Re: [ClusterLabs] how to setup single node cluster

[Ken Gaillot]

On Fri, 2021-04-09 at 08:20 +0300, Andrei Borzenkov wrote:
> On 08.04.2021 09:26, d tbsky wrote:
> > Reid Wahl 
> > > I don't think we do require fencing for single-node clusters.
> > > (Anyone at Red Hat, feel free to comment.) I vaguely recall an
> > > internal mailing list or IRC conversation where we discussed this
> > > months ago, but I can't find it now. I've also checked our
> > > support policies documentation, and it's not mentioned in the
> > > "cluster size" doc or the "fencing" doc.
> > 
> >since the cluster is 100% alive or 100% dead with single node, I
> > think fencing/quorum is not required. I am just curious what is the
> > usage case. since RedHat supports it, it must be useful in real
> > scenario.
> 
> 
> I do not know what "disaster recovery" configuration you have in
> mind,
> but if you intend to use geo clustering fencing can speed up fail-
> over
> so it is at least useful.
> 
> Even in single node cluster if resource failed to stop you are stuck
> -
> you cannot actually do anything from that point without manual
> intervention. Depending on configuration and requirements rebooting
> node
> may be considered as an attempt to automatically "reset" cluster
> state.

The use case for a single-node disaster recovery cluster is to have the
main cluster be a full, multi-node cluster with fencing, with a single-
node cluster at a remote site for disaster recovery when the main
cluster is down (possibly for just the most essential resources).

Fencing isn't critical for the DR site because if the DR site is being
used, the main site is already down.

The DR site could be activated automatically with booth (if a third
arbitrator site is available), or manually by an administrator (for
example by changing the target-role resource default, or manually
assigning tickets).

The advantages of using a cluster at all at a manual DR site are that
administrators can use the same cluster management commands they're
familiar with, and certain resources can always run at the DR site to
keep it ready (e.g. shared storage or a database replicant).

There are some ideas about making such a setup easier to manage, such
as being able to coordinate configuration changes (each site has a
separate cluster configuration), and maybe having "storage agents" to
manage shared storage across clusters.
-- 
Ken Gaillot 

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Re: [ClusterLabs] how to setup single node cluster

[Andrei Borzenkov]

On 08.04.2021 09:26, d tbsky wrote:
> Reid Wahl 
>> I don't think we do require fencing for single-node clusters. (Anyone at Red 
>> Hat, feel free to comment.) I vaguely recall an internal mailing list or IRC 
>> conversation where we discussed this months ago, but I can't find it now. 
>> I've also checked our support policies documentation, and it's not mentioned 
>> in the "cluster size" doc or the "fencing" doc.
> 
>since the cluster is 100% alive or 100% dead with single node, I
> think fencing/quorum is not required. I am just curious what is the
> usage case. since RedHat supports it, it must be useful in real
> scenario.


I do not know what "disaster recovery" configuration you have in mind,
but if you intend to use geo clustering fencing can speed up fail-over
so it is at least useful.

Even in single node cluster if resource failed to stop you are stuck -
you cannot actually do anything from that point without manual
intervention. Depending on configuration and requirements rebooting node
may be considered as an attempt to automatically "reset" cluster state.
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Re: [ClusterLabs] how to setup single node cluster

[Strahil Nikolov]

I always though that the setup is the same, just the node count is only one.
I guess you need pcs, corosync + pacemaker.If RH is going to support it, they 
will require fencing. Most probably sbd or ipmi are the best candidates.
Best Regards,Strahil Nikolov 
 
  On Thu, Apr 8, 2021 at 6:52, d tbsky wrote:   Hi:
    I found RHEL 8.2 support single node cluster now.  but I didn't
find further document to explain the concept. RHEL 8.2 also support
"disaster recovery cluster". so I think maybe a single node disaster
recovery cluster is not bad.

  I think corosync is still necessary under single node cluster. or
is there other new style of configuration?

    thanks for help!
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Re: [ClusterLabs] how to setup single node cluster

[d tbsky]

Reid Wahl 
> Disaster recovery is the main use case we had in mind. See the RHEL 8.2 
> release notes:
>   - 
> https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/8.2_release_notes/rhel-8-2-0-release#enhancement_high-availability-and-clusters
>
> I thought I also remembered some other use case involving MS SQL, but I can't 
> find anything about it so I might be remembering incorrectly.

thanks a lot for confirmation. according to the discussion above, I
think the setup procedure is similar for singe-node cluster. I should
still use corosync (although it seems sync nowhere).
I will try that when I have time.
thanks again for your kindly help!
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Re: [ClusterLabs] how to setup single node cluster

[Reid Wahl]

On Wed, Apr 7, 2021 at 11:27 PM d tbsky  wrote:

> Reid Wahl 
> > I don't think we do require fencing for single-node clusters. (Anyone at
> Red Hat, feel free to comment.) I vaguely recall an internal mailing list
> or IRC conversation where we discussed this months ago, but I can't find it
> now. I've also checked our support policies documentation, and it's not
> mentioned in the "cluster size" doc or the "fencing" doc.
>
>since the cluster is 100% alive or 100% dead with single node, I
> think fencing/quorum is not required. I am just curious what is the
> usage case. since RedHat supports it, it must be useful in real
> scenario.
>

Disaster recovery is the main use case we had in mind. See the RHEL 8.2
release notes:
  -
https://access.redhat.com/documentation/en-us/red_hat_enterprise_linux/8/html/8.2_release_notes/rhel-8-2-0-release#enhancement_high-availability-and-clusters

I thought I also remembered some other use case involving MS SQL, but I
can't find anything about it so I might be remembering incorrectly.


