[CentOS] RAID 5 setup?

[Robert Heller]

At Thu, 25 Mar 2010 17:07:47 -0400 CentOS mailing list <centos at centos.org> wrote:

> 
> On Thu, Mar 25, 2010 at 3:36 PM, Robert Heller <heller at deepsoft.com> wrote:
> > At Thu, 25 Mar 2010 12:24:57 -0700 (PDT) CentOS mailing list <centos at centos.org> wrote:
> >
> >>
> >>
> >> Can anyone provide a tutorial or advice on how to configure a software RAID 5 from the command-line (since I did not install Gnome)?
> >>
> >> I have 8 x 1.5tb Drives.
> >
> > mdadm --create /dev/md0 --level=5 --raid-devices=7 /dev/sd[abcdefg]1
> >
> > The above will create a level 5 RAID named /dev/md0 of /dev/sda1
> > /dev/sdb1 /dev/sda1 /dev/sdc1 /dev/sdd1 /dev/sde1 /dev/sdf1, with
> > hot-spare /dev/sdg1
> >
> > Note: RAID5 is not really recomended for such large disks. Â You run the
> > risk of a complete data loss if one disk fails and the another disk
> > fails during the rebuild.
> >
> >
> >>
> >> -Jason
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> >>
> >>
> >
> > --
> > Robert Heller             -- 978-544-6933
> > Deepwoods Software        -- Download the Model Railroad System
> > http://www.deepsoft.com/ Â -- Binaries for Linux and MS-Windows
> > heller at deepsoft.com       -- http://www.deepsoft.com/ModelRailroadSystem/
> >
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> >
> 
> Robert,
> 
> Why is the size a factor here? Why would this be OK with smaller
> disks? How would you partition this instead?

There was a thread some time back (a few weeks? Couple of months?) about
how as disk size got so much larger, the error rate hasn't really gotten
much better.  With such large disks, the number of I/O operations needed
to do a rebuild of a RAID 5 array is so large that one will be
increasingly likely to hit an error, at which point all bets are off. 
(There are some papers talking about this -- I don't have the links, but
I think they are in the list archives.)

The prefered way to go would be RAID10 (RAID1 (mirror) + RAID0 (stripe)).
Form pairs as RAID1, then strip the pairs.  With 8 disks, this would 4
pairs, 1.5TB/pair = 1.5*4 = 6TB total.

> 
> Thanks.
> 
> Boris.
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> 
>                                                                                           

-- 
Robert Heller             -- 978-544-6933
Deepwoods Software        -- Download the Model Railroad System
http://www.deepsoft.com/  -- Binaries for Linux and MS-Windows
heller at deepsoft.com       -- http://www.deepsoft.com/ModelRailroadSystem/