[rrd-users] Use of LAST within RRA

[A Darren Dunham]

On Thu, Feb 04, 2010 at 12:32:11PM -0500, Lyle Brooks wrote:
> I am wondering about the proper use of LAST within an RRA.
> 
> When I create an RRD file, I use the following definitions...
[...]
>     'RRA:MAX:0.5:1:576' \
>     'RRA:MAX:0.5:6:672' \
>     'RRA:MAX:0.5:24:744' \
>     'RRA:MAX:0.5:288:730' \
>     'RRA:LAST:0.5:1:576' \
>     'RRA:LAST:0.5:6:672' \
>     'RRA:LAST:0.5:24:744' \
>     'RRA:LAST:0.5:288:730'

> I admit, I simply used 'cut-n-paste' based off what I did for the
> AVERAGE and MAX consolidation functions, but as I reflect on the LAST
> consolidation function, I find my self asking...
> 
> 1) Do I need to keep so many rows of LAST points?  or could I get
> away with only 1?  I mean, there's only one "LAST" point, yes?

There's only one LAST point just like there's only one MAX and one MIN
(and one AVERAGE).   This is a consolidation function, so you're telling
it when it needs to create a CDP from multiple PDPs, how it does so.  

I would assume rather than creating so many RRAs, you'd think about why
you were creating the RRAs and what you hope to see with them.  In
general, I have no need for MIN or LAST consolidations in what I'm
doing.  If you do, they're great, but I wouldn't bother calculating and
storing more than I need.

> Could I use these lines instead?
> 
>     'RRA:LAST:0.5:1:1' \
>     'RRA:LAST:0.5:6:1' \
>     'RRA:LAST:0.5:24:1' \
>     'RRA:LAST:0.5:288:1'

That wouldn't be very useful.  The 'rows' is telling the RRA how many
CDPs to store.  Your graph will be very boring if it only stores one
CDP.  LAST is how the CDPs are created, not how they're stored.

> 2) Do I need 4 RRA's with LAST?  Or do I really only need one?

Same answer as any other CF.  If you're often graphing the data at
particular resolutions, having the RRA around helps you.  

-- 
Darren