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you serious about your data. Investing in a RAID system can
help keep your data safe and sound. by Suryakant
extremely low on disk space'! Network administrators cringe
when these words pop up on the screen. Perhaps they already
knew they were running out of storage. And now, to upgrade/install
additional storage capacity, the server has to be shut down
creating service blackouts for employees and customers. In
a 24x7 service world, where downtime means lost productivity
or business, an event like this is unacceptable.
To avoid a situation like this, some companies anticipate
their storage needs well. That often results in overspending
on equipment. This attitude and the increasing storage appetite
of corporate networks has led to a boom in the data storage
market. Sales of storage disk systems will climb from $39
billion this year to $53 billion in 2004, according to market
research firm IDC.
DAS (Direct Attached Storage) is mainly concerned with the
storage that is attached directly to a server. Today, more
than 90 percent
of all computer storage devices like disk drives and disk
arrays are directly attached to a client computer through
various adapters with standardized protocols like SCSI (Small
Computer Serial Interface) and FC (Fiber Channel). This type
of storage is also known as sever attached storage.
In PC configuration, storage is usually integrated in the
same cabinet within the processor system. In mainframes and
large servers, the storage (disk subsystem) is typically located
in a separate unit at some distance from hosts. This subsystem
may be configured either as separate and independent disks,
typically called JBOD (Just a Bunch OF Disks), or as a fault
tolerant arrays of disks, popularly known as RAID (Redundant
Array of Independent Disks) subsystem. This plays an important
role for DAS selection.
RAID arrays for data redundancy
RAIDinitially called Redun-dant Array of Inexpensive
Diskswas originally conceived by a research team at
the University of California Berkeley in 1988. By combining
a collection of disks into a single logical array, an administrator
can store redundant data across these multiple disks in order
to provide fault tolerance by enabling quick recovery from
a disk failure.
To choose a RAID system
Should one go for a SCSI RAID array or FC? Here are five main
categories to be considered while choosing a RAID system.
Performance: The highest available SCSI standard is
wide Ultra2 SCSI whose bandwidth is 80 Mbps, whereas FC's
bandwidth is 100 Mbps. For example, if you are configuring
a RAID system that has no more than ten drives, that is nine
data drives and one parity drive, then the performance benefit
of FC's high bandwidth is not fully utilized. Although ten
drives will swamp the bandwidth of Ultra2 SCSI, they will
not use the full available bandwidth of FC.
Reliability: It is important to know that FC was designed
for fault tolerance and high reliability. The following list
describes some of the in-built FC standards which make the
technology more reliable than SCSI.
FC supports dual ports and dual loops while wide Ultra2
SCSI does not.
FC guarantees data delivery.
FC has less than 10x12 bit error rate far exceeding wide
FC standard contains 32-bit CRC (Cyclic Redundancy Check)
error detection while wide Ultra SCSI has only parity.
FC with copper interconnects uses only four wires between
each FC enclosure. Ultra SCSI uses 68 wires per SCSI channel
FC has a higher level of resilience and reliability. This
becomes more important when looking at higher bandwidth and
greater distance requirements.
Cost: Wide Ultra2 SCSI is more cost effective per MB for RAID
systems up to 500 GB on PCI (Peripheral Component Intercon-nect)-based
platforms. FC becomes more cost effective per MB for RAID
systems over 500 GB.
Cost certainly needs to be balanced against performance in
any RAID system. As seen in the above discussion regarding
performance, in RAID systems with more than ten drives, FC
takes the high performance lead. And, when multiple enclosures
are cascaded together into large storage systems, FC becomes
more cost effective than Ultra SCSI.
Installation: With single enclosure RAID systems, there
is little difference in installation issues between wide Ultra
SCSI and FC. When your system expands to multiple RAID enclosure
configurations, the installation advantages of FC become readily
installation, SCSI requires an ID be set up for each disk
drive/device in the RAID. FC devices have a WWN (World Wide
Name), like Ethernet, which eliminates the ID set up. SCSI
requires term power and termination to be applied correctly.
FC has no such requirements. All of these factors make an
FC installation faster,
simpler, and more reliable than Ultra SCSI.
FC is undoubtedly a more flexible technology than Ultra2 SCSI.
The following list describes some of the reasons for this
FC-AL (Fibre Channel-Arbitrated Loop) supports up to 126
devices per loop. Ultra SCSI supports 15 per channel. Essentially,
one FC controller is equivalent to eight parallel SCSI controllers.
FC-AL storage devices, can be located up to thirty meters
apart when using four-wire copper cable. This distance can
be increased up to ten kilometres when using fiber optic
cable, which greatly facilitates off site expansion. Ultra
SCSI can be 3 meters apart with standard connections and
12 meters apart with differential connections.
FC drives are standard with dual loop capability. This allows
complete fault tolerance to be implemented in a RAID subsystem.
Ultra SCSI has a single bus connection. Therefore, in Ultra
SCSI RAID systems, mirroring drives need to be implemented
to attain complete fault tolerance. This increases costs
FC RAID systems are simple to scale to Terabytes of storage.
With Ultra SCSI, scaling to such capacities is extremely
Both FC and Ultra SCSI has its place in the RAID system market.
Ultra SCSI is a good solution for single enclosure and server
embedded RAID systems with low to medium performance requirements.
For large capacity, high performance systems with requirements
for high fault tolerance and data integrity, FC is a superior
Disk mirroring involves the simultaneous writing of the same
data over one RAID controller on two separate hard disks
Similar to mirroring, disk duplexing is the simultaneous writing
of the data over two RAID controllers on two separate disks
This involves breaking data into small pieces and distributing
it across multiple disks. Depending on the RAID implementation,
data is broken up into bits, bytes, and blocks
Parity is logical information about the data you're storing,
and it's use to re-create lost data in the event of a disk
failure. It is used in combination with striping and usually
involves at least three disks
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