[rrd-developers] RRD PostgreSQL extension

Dustin Fisch fooker at lab.sh
Wed Sep 12 01:44:21 CEST 2012

Tobias Oetiker <tobi at oetiker.ch> wrote:

> Hi Dustin,
> Yesterday Dustin Fisch wrote:
> > Hi,
> >
> > I am currently thinking about building an RRD extension for
> > PostgreSQL.
> >
> > The extension will provide a RRD data type which represents a
> > complete database and functions for all the existing functions of
> > the RRD tool, like create, update, first and graph...
> >
> > PostgreSQL has a nice API for LOB, which allows to store the whole
> > RRD database using the PostgreSQL file management.
> >
> > I already have taken a look to the RRD code. As far as I can see,
> > there are functions like rrd_create_r(...) which provides the
> > functionality I need for such an extension.
> >
> > The PostgreSQL LOB API currently provides a file descriptor for the
> > access to the data stored in the LOB.
> >
> > Unfortunately, it looks like the RRD API only allows the usage of a
> > filename.
> >
> > Is there any way to use a file descriptor instead of the filename?
> > Or is there any chance such a patch would be accepted?
> if done nicely, I'll be glad to integrate such a patch ... I guess
> you would have to compile without mmap support ...
I'm not sure about mmap. I think I have to try it.

The plan for the patch is to refactoring the rrd_*_r methods by moving
out the real work of the method into something called rrd_*_x. The only
thing remaining in the original method are the calls to rrd_open and
rrd_close and the call to the new method.

The new rrd_*_x methods will mirror the interface of there rrd_*_r
counterparts expect the filename parameter, which will be replaced by
the already opened rrd_file_t.

> how would this pg integration work, would the rrd data be stored in
> a blob ? wouldn't this get read from disk entirely on every update
> and make things rather slow ?
Right. The idea is to store the RRD data in a BLOB.

But I don't see any performance problem with this as a PostgreSQL
extension is allowed to work on the file backing the BLOB directly
using a file descriptor.

This makes no difference to the way RRDtool currently works. Maybe this
allows to to use the current mmap code, too.

> cheers
> tobi


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