[Gluster-users] How Does Gluster Failover

Péter Károly JUHÁSZ
Wed, 31 Aug 2022 15:26:44 -0000

You can also add the mount option: backupvolfile-server to let the client
know the other nodes.

Matthew J Black <duluxoz at gmail.com> ? 2022?8?31??? 17:21???

> Ah, it all now falls into place: I was unaware that the client receives
> that file upon initial contact with the cluster, and thus has that
> information at hand independently of the cluster nodes.
>
> Thank you for taking the time to educate a poor newbie - it is very much
> appreciated.
>
> Cheers
>
> Dulux-Oz
> On 01/09/2022 01:16, Joe Julian wrote:
>
> You know when you do a `gluster volume info` and you get the whole volume
> definition, the client graph is built from the same info. In fact, if you
> look in /var/lib/glusterd/vols/$volume_name you'll find some
".vol" files.
> `$volume_name.tcp-fuse.vol` is the configuration that the clients receive
> from whichever server they initially connect to. You'll notice that
file
> has multiple "type/client" sections, each establishing a tcp
connection to
> a server.
>
> Sidenote: You can also see in that file, how the microkernels are used to
> build all the logic that forms the volume, which is kinda cool. Back when I
> first started using gluster, there was no glusterd and you have to build
> those .vol files by hand.
> On 8/31/22 8:04 AM, Matthew J Black wrote:
>
> Hi Joe,
>
> Thanks for getting back to me about this, it was helpful, and I really
> appreciate it.
>
> I am, however, still (slightly) confused - *how* does the client
"know"
> the addresses of the other servers in the cluster (for read or write
> purposes), when all the client has is the line in the fstab file:
"gfs1:gv1
> /data/gv1  glusterfs  defaults  0 2"? I'm missing something,
somewhere,
> in all of this, and I can't work out what that "something"
is.  :-)
>
> Your help truely is appreciated
>
> Cheers
>
> Dulux-Oz
> On 01/09/2022 00:55, Joe Julian wrote:
>
> 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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