[ILUG] Why RAID
bigbro at skynet.ie
Wed Jul 14 18:05:41 IST 2004
Actually, it's a very valid point, and one that seems to be missed
for the greater part in this discussion - even though I'm sure all of
you are aware of it :-)
RAID is NOT for backup - it's for resilience. I think that's been
adequately covered in this discussion.
It's also for massive storage and high speed access / massive
bandwidth. Show me an alternate method of having a massive and
dynamically extensible storage platform without RAID4 / WAFL (a la
NetApp) or similar? Running out of space? Throw some more disks at it.
Problem solved. Without RAID you'd be looking at buying a bigger drive,
and dealing with the downtime as you installed it, configured it, copied
the data across from the old drive and made it live. Not really an
option in many cases.
Historically, hard disk sizes were small compared to the large
storage requirements required by large databases and filesets. In fact,
a single dataset (say, table) would be larger than could be stored on a
single disk - so rather than break up the table in some unnatural
fashion, along comes RAID striping. w00t! Problem solved. Let the RAID
hardware deal with making the 30GB required storage out of 9GB disks.
AND add a few more for resilience (NOT backup! - I agree with you there
Rick ;-) ) Google will be happy to find you the mathematical
probabilities of drive failures as number of drives in an array
increases, and you should read that in the context that the larger an
array is, the more that is lost as a consequence of a drive in the array
being lost (in the case of no resilience, the entire array is lost. This
is a bad-bad-bad thing.) There are striped drives around here (I work
for an ISP) in the multi-terrabyte range. Without RAID that wouldn't be
Dealing with digital audio and video, RAID provides a hardware
method of abstracting the bandwidth required across multiple disks.
You've all heard of SCSI320. How many of you thought you could actually
get 320MB/s from a single disk? However, put a (RAID) array of them
together and you start to make better use of the bandwidth available on
the bus :-)
The original question, posed by Timothy was, "Reading about all the
problems with RAID, I wonder why people bother with it? Is it worth the
hassle?" In response, Yes! Yes! And wholeheartedly, yes! But you have to
use RAID for what it was made to be used for.
>On Wed, 14 Jul 2004 09:20:47 +0100, Greg Farrell <greg at gregfarrell.org> wrote:
>>You're all missing the key usage of raid - having 2 fast disks with data
>>striped across them, so planetside or other big games have shorter load
>Ah Greg, i knew you'd say something like that ;p
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