Re: Client Handling of Elastic Clusters

Timothy Orme <torme@xxxxxxxxxxxx> · Wed, 16 Oct 2019 18:41:03 +0000

Yeah there are somewhat dirty ways to work around it, and I hadn't thought of this one.  Another option for us is to try and tag certain instances as volfile servers, and always prevent the autoscaler from removing them.  It would be nice though if this behavior
 could be added to gluster itself as in cloud environments we don't typically rely on having hostnames.  I think I'd also sleep better if it would error out if it couldn't retrieve that info.  Ill throw in the feature request.  Probably out of my element, but
 Ill also take a shot at adding in a PR for it as well.

Thanks!

Tim

From: Amar Tumballi <amarts@xxxxxxxxx>

Sent: Tuesday, October 15, 2019 8:51 PM

To: Timothy Orme <torme@xxxxxxxxxxxx>

Cc: gluster-users <gluster-users@xxxxxxxxxxx>

Subject: [EXTERNAL] Re:  Client Handling of Elastic Clusters

Hi Timothy,

Thanks for this report. This seems to be a genuine issue. I don't think we have a solution for this issue for now, other than may be making sure we point 'serverD' (or new server's IPs) as ServerA in /etc/hosts on that particular client as a hack.

Meantime, it would be great if you copy paste this in an issue (https://github.com/gluster/glusterfs/issues/new),
 it would be good to track this.

Regards,
Amar

On Wed, Oct 16, 2019 at 12:35 AM Timothy Orme <torme@xxxxxxxxxxxx> wrote:

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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