Re: Client Handling of Elastic Clusters

Timothy Orme <torme@xxxxxxxxxxxx> · Wed, 16 Oct 2019 19:29:24 +0000

I did explore CEPH a bit, and that might be an option as well, still doing exploration on gluster.   Hopefully no one hates you for making the suggestion
🙂

I haven't tried NFS Ganesha yet.  I was under the impression it was maybe a little unstable yet, and found the docs a little limited for it.  If that solves the issue that might also be a good option.  I've heard others suggest performance is better for it
 than the FUSE client as well.

I don't know how other systems deal with it currently, but it seems like even just leveraging the volfile itself as a source for backups would work well.  There are still likely issues where things could lapse, but that seems like an improvement at least. 
 I'll try and dig into what other's are using, though maybe they don't have this issue at all since they tend to use metadata servers?

Thanks,

Tim

From: Strahil Nikolov <hunter86_bg@xxxxxxxxx>

Sent: Wednesday, October 16, 2019 11:49 AM

To: gluster-users <gluster-users@xxxxxxxxxxx>; Timothy Orme <torme@xxxxxxxxxxxx>

Subject: [EXTERNAL] Re: Re: [Gluster-users] Client Handling of Elastic Clusters

Most probably current version never supported (maybe there was no such need until now) such elasticity and the only option is to use Highly-Available NFS Ganesha as the built-in NFS is deprecated.
What about scaling on the same system ? Nowadays , servers have a lot of hot-plug disk slots and you can keep the number of servers the same ... still the server bandwidth will be a limit at some point .

I'm not sure how other SDS deal with such elasticity . I guess many users in the list will hate me for saying this , but have you checked CEPH for your needs ?

Best Regards,
Strahil Nikolov

В сряда, 16 октомври 2019 г., 21:13:58 ч. Гринуич+3, Timothy Orme <torme@xxxxxxxxxxxx> написа:

Yes, this makes the issue less likely, but doesn't make it impossible for something that is fully elastic.

For instance, if I had instead just started with A,B,C and then scaled out and in twice, all volfile servers would have potentially be destroyed and replaced.  I think the problem is that the selection of volfile servers is determined at mounting, rather than
 updating as the cluster changes.  There are ways to greatly reduce this issue, such as adding more backup servers, but it's still a possibility. 

I think more important then, for me at least, is to have the option of failing when no volfile servers are remaining as it can produce incomplete views of the data.

Thanks!

Tim

From: Strahil <hunter86_bg@xxxxxxxxx>

Sent: Tuesday, October 15, 2019 8:46 PM

To: Timothy Orme <torme@xxxxxxxxxxxx>; gluster-users <gluster-users@xxxxxxxxxxx>

Subject: [EXTERNAL] Re: [Gluster-users] Client Handling of Elastic Clusters

Hi Timothy,
Have you tried to mount on the client  via all servers :
mount -t glusterfs -o backup-volfile-servers=B:C:D:E:F A:/volume  /destination
Best Regards,

Strahil Nikolov
On Oct 15, 2019 22:05, 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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