Guy Watkins wrote:
} -----Original Message-----
} From: linux-raid-owner@xxxxxxxxxxxxxxx [mailto:linux-raid-
} owner@xxxxxxxxxxxxxxx] On Behalf Of Bill Davidsen
} Sent: Saturday, May 09, 2009 7:08 PM
} To: Goswin von Brederlow
} Cc: linux-raid@xxxxxxxxxxxxxxx
} Subject: Re: Requesting replace mode for changing a disk
}
} Goswin von Brederlow wrote:
} > Hi,
} >
} > consider the following situation: You have a software raid that runs
} > fine but one disk is suspect (e.g. SMART says failure imminent or
} > something). How do you replace that disk?
} >
} > Currently you have do fail/remove the disk from the raid, add a
} > fresh disk and resync. That leaves a large window in which redundancy
} > is compromised. With current disk sizes that can be days.
} >
} > It would be nice if one could tell the kernel to replace a disk in a
} > raid set with a spare without the need to degrade the raid.
} >
} > Thoughts?
} >
}
} This is one of many things proposed occasionally here, no real
} objection, sometimes loud support, but no one actually *does* the code.
}
} You have described the problem exactly, and the solution is still to do
} it manually. But you don't need to fail the drive long term, if you can
} stop the array for a few moments. You stop the array, remove the suspect
} drive, create a raid1 of the suspect drive marked write-mostly and the
} new spare, then add the raid1 in place of the suspect drive. For any
} chunks present on the new drive the reads will go there, reducing
} access, while data is copied from the old to the new in resync, and
} writes still go to the old suspect drive so if the new drive fails you
} are no worse off. When the raid1 is clean you stop the main array and
} back the suspect drive out.
}
} This is complicated enough that I totally agree a hot migrate would be
} desirable. This is why people use lvm, although I make zero claims that
} this same problem will solve more easily, I'm just not an lvm guru (or
} even a newbie, just an occasional user).
If the disk is suspect, I would expect read errors!
If you have 1 bad block on the suspect disk, this process will fail.
The raid1 is part of the original raid5, so the error should go to that
level, where it will be recovered, and hopefully then rewritten. I have
actually done this, and it has always completed, so I haven't researched
why it worked, just noted that it did.
If the logic was built-in to md, then any read errors while replacing could
be recovered from another disk or disks.
--
bill davidsen <davidsen@xxxxxxx>
CTO TMR Associates, Inc
"You are disgraced professional losers. And by the way, give us our money back."
- Representative Earl Pomeroy, Democrat of North Dakota
on the A.I.G. executives who were paid bonuses after a federal bailout.
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