[Gluster-users] 2 nodes volume with existing data

Atin Mukherjee amukherj at redhat.com
Tue May 17 16:32:55 UTC 2016

On 05/17/2016 01:59 PM, CHEVALIER Pierre wrote:
> Hi everyone,
> I have found a similar question in the ML but didn't find the answer I
> wanted.
> My situation is  the following :
> We have 2 existing data storage unit with 72TB each, with 10TB left.
> Currently these storage unit are synced using rsync periodically. We
> want to tansform these 2 nodes into glusterfs bricks without loosing any
> data or deleting a node.
> I was thinking setting up the 2 nodes with glusterfs in replica this way :
> 1) Rsync server1/brick1 and server2/brick2 content
> 2) Create the gluster replica and start :
> gluster volume create volume1 replica 2 transport tcp server1:/brick1
> server2:/brick1
> gluster volume start volume1
Gluster heavily depends on extended attributes for its business logic
and hence configuring a brick with existing data means that the extended
attributes would not be set for these files and hence this may not work
as per the expectation.

Given that you have a restriction using another brick, I'd suggest you
to do the following:

1. Delete all the contents of server2/brick as this is a secondary copy
2. Create a plain distributed volume in server2 with server2/brick1
2. Start the volume
3. Mount the volume and then do copy the content from server1/brick1 to
the mount point.
4. Probe server2 into the cluster
5. Perform an add-brick with server1/brick by gluster volume add-brick
<volname> replica 2 <brick name> (Again make sure the content is deleted
6. This should trigger the self heal

However since the data volume is very high, self heal can take a longer
time to sync and finish.

Also with a two node setup prevention of split brains is not guaranteed.
An arbiter volume or a 3 way replica would be the solution to tackle
this problem.

Since I am not an AFR expert, I'd like to get a sign off from AFR team
(In cc) before you try this out.

> Then, should I run a healing process, or stat each files in order to
> self heal ?
> What happens if there is a slight difference between the 2 nodes,
> mathematically if :
> server1/brick1= server2/brick1+delta
> Will the delta part impact server1, or will it be lost  ?
> Maybe I worry for nothing, but as these 2 nodes host important data, I
> wanted to make sure that eveything is OK.
> Thanks for your help !
> -- 
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