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Old 06-16-2010, 11:37 AM   #1
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RAID1 with 3 live drives?

I'm not entirely a newbie, but this seems like such a simple question I'm not sure where else to ask it. I checked through the various HOWTOs and searched already and didn't find a clear answer, and I want to know for sure before we start investing in hardware.

Is is possible to create a RAID1 (mirroring only) array with 3 live drives, rather than with 2 live plus a spare? Our goal is to have 3 drives in a hot-swap bay, and be able to pull and replace one drive periodically as a full backup. If I do:
mdadm --create /dev/md0 --level=1 --raid-devices=2 --spare-devices=1 /dev/sda1 /dev/sdb1 /dev/sdc1
then I assume that only sda1 and sdb1 will be mirrored, and sdc1 will just "hang out" until it is needed, but won't get any data on it. Furthermore, only someone who knows how to read /proc/mdstat will be able to tell which drive they can pull to get the current data.

What I believe we want to do is:
mdadm --create /dev/md0 --level=1 --raid-devices=3 /dev/sda1 /dev/sdb1 /dev/sdc1
but I can't see anyone actually giving this example as valid. It would be dumb to invest in the hardware and then discover that it doesn't work that way, right?

Additionally, we right now have the mirror set up with just two of the drives, like this:
mdadm --create /dev/md0 --level=1 --raid-devices=2 /dev/sda1 /dev/sdb1
Is it then possible to say:
mdadm --grow /dev/md0 --raid-devices=3
mdadm --manage /dev/md0 --add /dev/sdc1
to safely add the third device, and will it ensure that all three devices are mirroring the way we want? The manual pages say something about "only on kernels which provide necessary support" but doesn't mention if 2.6.12-1 (FC3) is one that does.

I'm hoping there is someone who has done this and can say "yep, it works."

Old 06-16-2010, 12:03 PM   #2
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Yes it works, but I've never done it myself.
You can have n mirrors in a mirrored array.
This doesn't specifically say it can be done but it can.

Search for n-way mirroring.

BTW, FC3 is OLD.


Last edited by smoker; 06-16-2010 at 12:07 PM.
1 members found this post helpful.
Old 06-16-2010, 06:25 PM   #3
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smoker sez:
BTW, FC3 is OLD.
Yeah. This server has been running since FC3 was new. I've tried some newer versions since then (on home machines, etc.) and everything just seems to keep going downhill. It is now moving to replace a machine that was running RH6.2.

We like stability around here, rather than new features. If it had been around at the time, I might have used one of the Ubuntu LTS releases, but I didn't know about them back then.



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