-- 
Regards,

Reid Wahl, RHCA
Senior Software Maintenance Engineer, Red Hat
CEE - Platform Support Delivery - ClusterHA
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Re: [ClusterLabs] how to setup single node cluster

[d tbsky]

Reid Wahl 
> I don't think we do require fencing for single-node clusters. (Anyone at Red 
> Hat, feel free to comment.) I vaguely recall an internal mailing list or IRC 
> conversation where we discussed this months ago, but I can't find it now. 
> I've also checked our support policies documentation, and it's not mentioned 
> in the "cluster size" doc or the "fencing" doc.

   since the cluster is 100% alive or 100% dead with single node, I
think fencing/quorum is not required. I am just curious what is the
usage case. since RedHat supports it, it must be useful in real
scenario.
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Re: [ClusterLabs] how to setup single node cluster

[Klaus Wenninger]

On 4/8/21 8:16 AM, Reid Wahl wrote:



On Wed, Apr 7, 2021 at 9:46 PM Strahil Nikolov > wrote:


I always though that the setup is the same, just the node count is
only one.

I guess you need pcs, corosync + pacemaker.
If RH is going to support it, they will require fencing. Most
probably sbd or ipmi are the best candidates.


I don't think we do require fencing for single-node clusters. (Anyone 
at Red Hat, feel free to comment.) I vaguely recall an internal 
mailing list or IRC conversation where we discussed this months ago, 
but I can't find it now. I've also checked our support policies 
documentation, and it's not mentioned in the "cluster size" doc or the 
"fencing" doc.


The closest thing I can find is the following, from the cluster size 
doc[1]:

~~~
RHEL 8.2 and later: Support for 1 or more nodes

  * Single node clusters do not support DLM and GFS2 filesystems (as
they require fencing).

~~~

To me that suggests that fencing isn't required in a single-node 
cluster. Maybe sbd could work (I haven't thought it through), but 
conventional power fencing (e.g., fence_ipmilan) wouldn't. That's 
because most conventional power fencing agents require sending a 
"power on" signal after the "power off" is complete.

And moreover you have to be alive enough to kick off
conventional power fencing to self-fence ;-)
With sbd the hardware-watchdog should kick in.

Klaus


[1] https://access.redhat.com/articles/3069031 




Best Regards,
Strahil Nikolov

On Thu, Apr 8, 2021 at 6:52, d tbsky
mailto:tbs...@gmail.com>> wrote:
Hi:
    I found RHEL 8.2 support single node cluster now.  but I
didn't
find further document to explain the concept. RHEL 8.2 also
support
"disaster recovery cluster". so I think maybe a single node
disaster
recovery cluster is not bad.

  I think corosync is still necessary under single node
cluster. or
is there other new style of configuration?

    thanks for help!
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--
Regards,

Reid Wahl, RHCA
Senior Software Maintenance Engineer, Red Hat
CEE - Platform Support Delivery - ClusterHA

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Re: [ClusterLabs] how to setup single node cluster

[Reid Wahl]

On Wed, Apr 7, 2021 at 9:46 PM Strahil Nikolov 
wrote:

> I always though that the setup is the same, just the node count is only
> one.
>
> I guess you need pcs, corosync + pacemaker.
> If RH is going to support it, they will require fencing. Most probably sbd
> or ipmi are the best candidates.
>

I don't think we do require fencing for single-node clusters. (Anyone at
Red Hat, feel free to comment.) I vaguely recall an internal mailing list
or IRC conversation where we discussed this months ago, but I can't find it
now. I've also checked our support policies documentation, and it's not
mentioned in the "cluster size" doc or the "fencing" doc.

The closest thing I can find is the following, from the cluster size doc[1]:
~~~
RHEL 8.2 and later: Support for 1 or more nodes

   - Single node clusters do not support DLM and GFS2 filesystems (as they
   require fencing).

~~~

To me that suggests that fencing isn't required in a single-node cluster.
Maybe sbd could work (I haven't thought it through), but conventional power
fencing (e.g., fence_ipmilan) wouldn't. That's because most conventional
power fencing agents require sending a "power on" signal after the "power
off" is complete.

[1] https://access.redhat.com/articles/3069031


> Best Regards,
> Strahil Nikolov
>
> On Thu, Apr 8, 2021 at 6:52, d tbsky
>  wrote:
> Hi:
> I found RHEL 8.2 support single node cluster now.  but I didn't
> find further document to explain the concept. RHEL 8.2 also support
> "disaster recovery cluster". so I think maybe a single node disaster
> recovery cluster is not bad.
>
>   I think corosync is still necessary under single node cluster. or
> is there other new style of configuration?
>
> thanks for help!
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>


-- 
Regards,

Reid Wahl, RHCA
Senior Software Maintenance Engineer, Red Hat
CEE - Platform Support Delivery - ClusterHA
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