Re: migrate single disk to RAID 1?

[berk walker <berk@panix.com>]

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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