[Gluster-users] Gluster FS Native Client Behaviour (3.7)

Ravishankar N ravishankar at redhat.com
Wed Feb 3 15:53:39 UTC 2016

On 02/03/2016 08:03 PM, Peter Spain wrote:
> Hello
> I am very new to GlusterFS and have been playing around with over the 
> last few weeks, with a view to using it in production. So far I found 
> Gluster to be very interesting and easy to get along with. However, 
> there seems to be a giant hole where the Gluster Native Client 
> documentation should live.
> After using it for a few weeks and playing around (inside various VMs) 
> I am still not entirely sure how the client behaves.
> From network captures it is clear that the client communicates to all 
> the nodes for a particular volume, and that the client gets this 
> information from a volfile (which it retrieves when mounting a 
> volume). Various blog posts confirm this and go on to mention that the 
> client is responsible for replicating data across nodes, and not the 
> nodes themselves. I assume this is still the case?
That is correct. The client volfile dictates what translators are loaded 
on the client process. You can look at the various volfiles in 
/var/lib/glusterd/vols/<volname> on the server to get an idea of what 
xlators are loaded for different processes and how they are stacked to 
form a graph. The graph is also printed in the log files of each process 
as it starts. Each xlator does a specific function. The replication 
xlator (AFR) is a client xlator that is loaded (among others) on the 
client graph in case of replicated volumes and has the replication logic.
> Beyond that I really have no idea how the client behaves in a 
> replicated volume.
If you specifically want to know how AFR works, you can see 
It is a bit dated but the most of it is still valid.

> My questions are:
> There is a "ping-timeout" option to adjust how long it takes the 
> client to connect to a different node, in case of node failure.
Actually, it is the time duration for which the client waits to check if 
the server is responsive. (see `gluster volume help`), even after 
connection is established.
> If the client knows about all nodes and actively communicates with all 
> of them, why does it need a time out at all?
You would need some sort of a heartbeat for the clients to know that its 
connection to servers are still intact or lost because a brick went down 
or there was a network disconnect etc.
> Why does the client "stick" to a particular node?
Not sure what you mean. The client process connects to all the bricks of 
the volume. Open one of the client volfiles (or the logfile) and study 
the graph, you'll see many 'protocol/client' xlators, one for each brick 
that the client needs to talk to.
> Does the client go back to the original node once it recovers?
> Is it possible to dictate which node a client will initially connect 
> to on mounting a volume?
Like I said, the client connects to all bricks of the volume.
> If all this information is contained in some documentation I would 
> love to be pointed to it, as so far I cannot find the answer to these 
> questions.
There are many presentations you can find online on glusterfs 
architecture. https://gluster.readthedocs.org/en/latest/ is another good 

Hope that helps,
> Regards
> Peter
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