Re: Requesting replace mode for changing a disk

Bill Davidsen
2009-05-10T14:33:49Z

Guy Watkins wrote:
> } -----Original Message-----
> } From: linux-raid-owner@vger.kernel.org [mailto:linux-raid-
> } owner@vger.kernel.org] On Behalf Of Bill Davidsen
> } Sent: Saturday, May 09, 2009 7:08 PM
> } To: Goswin von Brederlow
> } Cc: linux-raid@vger.kernel.org
> } 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@tmr.com>
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.