Re: [GENERAL] openvz and shared memory trouble

Zitat von Willy-Bas Loos <willybas(at)gmail(dot)com>:

> Hi,
>
> I have a problem with postgres in combination with openvz.
> My hot standby crashes on me when i give it a normal value for
> shared_buffers (4GB, which 25% of the memory in the container).
> When i tune down the shared_buffers, it works again. But of course this is
> far from desireable.

Have a look at the bean counters from OpenVZ. Some of them have
dependencies to each other and can lead to unexpected out-of-memory
condition inside the container. Check the logs for the reason
PostgreSQL crashed, maybe it was the OOM killer.

> Now, i've seen a couple more messages around the net about openVZ and
> postgres that weren't very encouraging:
>
> http://www.postgresql.org/message-id/CAHyXU0xa5EgvjeH=4vp-eZDJdS5kMQuiDivvTRLjY-uZ62Y44w@mail.gmail.com

This is about Postgres not able to read a block from a file, so it
looks like a filesystem issue to me.

> http://www.postgresql.org/message-id/23206.1254683408@sss.pgh.pa.us

This is about "hot" migration which is in general a dangerous task for
databases. Obviously it doesn't work in this case. We also had no
success some time ago trying "hot" migration with Oracle Databases on
OpenVZ.

> http://postgresql.1045698.n5.nabble.com/BUG-9721-Fatal-error-on-startup-no-free-slots-in-PMChildFlags-array-td5797390.html

This is also a related problem to the above. Freezing a running
database (which is also done for migration) must be atomic, else the
internal database states might get confused.

> But i am in a position where we already have a pretty strong binding with
> openVZ.
> I also have a colleague that has been using openVZ and postgres on
> production for quite a while now, without any problems. And already 1 of
> our servers is running a production database in openVZ that is well used,
> without any trouble of this kind.
>
> Now, what i would like to know is:
>
> - is there clarity about what the issues are at all
> - is it just 1 problem (in openVZ) that make openVZ and postgres so
> poorly compatible?
> - how can i stay out of trouble (when using postgres inside openVZ)
> - who else has trouble with openVZ in combination with postgres, and
> what constitutes the trouble?

From my point of view your problem is not related to the URLs shown.
You have to carefully check the Container settings and errors and
avoid freezing/migrating the database container if possible.

In general there is no Problem with databases inside Containers but
you have to carefully check the settings for the container in question
and test the advanced features like hot migration if you really have
to use them.

Regards

Andreas