[Gluster-users] How Does Gluster Failover

[Joe Julian]

With a replica volume the client connects and writes to all the replicas 
directly. For reads, when a filename is looked up the client checks with 
all the replicas and, if the file is healthy, opens a read connection to 
the first replica to respond (by default).

If a server is shut down, the client receives the tcp messages that 
close the connection. For read operations, it chooses the next server. 
Writes will just continue to the remaining replicas (metadata is stored 
in extended attributes to inform future lookups and the self-healer of 
file health).

If a server crashes (no tcp finalization) the volume will pause for 
ping-timeout seconds (42 by default). Then continue as above. BTW, that 
42 second timeout shouldn't be a big deal. The MTBF should be 
sufficiently far apart that this should still easily get you five or six 
nines.

On 8/30/22 11:55 PM, duluxoz wrote:
>
> Hi Guys & Gals,
>
> A Gluster newbie question for sure, but something I just don't "get" 
> (or I've missed in the doco, mailing lists, etc):
>
> What happens to a Gluster Client when a Gluster Cluster Node goes 
> off-line / fails-over?
>
> How does the Client "know" to use (connect to) another Gluster Node in 
> the Gluster Cluster?
>
> Let me elaborate.
>
> I've got four hosts: gfs1, gfs2, gfs3, and client4 sitting on 
> 192.168.1.1/24, .2, .3, and .4 respectively.
>
> DNS is set up and working correctly.
>
> gfs1, gs2, and gfs3 form a "Gluster Cluster" with a Gluster Volume 
> (gv1) replicated across all three nodes. This is all working correctly 
> (ie a file (file1) created/modified on gfs1:/gv1 is replicated 
> correctly to gfs2:/gv1 and gfs3:/gv1).
>
> client4 has an entry in its /etc/fstab file which reads: "gfs1:gv1  
> /data/gv1 glusterfs  defaults  0 2". This is also all working 
> correctly (ie client4:/data/gv1/file1 is accessible and replicated).
>
> So, (and I haven't tested this yet) what happens to 
> client4:/data/gv1/file1 when gfs1 fails (ie is turned off, crashes, etc)?
>
> Does client4 "automatically" switch to using one of the other two 
> Gluster Nodes, or do I have something wrong in clients4's /etc/fstab 
> file, or an error/mis-configuration somewhere else?
>
> I thought about setting some DNS entries along the lines of:
>
> ~~~
>
> glustercluster  IN  A 192.168.0.1
>
> glustercluster  IN  A  192.168.0.2
>
> glustercluster IN  A  192.168.0.3
>
> ~~~
>
> and having clients4's /etc/fstab file read: "glustercluster:gv1 
> /data/gv1  glusterfs  defaults  0 2", but this is a Round-Robin DNS 
> config and I'm not sure how Gluster treats this situation.
>
> So, if people could comment / point me in the correct direction I 
> would really appreciate it - thanks.
>
> Dulux-Oz
>
>
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>
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