Shrinking raid array

Shrinking raid array

[Phillip Susi]

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I'm working on shrinking a raid array and need some clarification on
where the metadata goes and how large it is.  I have used -G with -Z
and -z to reduce the used size of each disk, but the man page says you
need some extra space at the end of the disk for the metadata ( format
1.0 ), but doesn't specify exactly how much.  I need to know what the
exact size of the reduced partition should be.

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Re: Shrinking raid array

[Phillip Susi]

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On 2/12/2013 10:52 AM, Phillip Susi wrote:
> I'm working on shrinking a raid array and need some clarification
> on where the metadata goes and how large it is.  I have used -G
> with -Z and -z to reduce the used size of each disk, but the man
> page says you need some extra space at the end of the disk for the
> metadata ( format 1.0 ), but doesn't specify exactly how much.  I
> need to know what the exact size of the reduced partition should
> be.

I just noticed that reducing the used dev size has not moved the
metadata, so it is looking like it just isn't possible to shrink the
md partition with metadata format 1.0?

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Re: Shrinking raid array

[Mikael Abrahamsson]

On Tue, 12 Feb 2013, Phillip Susi wrote:

> I just noticed that reducing the used dev size has not moved the 
> metadata, so it is looking like it just isn't possible to shrink the md 
> partition with metadata format 1.0?

From "man mdadm"

"The different sub-versions store the superblock at different locations on 
the device, either at the end (for 1.0), at the start (for 1.1) or 4K from 
the start (for 1.2)"

I don't really see how it could be shrunk with a superblock at the end, 
then you wouldn't be able to find it next time...

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: Shrinking raid array

[Phillip Susi]

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On 2/12/2013 12:53 PM, Mikael Abrahamsson wrote:
> On Tue, 12 Feb 2013, Phillip Susi wrote:
> 
>> I just noticed that reducing the used dev size has not moved the 
>> metadata, so it is looking like it just isn't possible to shrink
>> the md partition with metadata format 1.0?
> 
> From "man mdadm"
> 
> "The different sub-versions store the superblock at different
> locations on the device, either at the end (for 1.0), at the start
> (for 1.1) or 4K from the start (for 1.2)"
> 
> I don't really see how it could be shrunk with a superblock at the
> end, then you wouldn't be able to find it next time...

You move the superblock, then shrink the partition...

But it looks like mdadm doesn't move the superblock even though it no
longer is using the full partition.

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Re: Shrinking raid array

[Mikael Abrahamsson]

On Tue, 12 Feb 2013, Phillip Susi wrote:

> You move the superblock, then shrink the partition...

As far as I know, mdadm has no code to shrink partitions?

> But it looks like mdadm doesn't move the superblock even though it no
> longer is using the full partition.

I think that is perfectly reasonable. Otherwise the superblock wouldn't be 
found next time if the partition wasn't resized.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: Shrinking raid array

[Phillip Susi]

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On 2/12/2013 1:43 PM, Mikael Abrahamsson wrote:
> On Tue, 12 Feb 2013, Phillip Susi wrote:
> 
>> You move the superblock, then shrink the partition...
> 
> As far as I know, mdadm has no code to shrink partitions?

Of course not, you would use fdisk or parted or whatever else to do that.

>> But it looks like mdadm doesn't move the superblock even though
>> it no longer is using the full partition.
> 
> I think that is perfectly reasonable. Otherwise the superblock
> wouldn't be found next time if the partition wasn't resized.

Yes, but the way it is now the superblock won't be found if the
partition *is* resized.  What is the use of reducing the amount of the
partition mdadm is using if you can't then shrink the partition itself?


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Re: Shrinking raid array

[Mikael Abrahamsson]

On Tue, 12 Feb 2013, Phillip Susi wrote:

> Yes, but the way it is now the superblock won't be found if the 
> partition *is* resized.  What is the use of reducing the amount of the 
> partition mdadm is using if you can't then shrink the partition itself?

It makes sense if you for some reason want to use a smaller component 
drive (thinking mostly of older drives where 36 or 73GB didn't mean an 
exact size between manufacturers and you happened to create the array on a 
larger drive than you now have available.

Anyhow, I can see that what you're looking for is useful as well but I 
also believe it should be done through a special option and I imagine it 
should also output information regarding where the partition should now 
end (to use in fdisk) so mdadm can find the superblock next time.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se

Re: Shrinking raid array

[Mikael Abrahamsson]

On Wed, 13 Feb 2013, Mikael Abrahamsson wrote:

> Anyhow, I can see that what you're looking for is useful as well but I 
> also believe it should be done through a special option and I imagine it 
> should also output information regarding where the partition should now 
> end (to use in fdisk) so mdadm can find the superblock next time.

Actually, in addition to this, I also believe that there should be some 
kind of superblock at the end of the current partition that points to the 
location of the "new" moved superblock, so that things work even if you do 
not change the partition size.

-- 
Mikael Abrahamsson    email: swmike@swm.pp.se