Re: What is Raid?
- From: Serpent77 <Serpent77.257voa@xxxxxxxxxxxxxxxxx>
- Date: Sat, 25 Mar 2006 07:25:14 +0000
With a JBOD configuration, however, all your attached disks will act as
large "virtual drive". So if you put a 30gb and a 40gb HDD on a RAID
you will have a 70gb drive.
Sorry, had to jump in here and clarify this.
JBOD is not a form of RAID in any way, and therefore it can not add
drive spaces together. One common meaning for this acronym is "justa
bunch of disks" and is basically just a way to turn off the RAID
functionality on a controller card. However that being said, a RAID
controller with a "Span" mode isn't actually using RAID either, but it
can logically connect two or more drives into a single data space.
This has an appearance of a RAID 0 in the respect that it takes two
drives and makes them look like a larger drive, but there is no
striping (for increase of speed) and there is no redundancy data such
as CRC (in other words if a one drive fails, both effectively fail,
just as in RAID 0).
In RAID mode, a controller card MUST be configured with compatible
drives using the lowest common denominator of available drives (ie, the
drive can only be as fast as the slowest drive (not completley accurate,
but a safe way to think of it), the largest array can only be as large
as the smallest drive in a RAID 1 array, or the largest array segment
in other RAID formats can only be as large as the smallest drive. If a
RAID array isn't configured the controler card can not present any
drives to the operating system. (ex: 2 HD's used, one a 5400 RPM
20GB, the other a 7200 RPM 30GB; In RAID 0 you could create a 40GB
array that could only operate about as fast as two 5400 RPM drives
together; RAID 1 allows for 20GB of redundant storage still operating
just a little faster than 5400 RPM speeds for writing, but for reading,
you may see a slightly higher boost in performance (depending on which
drive is used for the data read); finally in a Span you could have 50GB
of space with no redundancy, speed would be somewhat erratic since data
from the slower drive would come in slower but it would come in faster
from the faster drie, so depending on where the data is stored at and
coming from speed could be either 5400RPM or 7200RPM).
Switching over to JBOD, or turning off the RAID fetures of a controller
card (usually done in the BIOS by selecting RAID mode or
"Controller"/"Normal"/"No RAID"/etc mode for the SATA controllers
allows the controllers to present the individual drives to the OS.
Finally in "Span" mode, if available, you may attach multiple drives
together, regardless of their specs and lump all of their space into a
single virtual disk. Keep in mind though that this could promote data
loss if you're unsure of the health of the hard drives used, it also
has no speed benefit since you're at the mercy of whatever hard drive
holds the data, and the capabillities it posses. The only benefit to
this method is increasing the logical storage space you have available
to you. This might be important if you handle large files (say large
databases, or movie files in the realm of gigabytes). The only problem
is this "benefit" is pretty small when you consder there are more
effective solutions in true RAID. This should be considered a poor
man's method for increasing storage space, and as a matter of fact in
the sfotware RAID world (especially on Windows) is treated exactly as
such, where spanning a volume is used to increase avaialble space to a
volume by "borrowing" the space from a drive that has space available.
Hope this clears things up some.
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