From mboxrd@z Thu Jan  1 00:00:00 1970
From: Goswin von Brederlow <goswin-v-b@web.de>
Subject: Re: Requesting replace mode for changing a disk
Date: Wed, 13 May 2009 05:27:28 +0200
Message-ID: <87zldhvg0v.fsf@frosties.localdomain>
References: <20090513012112681.IEFQ19662@cdptpa-omta02.mail.rr.com>
Mime-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Return-path: <linux-raid-owner@vger.kernel.org>
In-Reply-To: <20090513012112681.IEFQ19662@cdptpa-omta02.mail.rr.com> (Leslie
	Rhorer's message of "Tue, 12 May 2009 20:21:14 -0500")
Sender: linux-raid-owner@vger.kernel.org
To: lrhorer@satx.rr.com
Cc: 'Linux RAID' <linux-raid@vger.kernel.org>
List-Id: linux-raid.ids

"Leslie Rhorer" <lrhorer@satx.rr.com> writes:

>> This is one of many things proposed occasionally here, no real
>> objection, sometimes loud support, but no one actually *does* the code.
>
> 	At the risk of being a metoo, I would really love this feature.
>
>> 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,
>
> Um, how, exactly?  That is to say, after stopping the array, how does one
> remove the drive?  From the next step in your suggestion, it doesn't seem
> tome you are talking about physically removing the drive, so how does one
> remove a drive from a stopped array for this purpose?  I didn't think that
> either
>
> 	mdadm -r <drive> <array>
> or
>
> 	mdadm -f <drive> <array>
>
> could be used on a stopped array.  Am I mistaken?
>
>> create a raid1 of the suspect drive marked write-mostly and the
>> new spare,
>
> But doesn't creating the array with the drive wipe the contents?  If so, it
> doesn't seem to me this provides much redundancy.
>
>> then add the raid1 in place of the suspect drive.
>
> Before starting the array?  If so, how?  Or should one do an assemble
> including the newly minted RAID1?  I thought mdadm would take the newly
> added drive to be blank, even if it isn't.
>
>> For any
>> chunks present on the new drive the reads will go there, reducing
>
> Huh?  Are you saying any read which finds one chunk missing will
> automatically write back the missing data (doing a spot rebuild), or
> something else?
>
>> access, while data is copied from the old to the new in resync, and
>
> See my query above.  It seems to me you are saying the RAID1 can be created
> without wiping the drive.
>
>> writes still go to the old suspect drive so if the new drive fails you
>> are no worse off.
>
> I think I would expect the old drive to be more likely to fail than the new.
>
>> When the raid1 is clean you stop the main array and
>> back the suspect drive out.
>
> OK, basically the same question.  How does one disassemble the RAID1 array
> without wiping the data on the new drive?

I think he ment this:

mdadm --stop /dev/md0
mdadm --build /dev/md9 --chunk=64k --level=1 --raid-devices=2 /dev/suspect /dev/new
mdadm --assemble /dev/md0 /dev/md9 /dev/other ...

MfG
        Goswin