[Gluster-users] Client Handling of Elastic Clusters

[Timothy Orme]

Hello,

I'm trying to setup an elastic gluster cluster and am running into a few odd edge cases that I'm unsure how to address.  I'll try and walk through the setup as best I can.

If I have a replica 3 distributed-replicated volume, with 2 replicated volumes to start:

MyVolume
   Replica 1
      serverA
      serverB
      serverC
   Replica 2
      serverD
      serverE
      serverF

And the client mounts the volume with serverA as the primary volfile server, and B & C as the backups.

Then, if I perform a scale down event, it selects the first replica volume as the one to remove.  So I end up with a configuration like:

MyVolume
   Replica 2
      serverD
      serverE
      serverF

Everything rebalances and works great.  However, at this point, the client has lost any connection with a volfile server.  It knows about D, E, and F, so my data is all fine, but it can no longer retrieve a volfile.  In the logs I see:

[2019-10-15 17:21:59.232819] I [glusterfsd-mgmt.c:2463:mgmt_rpc_notify] 0-glusterfsd-mgmt: Exhausted all volfile servers

This becomes problematic when I try and scale back up, and add a replicated volume back in:

MyVolume
   Replica 2
      serverD
      serverE
      serverF
   Replica 3
      serverG
      serverH
      serverI

And then rebalance the volume.  Now, I have all my data present, but the client only knows about D,E,F, so when I run an `ls` on a directory, only about half of the files are returned, since the other half live on G,H,I which the client doesn't know about.  The data is still there, but it would require a re-mount at one of the new servers.

My question then, is there a way to have a more dynamic set of volfile servers? What would be great is if there was a way to tell the mount to fall back on the servers returned in the volfile itself in case the primary one goes away.

If there's not an easy way to do this, is there a flag on the mount helper that can cause the mount to die or error out in the event that it is unable to retrieve volfiles?  The problem now is that it sort of silently fails and returns incomplete file listings, which for my use cases can cause improper processing of that data.  I'd rather have it hard error than provide bad results silently obviously.

Hope that makes sense, if you need further clarity please let me know.

Thanks,
Tim


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