[rrd-users] Re: Rrd file size

[Alex van den Bogaerdt]

Michael Phillips wrote:
> 
> Since MRTG has created my RRD files for me, how can I see how big they
> are?  I need them to store a months worth of data.
> Can someone put the rrdtool resize in to laymen's terms?

Using "rrdtool info" you can look at the way your RRD is built.  In
this RRD you find one or more RRAs.  In these RRAs the data is stored.

Each RRA has a number of rows.  Each row represents an amount of time
and the value in that row represents the rate during that time.

The amount of time in each row depends on two things:
1) the amount of PDPs in each row
2) the amount of time in each PDP: this is the "--step" value

The step time by the way is set for the entire RRD, this doesn't change.

One of the RRAs generated from MRTG is 600 rows, each row representing
1 PDP, each PDP representing 300 seconds.  The total amount of time in
that RRA is thus 600 * 1 * 300 = 180000 seconds (2 days 2 hours).

Another RRA is 700 rows with 6 PDPs per row.  The total amount of time
in that RRA is therefore 700 * 6 * 300 = 1260000 seconds ( 14 d 14 hrs)

When RRDtool needs data to plot or print, it fetches that data from one
of the available RRAs.  If the data is not available in an RRA (due to
the amount of time in each RRA or because it is storing other data than
you query) it won't be used.

Now, to answer your question:

What "rrdtool resize" does is alter the amount of rows in an RRA you
select.  If you change the amount of rows from 600 to 1200 it is easy
to see that the amount of time changes from 2 days 2 hours into 4d4h.

This part of the rrdtool program generates a new database.  It is
strongly advised that you use "rrdtool info" or even "rrdtool dump"
to check that the resulting database is what you ment it to be.  When
you're satisfied you can rename this database to the original name.

I hope this is simple enough without being too simple :)

cheers,
-- 
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