tolerance, but read/ write to
the disk will improve
RAID 1: Disk Mirroring
and Disk Duplexing.
Disk Mirroring requires at least two partitions
of same size. Each partition should be on a different
physical drive. In other words, minimum 2 disks are
required. For disk duplexing minimum 2 disks and
2 controllers are required. RAID 1 provides good fault
tolerance, though disk access is slower compared to disk
RAID 2: Disk Striping
with Error correction.
RAID 3: Disk striping
with error correction code stored as parity
takes a striped array as in RAID 0, then adds a
parity hard disk drive to the array.
This parity information is vital if one drive
fails. If a drive should fail in this situation, the
parity drive can be used to restore blocks that have
been broken due to data corruption.
RAID 4: Disk striping
with large blocks allocation.
RAID 5: Disk Striping
with Parity. Disk Striping with Parity requires at least
three partitions of same size, and each partition should
be on a different physical drive. In other words,
minimum 3 disks are required for disk striping
with parity. Fault tolerant, less expensive than disk
mirroring. If more than one disk fails, data can't be
recovered. You still need to depend on tape back up for
multiple disk failures.
5 is similar to RAID 3, except the parity information is
spread across all drives, this allows all drives to be
able to rebuild the array if a drive fails. In RAID 3
parity is written to only one drive.
that, compared to disk striping or disk striping with
parity; Disk mirroring has more overhead, as the entire
disk get copied to another disk.
RAID 10 describes Mirrored Striping. It is a mode 0
array, plus a mode 1 array, striped. For example, if you
have striped data into two drives, each drive is
mirrored in RAID 10. That is a total of 4 drives.
of cabling used for SCSI:
8-bit Internal device
SCSI connection: Uses Unshielded 50 pin ribbon cable
8-bit External device
SCSI connection: Uses DB-25 or Centronic-50 cable
8-bit External device
Fast SCSI connection: Uses mini sub-D connector cable
8-bit cables are
considered A-cable by SCSI-2 definition.
16-bit Wide SCSI uses
P-cable or (A-cable + 68 pin B-cable)
32-bit Wide SCSI uses
Q-cable plus P-cable
lengths used by SCSI devices are:
Single Ended (or un
balanced) electronic signaling: 6 m 0r 20ft (apx.)
(HVD-High Voltage Differential): 25 m or 82 ft (apx.)
(LVD - Low Voltage Differential): 12 m
SCSI system uses the
following types of terminations:
Passive: Uses passive
resistors, not recommended usually.
terminator has a voltage regulator that ensures the
correct termination voltage.
Termination (FPT):A better implementation of active
termination that clamps to the output of two regulated
Each device in a SCSI chain needs to have unique ID. For
a 16-bit Wide SCSI, there are 16 possible SCSI Ids,
0-15. A standard 8-bit SCSI can support only 8 devices
(including SCSI card), the Ids allowed are 0-7.
RAID is implemented across several platforms: SCSI,
FIBRE-SCSI and IDE are the most common implementations
of RAID technologies.
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