RE: Requesting replace mode for changing a disk

["Guy Watkins" <linux-raid@watkins-home.com>]

} -----Original Message-----
} From: Bill Davidsen [mailto:davidsen@tmr.com]
} Sent: Sunday, May 10, 2009 10:34 AM
} To: Guy Watkins
} Cc: 'Goswin von Brederlow'; linux-raid@vger.kernel.org
} Subject: Re: Requesting replace mode for changing a disk
} 
} 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.

It depends on who sees the error.  If the parent array is trying to read,
then yes.  But if the RAID1 is reading to sync, then no.  The RAID1 layer
does not know about the RAID5 (or whatever) just above.

} > If the logic was built-in to md, then any read errors while replacing
} could
} > be recovered from another disk or disks.
} >
} >