CentOS - Jul 2014 - Convert "bare partition" to RAID1 / mdadm?

I have a large disk full of data that I'd like to upgrade to SW RAID 1 
with a minimum of downtime. Taking it offline for a day or more to rsync 
all the files over is a non-starter. Since I've mounted SW RAID1 drives 
directly with "mount -t ext3 /dev/sdX" it would seem possible to flip 
the process around, perhaps change the partition type with fdisk or 
parted, and remount as SW RAID1?

I'm not trying to move over the O/S, just a data paritition with LOTS of 
data. So far, Google pounding has resulted in howtos like this one 
that's otherwise quite useful, but has a big "copy all your data
over"
step I'd like to skip:

http://sysadmin.compxtreme.ro/how-to-migrate-a-single-disk-linux-system-to-software-raid1/

But it would seem to me that a sequence roughly like this should work 
without having to recopy all the files.

1) umount /var/data;
2) parted /dev/sdX
     (change type to fd - Linux RAID auto)
3) Set some volume parameters so it's seen as a RAID1 partition 
"Degraded". (parted?)
4) ??? Insert mdadm magic here ???
5) Profit! `mount /dev/md1 /var/data`

Wondering if anybody has done anything like this before...

-Ben

On Thu, Jul 24, 2014 at 7:11 PM, Lists <lists at benjamindsmith.com>
wrote:
> I have a large disk full of data that I'd like to upgrade to SW RAID 1
> with a minimum of downtime. Taking it offline for a day or more to rsync
> all the files over is a non-starter. Since I've mounted SW RAID1 drives
> directly with "mount -t ext3 /dev/sdX" it would seem possible to
flip
> the process around, perhaps change the partition type with fdisk or
> parted, and remount as SW RAID1?
>
> I'm not trying to move over the O/S, just a data paritition with LOTS
of
> data. So far, Google pounding has resulted in howtos like this one
> that's otherwise quite useful, but has a big "copy all your data
over"
> step I'd like to skip:
>
>
http://sysadmin.compxtreme.ro/how-to-migrate-a-single-disk-linux-system-to-software-raid1/
>
> But it would seem to me that a sequence roughly like this should work
> without having to recopy all the files.
>
> 1) umount /var/data;
> 2) parted /dev/sdX
>      (change type to fd - Linux RAID auto)
> 3) Set some volume parameters so it's seen as a RAID1 partition
> "Degraded". (parted?)
> 4) ??? Insert mdadm magic here ???
> 5) Profit! `mount /dev/md1 /var/data`
>
> Wondering if anybody has done anything like this before...
>
Even if I found the magic place to change to make the drive think it
was a raid member, I don't think I would trust getting it right with
my only copy of the data.  Note that you don't really have to be
offline for the full duration of an  rysnc to copy it.  You can add
another drive as a raid with a 'missing' member, mount it somewhere
and rsync with the system live to get most of the data over.  Then you
can shut down all the applications that might be changing data for
another rsync pass to pick up any changes - and that one should be
fast.   Then move the raid to the real mount point and either (safer)
swap a new disk, keeping the old one as a backup or (more dangerous)
change the partition type on the original and add it into the raid set
and let the data sync up.

-- 
   Les Mikesell
     lesmikesell at gmail.com

On Fri, Jul 25, 2014 at 5:41 AM, Lists <lists at benjamindsmith.com>
wrote:
> I have a large disk full of data that I'd like to upgrade to SW RAID 1
> with a minimum of downtime. Taking it offline for a day or more to rsync
> all the files over is a non-starter. Since I've mounted SW RAID1 drives
> directly with "mount -t ext3 /dev/sdX" it would seem possible to
flip
> the process around, perhaps change the partition type with fdisk or
> parted, and remount as SW RAID1?
>
> I'm not trying to move over the O/S, just a data paritition with LOTS
of
> data. So far, Google pounding has resulted in howtos like this one
> that's otherwise quite useful, but has a big "copy all your data
over"
> step I'd like to skip:
>
>
http://sysadmin.compxtreme.ro/how-to-migrate-a-single-disk-linux-system-to-software-raid1/
For data partitions a lot of the stuff is not  applicable.

With respect to the madam steps, creating degraded arrays, filesystem
on those degraded arrays and then copy over the data etc. is spot on
IMO.

I would recommend the steps in the above tutorial to really be assured
that none of data is corrupted.
>
> But it would seem to me that a sequence roughly like this should work
> without having to recopy all the files.
>
> 1) umount /var/data;
> 2) parted /dev/sdX
>      (change type to fd - Linux RAID auto)
> 3) Set some volume parameters so it's seen as a RAID1 partition
> "Degraded". (parted?)
> 4) ??? Insert mdadm magic here ???
> 5) Profit! `mount /dev/md1 /var/data`
>
> Wondering if anybody has done anything like this before...
>
'mdadm' starts initializing the array (writing on the disk),
overwriting your file system on that partition.

I would not recommend it but you can try it and see what happens with
your experiment.  Should be a no brainer since you have secondary back
ups of the data elsewhere (stated in this thread).

-- Arun Khan