On Sun, Mar 7, 2010 at 12:22 AM, Keld Simonsen [off-list ref] wrote:

On Sun, Mar 07, 2010 at 03:10:18AM -0500, Guy Watkins wrote:

quoted

} -----Original Message-----
} From: Keld Simonsen [mailto:keld@keldix.com]
} Sent: Sunday, March 07, 2010 3:07 AM
} To: Neil Brown
} Cc: Guy Watkins; 'Greg Freemyer'; 'Mark Knecht'; 'Linux-RAID'
} Subject: Re: What RAID type and why?
}
} On Sun, Mar 07, 2010 at 01:21:13PM +1100, Neil Brown wrote:
} > On Sat, 06 Mar 2010 18:17:44 -0500
} > "Guy Watkins" [off-list ref] wrote:
} >
} > > }
} > > } At a minimum I would build a 3-disk raid 6.  raid 6 does a lot of
} i/o
} > > } which may be a problem.
} > >
} > > If he only needs 3 drives I would recommend RAID1.  Can still loose 2
} drives
} > > and you don't have the RAID6 I/O overhead.
} > >
} >
} > and as md/raid6 requires at least 4 drives, RAID1 is not just the best
} > solution to survive two failures on a 3-device array, it is the only
} solution.
}
} Raid10 can also do it.
}
} raid1 is in many ways obsolete and you should rather use raid10,
} which in my eyeys is just another way of doing the same conceptual thing
} as raid1.
}
} Best regards
} keld

Are you sure RAID10 can loose 2 of 3 drives?  I did not think it worked that
way.  I thought RAID10 maintained 2 copies, not 3.  But I have never used
RAID10.

If you ask mdadm to do it, yes. Example:

mdadm --create /dev/md3 --chunk=256 -R -l 10 -n 3 -p f3 /dev/sd[abc]1

the "-p f3" is the one that asks to have 3 copies.

best regards
keld
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Yes, that way would work, except in that case it would use more
complicated methods to split up the stripes among the drives.  Since
you're application seems to be read heavy, I agree with using 'far'
for the stripe method.

However the dis-advantage of mdadm raid10 has been two-fold compared
to raid1 (until kernel 2.6.33+).
1) Fixed in 2.6.33: Striped storage did not previously support
write-barriers (required for atomic write mechanisms/journals).
2) Still unsupported? : Reshape of raid10 arrays.
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