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Can my md-RAID setup be grown safely?
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Jarjar
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Location: Sweden

PostPosted: Wed Dec 07, 2011 7:41 pm    Post subject: Can my md-RAID setup be grown safely? Reply with quote

I have a 3-disk RAID5 array managed by mdadm, with an XFS filesystem on top.
However, unlike many, I don't run LVM or such; I don't even have a partition table on the array, but rather a filesystem.
In other words, I didn't do:
mdadm --create ...
cfdisk /dev/md0
...
but rather did:
mdadm --create ...
mkfs.xfs /dev/md0
mount /dev/md0 /...

Can a setup like this be grown safely? I'd add a fourth partition (same size as the existing, of course), grow the array, then use xfs_growfs - AFAIK, that is.
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wildbug
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Joined: 07 Oct 2007
Posts: 73

PostPosted: Fri Dec 09, 2011 5:28 pm    Post subject: Reply with quote

Yes, you should be able to do this by adding the new disk as a hot-spare, growing the array, then growing the filesystem.

Code:
mdadm /dev/md0 --add /dev/sde
mdadm --grow /dev/md0 -n4
...
xfs_growfs /your-xfs-mount-point


I just double-checked this in a VM, and the following was successful:
Code:
(Create the test array)

# mdadm --create /dev/md0 --level=5 -n3 /dev/sd[bcd]
# mkfs.xfs /dev/md0
# mount /dev/md0 /mnt
# df -h /mnt
Filesystem   Size  Used  Avail Use%  Mounted on
/dev/md0     394M  4.4M  390M  2%    /mnt

(Add the new drive)

# mdadm /dev/md0 --add /dev/sde
# mdadm --grow /dev/md0 -n4
# xfs_growfs /mnt
# df -h /mnt
Filesystem   Size  Used  Avail Use%  Mounted on
/dev/md0     595M  4.6M  590M  1%    /mnt
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Jarjar
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PostPosted: Fri Dec 09, 2011 6:01 pm    Post subject: Reply with quote

Awesome, thanks for testing and everything! Amidst all the stress of yesterday's troubleshooting, I didn't even think about testing it in a VM (despite having a clone of that computer as a VM!).
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drescherjm
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PostPosted: Sat Dec 10, 2011 5:37 pm    Post subject: Reply with quote

I have grown mdadm arrays a dozen (mostly raid6) or so times at work without problems.
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Jarjar
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PostPosted: Sun Dec 11, 2011 5:59 pm    Post subject: Reply with quote

In case someone finds this by searching:
I just finished this process, and it worked great for me as well. Adding the disk took a long time (a bit over 20 hours), but xfs_growfs finished in 1.9 seconds!
Capacity increased as expected (by 50% despite going from 3->4 disks, a very nice side effect of RAID5).
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Mad Merlin
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PostPosted: Wed Dec 14, 2011 12:26 am    Post subject: Reply with quote

FWIW, non-partitioned mdadm arrays are actually quite common. It's far more common to have /dev/md{0..n} than a single /dev/md0 with partitions inside it.
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