[K12OSN] why RAID?

[Jack]

On Fri, 2004-12-03 at 09:36 -0600, Jim Kronebusch wrote:
> > I see a lot of posts here asking about setting up a RAID 
> > system.  My ignorance and inexperience will no doubt show 
> > through here, but why is it so important to get RAID set up?  
> > Why not just have a cron job that makes a copy of /home or 
> > any other important directories?  It could be set to run 
> > every night, or even after every class period.  It could be 
> > set with a low priority via the "nice" command so that it 
> > doesn't interfere much with the read/write operations of 
> > users who are currently logged in.
> 
> In addition to Jim McQuillan's point you also get the performance
> benefits with RAID0 or RAID5 via striping.  Say with a single drive your
> platters need to make a full revolution to write every bit of data.  If
> you stripe with 2 drives you cut the time in half (theoretically), 3
> drives into a third and so on.  Because while the first drive is making
> is revolution after writing the bit the next drive has the ability to
> write a bit etc.  
> 
> So bottom line is RAID1 and RAID5 get you redundancy with immediate
> failover, no downtime.  And RAID0 and RAID5 gain performance with write
> speeds.  My preference is RAID5 when possible because it gains the best
> of both worlds, but unfortunately you pay more for the controllers that
> handle it.  If you are on a budget crunch RAID0+1 in my opinion is the
> next best option.  
> 
> And I personally never trust backups, they are my insurance.  But when
> there is a server down and users on my back I want immediate results
> that allow me to do repairs in the off hours, and leave the backups on
> the shelf.
> 
> Another note is that with RAID0 you get the full total of all of the
> drive amounts added together (80GB+80GB+80GB=240GB).  With RAID1 you
> only get the total of the one drive (80GB+80GB=80GB).  And with RAID5
> you loose the size of at least one of the drives in the raid (RAID5 uses
> 3 drives minimum)(80GB+80GB+80GB=160GB).  So RAID5 also makes the most
> efficient use of your storage capacities while maintaining redundancy
> and failover.
> 
> Also with RAID you can buy "Hot Swap" controllers and drive trays (at an
> extra cost of course).  This allows you to remove the bad drive while
> the machine is running and the RAID rebuilds itself on the fly.  Your
> users never see a difference and your not in after hours.
> 
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> 
RAID5 is a very good way to protect your data due to the parity but when
using RAID5 you take pretty bit (up to 75%)  WRITE performance hit.
Only READ performace is gained when using RAID5.  Also depending on how
many drives you use with RAID5 you loose storage.  You need a minumum of
3 drives to do RAID5

RAID0 gives the best performance by far but it is not fault tolerant at
all.  

Here is a good site for some information on RAID.  
http://www.acnc.com/04_00.html

Jack