My 2 cents...
RAID - Redundant Array of Inexpensive Disks. Often (incorrectly), the word
"Inexpensive" is substituted with "Independent". I believe this error first
started when a well known computer magazine published an article with the word
"Independent" and the confusion began. As stated before, other devices can
also be configured "RAID-like". One common example are RAID Tape arrays. I've
seen multi-drive terrabyte LTO arrays at conventions. Impressive!
RAID levels 0, 1, and 5 are most common.
RAID 0 - Stripping across two or more drives with no redundancy (isn't this an
oxy-moron? <g>). RAID-0 allows you to combine several drives of the same
capacity and present them to the system as one logical drive. Somewhat risky,
but fast. When properly configured with drives of known quality (e.g.
Maxtor/Quantum Atlas 10K III series) and with a backup system in place, RAID-0
is hard to beat.
RAID 0 - JBOD. Typically, all RAID configurations use drives of the same
capacity. If you use different capacity drives, then the RAID controller
treats all drives as the same capacity of the smallest drive in the array. For
example, if you have two 10GB drives and a 4GB drive, then the entire array is
treated as three 4GB drives (12GB RAID-0). Some controllers support JBOD -
"Just a Bunch Of Disks". This RAID-0 implementation allows the full capacity
of the drives to be utilized - so in the above example, the array would have a
total capacity of 24GB.
RAID 1 - Drive mirroring. Data is written identically to two volumes. If one
volume fails, data can be retrieved from the other volume. In higher-end
systems, the failed drive can be "hot-swapped" (replaced) without powering down
the system while the other drive services requests. Replacement drive data is
re-built in the background.
RAID 1 - Duplexing. Same concept as mirroring, but drives are on two separate
controllers. Duplexing basically adds a redundant controller.
RAID 5 - Data striping (distributed parity). The data is written in "stripes"
(or blocks/chunks/etc) across 3 or more drives. The "parity data" is also
striped across all the drives. This speeds up READ requests as drive heads can
be positioned to a "ready" state other drives while another drive is actually
transferring data (lowers total "Latency" times). May also speed up writes,
but often negated by read-after-write verify integrity checks. The entire
array can tolerate a failure of one or two drives, depending on the total
number of drives in the array, but the array will operate much slower until the
failed drive is replaced and the data rebuilt. Failed drives can be
"hot-swapped" in supported systems. The total capacity of the array is
"Drivesize * (N-1)" where "N" is the number of drives in the array. So, four
10GB drives would have a RAID-5 capacity of 30GB. RAID-4 is similar to RAID-5,
but used a dedicated parity drive (RAID-5 is better).
One can also combine RAID configurations in the same system for additional
redundancy. RAID 0/1 (mirrored RAID-0 stripe array) and 1/0 (striped RAID-1
mirror array) are popular (btw, 0/1 and 1/0 may look the same, but they're
not...1/0 is better). RAID 5/1 (mirrored RAID-5 array) is commonly found in
mission critical enterprise-sized servers and storage arrays. RAID 5/0 is also
used where speed and capacity are important, but it's rare.
ZIP drive reliability and MTBF...good, convenient drive. However, early ZIP
drives suffered from a "click of death". The drive would litterally commit
suicide and sever it's R/W head trying to read a disk. Iomega eventually fixed
the problem, but the stigma remained. The "click of death" is probably why you
see so many refurb ZIP drives available for sale. Also, be aware that the ZIP
250 drive will correctly read a 100MB disk, but if you write to a 100MB disk, a
100MB drive may no longer be able to read the disk. This is due to R/W head
size. Similar problem existed with the old 5.25" 1.2MB floppy drive writing to
360K disks. I believe the new ZIP 750 has the same problem with 250MB disks
(it will only read 100MB disks, not write). As to a statement earlier about
the ZIP drive's MTBF, I would say that it's actually *shorter* due to the
repeatitive spin-up spin-down cycles - more strain on the motor.
Drive companies...All drive companies have had their lemons. All companies
have had their shining stars. IBM recently had a lot of problems with their
popular 75GXP(?) line. My co-worker had one that failed after 3 months, got a
replacement under warranty and it failed 3 weeks later. A 2nd replacement
drive also failed. There are many more stories like this about this particular
line of drives. Western Digital did a recall a couple of years ago due to a
faulty component. Currently, Fujitsu is recalling some drives due to a faulty
component (get this, some chip manufacturer used phospherous in the plastic
used to make an IC chip and now they're failing. Phospherous? Isn't that
corrosive???). And Fujitsu had a track record of making some of the most
reliable drives on the market.
Warranties - For a while, drives were near perfection and reliability was high
so companies offered longer warranties as a marketing incentive. Alas, as the
HD market tumbled along with prices, the QC fell too. Companies started
replacing more drives under warranty than originally anticipated. Combine that
with the currenty economic conditions and drive companies are now shortening
the warranties back to 1 year. Beware...some of these warranties are from date
of manufacture, not date of sale. Also, in general, a drive company might have
several different "lines" of drives. The higher-end lines often come with a
longer warranty. Most "enterprise" class SCSI drives still come with a 5 year
warranty (but cost quite a bit more!).
But, alas, I've digressed long enough. Just though I'd offer some
clarification to some topics recently brought up.
73,
- Aaron Hsu, NN6O
(athsu)@unistudios.com
(nn6o)@arrl.net
No-QRO Int'l #1,000,006
All contents of this message copywright Aaron Hsu, NN6O
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