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Old 07-25-2010, 08:01 AM   #1
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Growing a raid 5 array


I have 3 x 1.5TB drives is a raid 5 array, at the moment the storage capacity is enough, but soon I will need to expand this. I have a spare 1.5TB drive that I am thinking about adding to the array.

So far I have read mixed articles/posts, some saying it can be done, others say it can't.

From what I understand I use:
mdadm --add /dev/md0 /dev/sde1
mdadm --grow /dev/md0 --raid-devices=4

But is it really that simple? Does the existing data really stay intact?
How dangerous is this to the data?

I know I should back everything up before doing something like this, but the raid is my back up, and due to the large amount of data, I can't afford another back-up while I play with this.

Has anyone done this, did it work... and are there any pit falls I need to be aware of before I try?

Just for info, this is on a Mandriva 2010 home server, and I am using a lvm.

Any comments are more than welcome.

Old 07-25-2010, 12:21 PM   #2
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Location: Massachusetts, USA
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I don't have any firsthand experience with this. I've been told by someone I trust (our local Linux Guru) that it works. The risk, as I understand it, is that the re-striping is an extreme stress test of the drives and is the most likely circumstance for drive failures. This is why people run into data loss. They have a failed drive, they replace it, and in the re-striping they get a second drive failure. It seems to be more of a problem with larger (TB+) drives, and, if all the drives were bought from the same lot at the same time, it seems that failures can be correlated, which is bad for RAID.

In your particular case, you are not dealing with a failed drive. So, you should be more secure than if you had a failed drive and were trying to replace it.

In any case, this is why people who can't afford failure are going to RAID 6 and other options rather than relying on RAID 5 with very large drives. I have an external array configured with ZFS raidz2, which is 2 parity drives and roughly equivalent to RAID 6. With ZFS, I cannot expand a raid. I can add raids to a zpool, but not expand an individual one.
Old 07-28-2010, 05:24 AM   #3
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In future you might like to use LVM on top of RAID
Old 07-28-2010, 07:32 AM   #4
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LVM answers a different set of requirements, though. It doesn't provide the parity based redundancy of raid, nor a way of expanding the raid. For larger settings, where you have multiple raid sets, or different sized drives that can't be put together in raid, it provides a way of pooling those into larger volumes of storage. It also provides snapshots.

If you used LVM to add a drive to an existing volume containing a raid, that drive would not have any inherent parity redundancy. Just wanted to be clear on that. I don't know how resilient LVM is when a component fails. I've never been able to get a clear answer on that for zpools in zfs either. But, if the drive fails, you will lose data.


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