Re: Hot-replace for RAID5

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(I accidentally sent my first reply directly to the OP, and forgot the mailing list - I'm adding it back now, because I don't want the OP to follow my advice until others have confirmed or corrected it!)

On 09/05/2012 21:53, Patrik Horník wrote:
> Great suggestion, thanks.
> So I guess steps with exact parameters should be:
> 1, add spare S to RAID5 array
> 2, mdadm --grow /dev/mdX --level 6 --raid-devices N+1 --layout=preserve
> 3, remove faulty drive and add replacement, let it synchronize
> 4, possibly remove added spare S
> 5, mdadm --grow /dev/mdX --level 5 --raid-devices N

Yes, that's what I was thinking.  You are missing "2b - let it synchronise".

Of course, another possibility is that if you have the space in the system for another drive, you may want to convert to a full raid6 for the future. That way you have the extra safety built-in in advance. But that will definitely lead to a re-shape.

> My questions:
> - Are you sure steps 3, 4 and 5 would not cause reshaping?

I /believe/ it will avoid a reshape, but I can't say I'm sure. This is stuff that I only know about in theory, and have not tried in practice.

> - My array has now left-symmetric layout, so after migration to RAID6
> it should be left-symmetric-6. Is RAID6 working without problem in
> degraded mode with this layout, no matter which one or two drives are
> missing?

The layout will not affect the redundancy or the features of the raid - it will only (slightly) affect the speed of some operations.

> - What happens in step 5 and how long does it take? (If it is without
> reshaping, it should only upgrade superblocks and thats it.)

That is my understanding.

> - What happens if I dont remove spare S before migration back to
> RAID5? Will the array be reshaped and which drive will it make into
> spare? (If step 5 is instantaneous, there is no reason for that. But
> if it takes time, it is probably safer.)

I /think/ that the extra disk will turn into a hot spare. But I am getting out of my depth here - it all depends on how the disks get numbered and how that affects the layout, and I don't know the details here.

> So all and alll, what guys do you think is more reliable now, new
> hot-replace or these steps?

I too am very curious to hear opinions. Hot-replace will certainly be much simpler and faster than these sorts of re-shaping - it's exactly the sort of situation the feature was designed for. But I don't know if it is considered stable and well-tested, or "bleeding edge".



> Thanks.
> Patrik
> On Wed, May 9, 2012 at 8:09 AM, David Brown<david.brown@xxxxxxxxxxxx> wrote:
>> On 08/05/12 11:10, Patrik Horník wrote:
>>> Hello guys,
>>> I need to replace drive in big production RAID5 array and I am
>>> thinking about using new hot-replace feature added in kernel 3.3.
>>> Does someone have experience with it on big RAID5 arrays? Mine is 7 *
>>> 1.5 TB. What do you think about its status / stability / reliability?
>>> Do you recommend it on production data?
>>> Thanks.
>> If you don't want to play with the "bleeding edge" features, you could add >> the disk and extend the array to RAID6, then remove the old drive. I think >> if you want to do it all without doing any re-shapes, however, then you'd >> need a third drive (the extra drive could easily be an external USB disk if
>> needed - it will only be used for writing, and not for reading unless
>> there's another disk failure).  Start by adding the extra drive as a hot
>> spare, then re-shape your raid5 to raid6 in raid5+extra parity layout. Then >> fail and remove the old drive. Put the new drive into the box and add it as >> a hot spare. It should automatically take its place in the raid5, replacing >> the old one. Once it has been rebuilt, you can fail and remove the extra
>> drive, then re-shape back to raid5.
>> If things go horribly wrong, the external drive gives you your parity
>> protection.
>> Of course, don't follow this plan until others here have commented on it,
>> and either corrected or approved it.
>> And make sure you have a good backup no matter what you decide to do.
>> mvh.,
>> David

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