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I have a Unix server running with 2 disks with the same capacity, but 1 is being used only for backups storage.
I would like to temporarily use that secondary disk as a mirror of the first, and move the backups to the main disk (which will obviously then be mirrored to the secondary disk too).

Is this a good idea? If yes, I am just not sure on how to install the RAID 1 (as software) to start using the secondary disk as a mirror of the first. Could you please help me here?

Thank you!

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up vote 1 down vote accepted

Maybe this would be a starting point?

You will lose all the data on the second drive. Make sure you have backups of the system first, in case something goes wrong, then move the data from disk two to disk one, and see if you can create the RAID volume.

Normally it's easier in logistics, to make sure things go smoothly, to make a backup and then wipe the volumes and create the RAID from a clean slate then restore your data to the new volume, but you might be able to create it without accidentally wiping the drives. Make backups first.

Otherwise Google for "linux software raid 1" and see what resources come up for managing mdadm.

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Migrating a running Linux install from single disks to software RAID-1 isn't typically non-trivial. It's not that hard, but it's also not just "plug in the second disk and choose 'Mirror'", either. Be absolutely sure you've got good backups (off of these disks) before you proceed. – Evan Anderson Oct 21 '10 at 18:40
I'd personally not even try moving a live server like that. Asking for too much chance of loss of data. One wrong partition number and gone. That's why I prefer the approach of back up, wipe, create the RAID, format, restore data... – Bart Silverstrim Oct 21 '10 at 18:54
@Bart I migrated live systems from single disk to RAID1 about 4 or 5 times, once remotely. While I did sometimes have to reboot them few times to get going I never lost a single bit of data during that process. Besides, you should have a good backup at all times. – Hubert Kario Oct 21 '10 at 20:17
I didn't say it wasn't possible, just that personally I wouldn't want to take that risk. It just seems like a cleaner procedure to clean it up and do a "restore" install. And of course you need backups :-) – Bart Silverstrim Oct 21 '10 at 20:55
Thank you Bart. My server had already too much downtimes, I would love if I could avoid one more. Things are being stabler now, one more downtime would affect everything again. That article is pretty useful! Thank you again! – Nuno Peralta Oct 21 '10 at 21:37

You can install a RAID volume utility that will do the mirroring during runtime, but to get there you'll need to copy the entire disk (not only files but the whole disk in raw like with dd command in unix).

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Thank you for your hint :) – Nuno Peralta Oct 21 '10 at 21:34

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