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Re: RAID greater than 2TB on Fedora Core 3



On Mon, 2005-04-04 at 11:13 -0400, Scot L. Harris wrote:
> On Mon, 2005-04-04 at 10:45, Masopust Christian wrote:
> > Dear all,
> > 
> > maybe this is a dumb question from an newbie, but...  ;-)
> > 
> > I've connected a RAID with appr. 2,6TB to my new installed
> > Fedora-System (Fedora Core 3, Adaptec 39160 and RAID is an
> > EonStore).
> > 
> > When running fdisk the biggest partition i can create is 2TB!
> > 
> > when trying to create the filesystem with mkfs.xfs directly
> > on /dev/sda i also get only 2TB !
> > 
> > so, what am i doing wrong?   is the adaptec 39160 the problem?
> > (this because i read something about such big raids working
> > with 3ware 9000 RAID cards)
> 
> What raid level are you trying to setup?
> 
> Each raid level utilizes the available drives in different ways which
> result in the usable space being different.  If you have 2.6 TB of raw
> disk space and use Raid 1 (simple mirroring) you would only be able to
> create a file system with a size of 1.3 TB.  
> 
> The different raid levels provide some trade offs in usable space vs.
> speed of writes vs. speed of reads vs. resiliency.
> 
> Raid 5 will have a 20% overhead  due to the way it replicates data
> across multiple drives.  
> 

Your description is good, but slightly flawed in the raid5 description.

A raid5 configuration loses exactly one drive for available space no
matter how many drives are involved.
If the array contains 3 drives it loses 1/3 of the total drive space.
If it instead contains 10 drives it only loses 1/10 the drive space.
Thus the 20% figure is correct only in a case where the array contains 5
physical drives.

This is because of the parity redundancy on raid5 that makes it possible
to totally lose 1 drive and still lose no data.

The OP did not state how many physical drives he has in the array, so I
cannot conjecture whether the usable space (2TB) is correct or not.


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