Re: Requesting replace mode for changing a disk

From: Goswin von Brederlow <goswin-v-b@web.de>
To: Bill Davidsen <davidsen@tmr.com>
Cc: Goswin von Brederlow <goswin-v-b@web.de>, linux-raid@vger.kernel.org
Subject: Re: Requesting replace mode for changing a disk
Date: Sun, 10 May 2009 03:22:58 +0200	[thread overview]
Message-ID: <87ab5lycnh.fsf@frosties.localdomain> (raw)
In-Reply-To: <4A060CBE.9090308@tmr.com> (Bill Davidsen's message of "Sat, 09 May 2009 19:07:42 -0400")

Bill Davidsen <davidsen@tmr.com> writes:

> 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).

The difference, appart from simpler usage, would be that the raid does
not have to be stoped. Stopping the raid that contains / or /usr means
some downtime.

In the case of LVM there is the fact that you can suspend a
device-mapper device and alter its mapping any way you wish. So you
can do things manually without umounting the filesystems. But lvm /
device-mapper doesn't have all the raid stuff so one can't just
switch.