Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Krzysztof Adamski]

I have a system with two drives like this:
/ RAID1 on sda3 and sdb3
/boot RAID1 on sda1 and sdb1

I would like to migrate to
/ RAID6 on sdi3, sdj3 and sdh3
/boot RAID1 on sdi1, sdj1 and sdh1

The /boot already done.
How would I do the migration to RAID6?

The new drives i,j and h are larger then a and b, after the migration I
will reshape the raid to use all the space on the new raid, unless it is
possible to do it in one step.

The reason I want RAID6 and not RAID1 with 3 drives is that later I may
add more drives to the array.

Currently I don't trust any RAID that has only one parity drive, it has
to survive a failure of any two drives. So RAID6 or triple RAID1 are a
minimum.

Thanks in advance,
K

Re: Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Pieter De Wit]

On 1/01/2014 13:51, Krzysztof Adamski wrote:
> I have a system with two drives like this:
> / RAID1 on sda3 and sdb3
> /boot RAID1 on sda1 and sdb1
>
> I would like to migrate to
> / RAID6 on sdi3, sdj3 and sdh3
> /boot RAID1 on sdi1, sdj1 and sdh1
>
> The /boot already done.
> How would I do the migration to RAID6?
>
> The new drives i,j and h are larger then a and b, after the migration I
> will reshape the raid to use all the space on the new raid, unless it is
> possible to do it in one step.
>
> The reason I want RAID6 and not RAID1 with 3 drives is that later I may
> add more drives to the array.
>
> Currently I don't trust any RAID that has only one parity drive, it has
> to survive a failure of any two drives. So RAID6 or triple RAID1 are a
> minimum.
>
> Thanks in advance,
> K
>
> --
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Hi,

RAID Level 6 requires a minimum of 4 drives to implement

Cheers,

Pieter

Re: Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Krzysztof Adamski]

On Wed, 2014-01-01 at 14:02 +1300, Pieter De Wit wrote:
> On 1/01/2014 13:51, Krzysztof Adamski wrote:
> > I have a system with two drives like this:
> > / RAID1 on sda3 and sdb3
> > /boot RAID1 on sda1 and sdb1
> >
> > I would like to migrate to
> > / RAID6 on sdi3, sdj3 and sdh3
> > /boot RAID1 on sdi1, sdj1 and sdh1
> >
> > The /boot already done.
> > How would I do the migration to RAID6?
> >
> > The new drives i,j and h are larger then a and b, after the migration I
> > will reshape the raid to use all the space on the new raid, unless it is
> > possible to do it in one step.
> >
> > The reason I want RAID6 and not RAID1 with 3 drives is that later I may
> > add more drives to the array.
> >
> > Currently I don't trust any RAID that has only one parity drive, it has
> > to survive a failure of any two drives. So RAID6 or triple RAID1 are a
> > minimum.
> >
> > Thanks in advance,
> > K
> >
> > --
> > To unsubscribe from this list: send the line "unsubscribe linux-raid" in
> > the body of a message to majordomo@vger.kernel.org
> > More majordomo info at  http://vger.kernel.org/majordomo-info.html
> Hi,
> 
> RAID Level 6 requires a minimum of 4 drives to implement
> 
> Cheers,
> 
> Pieter

I have seen messages by Neil from 2011 that he was planning to remove
this silly minimum requirement of 4 drive for RAID6.

Re: Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Tomas M]

>> RAID Level 6 requires a minimum of 4 drives to implement
>>
> I have seen messages by Neil from 2011 that he was planning to remove
> this silly minimum requirement of 4 drive for RAID6.

What is silly on that? :-)
IMHO if you need redundancy, you can simply use RAID1 on the three
drives, and then you can add more drives to the array.

I'm not sure what is your real intention here, why do you insist on
RAID6, you said you'd like to add another drive maybe later to the
array, but I'm not sure if RAID6 even supports adding new drives
(while growing its size). As far as I tested, it reports error: mdadm:
can only add devices to linear arrays (I assume RAID6 is not a linear
array)

Tomas M

Re: Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Krzysztof Adamski]

On Wed, 2014-01-01 at 08:15 +0100, Tomas M wrote:
> >> RAID Level 6 requires a minimum of 4 drives to implement
> >>
> > I have seen messages by Neil from 2011 that he was planning to remove
> > this silly minimum requirement of 4 drive for RAID6.
> 
> What is silly on that? :-)

Because it is silly, RAID6 definition is DATA + 2 PARITY. So DATA can be
1 drive.

> IMHO if you need redundancy, you can simply use RAID1 on the three
> drives, and then you can add more drives to the array.

Yes. I can do 3 drive RAID1. I guess this is what I will have to do for
now.

