migrate single disk to RAID 1?

migrate single disk to RAID 1?

[Ben Beuchler]

I'd like to convert an existing partition to RAID 1 using a portion of
a new, blank drive.  All of the examples I've seen involve creating
the RAID device with the *empty* partition (sda4), copying the data
from the old partition to the new RAID, then adding the old partition
to the RAID set.

From my modest understanding of how linux software RAID works, it
seems I should be able to take an existing partition containing data
(in this case sdb4 mounted as /mail) and build a RAID 1 array with
something similar to this:

umount /dev/sdb4
mdadm --build /dev/md0 --level=1 -n2 /dev/sdb4 missing
mount /dev/md0 /mail
mdadm --add /dev/md0 /dev/sda4

Will that work?  Or am I misunderstanding something fundamental?

Thanks!

-Ben

Re: migrate single disk to RAID 1?

[John Robinson]

On 14/07/2009 08:16, Ben Beuchler wrote:
> I'd like to convert an existing partition to RAID 1 using a portion of
> a new, blank drive.  All of the examples I've seen involve creating
> the RAID device with the *empty* partition (sda4), copying the data
> from the old partition to the new RAID, then adding the old partition
> to the RAID set.
> 
>>From my modest understanding of how linux software RAID works, it
> seems I should be able to take an existing partition containing data
> (in this case sdb4 mounted as /mail) and build a RAID 1 array with
> something similar to this:
> 
> umount /dev/sdb4
> mdadm --build /dev/md0 --level=1 -n2 /dev/sdb4 missing
> mount /dev/md0 /mail
> mdadm --add /dev/md0 /dev/sda4
> 
> Will that work?  Or am I misunderstanding something fundamental?

It'll work but you'll be using a legacy array without superblocks, so 
you can't have various features that come with superblocks like being 
recognisable or having a write-intent bitmap, which is why most guides 
suggest you --create an array with a blank partition then copy the data. 
The man page says of build mode:
   Build an array that doesn’t  have  per-device  superblocks.
   For  these  sorts  of  arrays,  mdadm  cannot differentiate
   between initial creation  and  subsequent  assembly  of  an
   array.   It also cannot perform any checks that appropriate
   components have been requested.  Because of this, the Build
   mode  should  only  be used together with a complete under-
   standing of what you are doing.

Cheers,

John.
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Re: migrate single disk to RAID 1?

[berk walker]

John Robinson wrote:
> On 14/07/2009 08:16, Ben Beuchler wrote:
>> I'd like to convert an existing partition to RAID 1 using a portion of
>> a new, blank drive.  All of the examples I've seen involve creating
>> the RAID device with the *empty* partition (sda4), copying the data
>> from the old partition to the new RAID, then adding the old partition
>> to the RAID set.
>>
>>> From my modest understanding of how linux software RAID works, it
>> seems I should be able to take an existing partition containing data
>> (in this case sdb4 mounted as /mail) and build a RAID 1 array with
>> something similar to this:
>>
>> umount /dev/sdb4
>> mdadm --build /dev/md0 --level=1 -n2 /dev/sdb4 missing
>> mount /dev/md0 /mail
>> mdadm --add /dev/md0 /dev/sda4
>>
>> Will that work?  Or am I misunderstanding something fundamental?
>
> It'll work but you'll be using a legacy array without superblocks, so 
> you can't have various features that come with superblocks like being 
> recognisable or having a write-intent bitmap, which is why most guides 
> suggest you --create an array with a blank partition then copy the 
> data. The man page says of build mode:
>   Build an array that doesn’t  have  per-device  superblocks.
>   For  these  sorts  of  arrays,  mdadm  cannot differentiate
>   between initial creation  and  subsequent  assembly  of  an
>   array.   It also cannot perform any checks that appropriate
>   components have been requested.  Because of this, the Build
>   mode  should  only  be used together with a complete under-
>   standing of what you are doing.
>
> Cheers,
>
> John.
> .
I think that after a --create, missing on the new drive, you would want 
to copy the contents of the original drive ,. Then add the original to 
the raid..
berk


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Re: migrate single disk to RAID 1?

[Ben Beuchler]

>> umount /dev/sdb4
>> mdadm --build /dev/md0 --level=1 -n2 /dev/sdb4 missing
>> mount /dev/md0 /mail
>> mdadm --add /dev/md0 /dev/sda4
>>
>> Will that work?  Or am I misunderstanding something fundamental?
>
> It'll work but you'll be using a legacy array without superblocks, so you
> can't have various features that come with superblocks

Right; for this particular application I'm willing to give up
superblocks.  Just to be 100% sure, thought:  the way I've described
the process would be non-destructive of the data on sdb4?

Thanks!

-Ben
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Re: migrate single disk to RAID 1?

[Robin Hill]

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On Tue Jul 14, 2009 at 10:44:15AM -0500, Ben Beuchler wrote:

> >> umount /dev/sdb4
> >> mdadm --build /dev/md0 --level=1 -n2 /dev/sdb4 missing
> >> mount /dev/md0 /mail
> >> mdadm --add /dev/md0 /dev/sda4
> >>
> >> Will that work?  Or am I misunderstanding something fundamental?
> >
> > It'll work but you'll be using a legacy array without superblocks, so you
> > can't have various features that come with superblocks
> 
> Right; for this particular application I'm willing to give up
> superblocks.  Just to be 100% sure, thought:  the way I've described
> the process would be non-destructive of the data on sdb4?
> 
Yes - there'll be no need to write to sdb4 at all.  Be aware that,
without a superblock, the array will need to do a full resync every
single time you start it (unless you use the --assume-clean switch, in
which case you could be risking corruption).

Cheers,
    Robin
-- 
     ___        
    ( ' }     |       Robin Hill        <robin@robinhill.me.uk> |
   / / )      | Little Jim says ....                            |
  // !!       |      "He fallen in de water !!"                 |

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Re: migrate single disk to RAID 1?

[Michal Soltys]

Ben Beuchler wrote:
>>> umount /dev/sdb4
>>> mdadm --build /dev/md0 --level=1 -n2 /dev/sdb4 missing
>>> mount /dev/md0 /mail
>>> mdadm --add /dev/md0 /dev/sda4
>>>
>>> Will that work?  Or am I misunderstanding something fundamental?
>>
>> It'll work but you'll be using a legacy array without superblocks, so you
>> can't have various features that come with superblocks
> 
> Right; for this particular application I'm willing to give up
> superblocks.  Just to be 100% sure, thought:  the way I've described
> the process would be non-destructive of the data on sdb4?
> 
> Thanks!

Actually, if on that sdb4 is a shrinkable filesystem, you can create md 
raid with superblock at the end. So the process would be something like:

- resize2fs filesystem on sdb4 a bit (shrink)
- mdadm stuff with e.g. 1.0 superblock
- resize2fs filesystem again, this time on md0

According to md(4) you shouldn't need to shrink more than 128KB, but be 
sure to get a healthy margin. Or do simple test with some tiny 
partition, or md-on-lvm.