Re: [GENERAL] Keycloak and Postgres

[Bill Moran]

On Wed, 5 Apr 2017 07:24:32 +
Marc Tempelmeier  wrote:
> 
> Can you elaborate a bit on this part:
> " Because of how Postgres caches changes, you may find that a failover 
> requires some time in recovery mode."

https://www.postgresql.org/docs/9.6/static/wal-intro.html

The WAL requires that any unexpected shutdown of Postgres (where it doesn't get 
to explicitly
flush data pages to disk) go through a recovery cycle to fix anything in the 
WAL that is not
yet in the data pages.

Doing disk level replication and using that as a failover essentially 
duplicates a crash
on the PostgreSQL end when you failover.

-- 
Bill Moran 


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Re: [GENERAL] Keycloak and Postgres

[Marc Tempelmeier]

Hi,

Thanks for your answer!

Can you elaborate a bit on this part:
" Because of how Postgres caches changes, you may find that a failover requires 
some time in recovery mode."

Thanks!

-Ursprüngliche Nachricht-
Von: Bill Moran [mailto:wmo...@potentialtech.com] 
Gesendet: Saturday, April 1, 2017 12:57 PM
An: Marc Tempelmeier <marc.tempelme...@flane.de>
Cc: pgsql-general@postgresql.org
Betreff: Re: [GENERAL] Keycloak and Postgres

On Thu, 30 Mar 2017 13:58:36 +
Marc Tempelmeier <marc.tempelme...@flane.de> wrote:

> Hi,
> 
> I have a replication question, we have some big Cisco UCS VM thingy, where 
> VMs are snapshotted, the drives are abstracted etc. If a VM crashes it will 
> be resumed in 1 min from another rack. What brings us master slave 
> replication or some other kind of replication in this setup? Should we do it 
> because of other failures?

Because of how Postgres caches changes, you may find that a failover requires 
some time in recovery mode. Those VM snapshot systems are great, but they 
aren't quite perfect if they don't know what is being done with the data on the 
drives.

Whether it's good enough depends heavily on what your expectation is.
Before trusting it to meet your needs, I would spend some time simulating 
failures and seeing what actually happens.

--
Bill Moran <wmo...@potentialtech.com>


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Re: [GENERAL] Keycloak and Postgres

[Bill Moran]

On Thu, 30 Mar 2017 13:58:36 +
Marc Tempelmeier  wrote:

> Hi,
> 
> I have a replication question, we have some big Cisco UCS VM thingy, where 
> VMs are snapshotted, the drives are abstracted etc. If a VM crashes it will 
> be resumed in 1 min from another rack. What brings us master slave 
> replication or some other kind of replication in this setup? Should we do it 
> because of other failures?

Because of how Postgres caches changes, you may find that a failover
requires some time in recovery mode. Those VM snapshot systems are great,
but they aren't quite perfect if they don't know what is being done with
the data on the drives.

Whether it's good enough depends heavily on what your expectation is.
Before trusting it to meet your needs, I would spend some time simulating
failures and seeing what actually happens.

-- 
Bill Moran 


-- 
Sent via pgsql-general mailing list (pgsql-general@postgresql.org)
To make changes to your subscription:
http://www.postgresql.org/mailpref/pgsql-general

[GENERAL] Keycloak and Postgres

[Marc Tempelmeier]

Hi,

I have a replication question, we have some big Cisco UCS VM thingy, where VMs 
are snapshotted, the drives are abstracted etc. If a VM crashes it will be 
resumed in 1 min from another rack. What brings us master slave replication or 
some other kind of replication in this setup? Should we do it because of other 
failures?

Best regards

Marc