Linux RAID

[Rick Stevens]

Kligh Darsono wrote:
> Thanks Rick,
> 
> Ok, i will try it later. But i have a question to you
> regarding RAID. Let say that we can do with LVM, so
> what is the different betwin RAID & LVM ?.

RAID is "Redundant Array of Inexpensive Disks".  It is a mechanism where
data is spread across multiple disks to accomplish several tasks.  One
is speed of data access because parts of files are spread over the disks
so you can read a file faster (you don't have to wait for one disk to
deliver the data).  The second (and more common) use is to spread and
duplicate the data across several drives so that the failure of a single
(or multiple) drives doesn't cause the data to fail.

LVM (Logical Volume Management) allows you to string together multiple
devices (e.g. RAID systems) to make up a logical drive.  For example,
say you use LVM and put your "/usr" filesystem on an LVM device.  Now,
if /usr fills up you can put in an additional drive, add it to the
volume on the fly and grow the /usr filesystem.  Note that you can do
this WITHOUT repartitioning and reformatting.

LVM can also be used to stripe data across multiple devices to enhance
performance, sort of like what some RAID configurations do.  However,
LVM is pure software, while RAID is _generally_ done by a dedicated
hardware controller.  There is software RAID also, but I am not as
comfortable using it as I am hardware RAID, since it is possible that a
rogue process can corrupt the RAID software and cause lots of problems.
It rather depends on what your acceptable level of risk is.  In our
platforms, our tolerance of risk is pretty low, so we generally use
hardware RAID (typically RAID 5) in our systems.
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- Rick Stevens, Senior Systems Engineer     rstevens at vitalstream.com -
- VitalStream, Inc.                       http://www.vitalstream.com -
-                                                                    -
-                       When in doubt, mumble.                       -
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