> 
> I'm not sure what is your real intention here, why do you insist on
> RAID6, you said you'd like to add another drive maybe later to the
> array, but I'm not sure if RAID6 even supports adding new drives
> (while growing its size). As far as I tested, it reports error: mdadm:
> can only add devices to linear arrays (I assume RAID6 is not a linear
> array)

My real requirement is having 2 parity drives, so the array can survive
2 drive failure. This is why I insist on RAID6. On other systems I have
added drives to RAID5 and RAID6 without any problems, this is the beauty
of Linux RAID. Neil has done an incredible job with it.

As far as adding more drives, I have 5 drive hot swap bay currently, 2
drives are the old drives and 3 are new drives. I need to migrate to the
new drives before I add more drives. Also I don't have a 4th drive yet
and also I don't need the space yet.

> Tomas M
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Re: Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Pieter De Wit]

On 1/01/2014 17:54, Krzysztof Adamski wrote:
> On Wed, 2014-01-01 at 14:02 +1300, Pieter De Wit wrote:
>> On 1/01/2014 13:51, Krzysztof Adamski wrote:
>>> <snip>
>> Hi,
>>
>> RAID Level 6 requires a minimum of 4 drives to implement
>>
>> Cheers,
>>
>> Pieter
> I have seen messages by Neil from 2011 that he was planning to remove
> this silly minimum requirement of 4 drive for RAID6.
>
>
IMHO, that "silly minimum requirement" *is* RAID6, 2xDATA, 2xPARITY. If 
you make it 1 DATA, then you might as well have RAID1 with a hot spare 
(not even sure how the calc's will work - for RAID6 like that, RAID1 is 
easy :) )

I would use RAID5 for now, then migrate to RAID6 when you get a forth 
drive. IIRC, you can always upgrade "up", not down (RAID numbers that is)

Cheers,

Pieter

Re: Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Krzysztof Adamski]

On Wed, 2014-01-01 at 23:33 +1300, Pieter De Wit wrote:
> On 1/01/2014 17:54, Krzysztof Adamski wrote:
> > On Wed, 2014-01-01 at 14:02 +1300, Pieter De Wit wrote:
> >> On 1/01/2014 13:51, Krzysztof Adamski wrote:
> >>> <snip>
> >> Hi,
> >>
> >> RAID Level 6 requires a minimum of 4 drives to implement
> >>
> >> Cheers,
> >>
> >> Pieter
> > I have seen messages by Neil from 2011 that he was planning to remove
> > this silly minimum requirement of 4 drive for RAID6.
> >
> >
> IMHO, that "silly minimum requirement" *is* RAID6, 2xDATA, 2xPARITY. If 
> you make it 1 DATA, then you might as well have RAID1 with a hot spare 
> (not even sure how the calc's will work - for RAID6 like that, RAID1 is 
> easy :) )

Based of what you are saying you probably would not believe that RAID5
can be made with 2 drives, DATA + 1 PARITY. But Linux RAID supports that
just fine (thanks Neil).

Hot spares are a joke, you have to wait for a drive to fail before you
use them, that can be to late. A 3 drive RAID1 make more sense. On one
system I have a 8 drive RAID1 (the /boot partition) the / (root)
partition is a 8 drive RAID6.

> I would use RAID5 for now, then migrate to RAID6 when you get a forth 
> drive. IIRC, you can always upgrade "up", not down (RAID numbers that is)

Using RAID5 defeats the purpose of having 2 parity drives.

If you want to know why I'm so paranoid, if that I had 5 enterprise
series drives fail within a month period of each other. I didn't lose
any data, but from now I don't trust any drive and I expect triple
redundancy. 

> Cheers,
> 
> Pieter
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Re: Looking for help how to migrate 2disk RAID1 to 3 disk RAID6

[Pieter De Wit]

On 2/01/2014 04:10, Krzysztof Adamski wrote:
> On Wed, 2014-01-01 at 23:33 +1300, Pieter De Wit wrote:
>
> Based of what you are saying you probably would not believe that RAID5
> can be made with 2 drives, DATA + 1 PARITY. But Linux RAID supports that
> just fine (thanks Neil).
Yeah - you right there - I can't see why you would have a "RAID1 set 
with parity", nor can I see how the calculations would work. Then again, 
I am not a MD developer and I am not going to look at the source code. I 
would like to know what the command line would look like to create this, 
for educational purposes.
> Hot spares are a joke, you have to wait for a drive to fail before you
> use them, that can be to late. A 3 drive RAID1 make more sense. On one
> system I have a 8 drive RAID1 (the /boot partition) the / (root)
> partition is a 8 drive RAID6.
>
>
IMHO, you have answered your question of what RAID level you need to 
run, RAID1. As for your original question of the data migration, I would 
move the data over on a filesystem level. I used to have this neat trick 
where by I created soft symlink between the "new" and "old" locations, 
from memory, it only works between different block devices. You then 
instruct linux to copy the data to "itself". Since it's a symlink it 
copies the data to the new device. It was something along those lines. 
You need to mount the source as read only, iirc. Contact me off list if 
you want more details.

Cheers,

Pieter