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I have a RAID5 array that consists of 3, 2TB drives. I want to add 2 more drives. The tutorials involve adding a single drive to an array, such as this:

https://raid.wiki.kernel.org/index.php/Growing

Can I add both drives at the same time? Can I do something like (going by the example in the wiki):

mdadm --add /dev/md1 /dev/sdb3 /dev/sdb4
mdadm --grow --raid-devices=5 /dev/md1

Or is it better to just add a single drive at a time?

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up vote 5 down vote accepted

Your array isn't any more vulnerable during 1 disk or 2 disk grow, there's only a small critical section in which a power outage can destroy the array.

Considering, that growing by 2 disks at once will finish much faster, I'd suggest going this way, there's no downside to it that I know of.

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Unless you find a robust documentation that clearly indicate that you can add two drives at the same time, do it - otherwise take your time and add them one at a time. Unless you have a 4 TB backup of the existing data... –  ring0 Oct 3 '10 at 18:10
    
I'm not sure where I read about the critical section, but it probably was lwn.net or kernel mailing list. –  Hubert Kario Oct 3 '10 at 21:58
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Thanks, I grew it as specified above. It took 3 days, but I'm now at 8Tb and nothing was lost. –  Geoffrey Oct 7 '10 at 2:44
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