Re: [rrd-users] Fetch the time of the first entry

[Steve Shipway]

Look in the documentation for 'rrdtool first' and 'rrdtool lastupdate'.
http://oss.oetiker.ch/rrdtool/doc/rrdfirst.en.html
This returns the first (oldest) entry for a given RRA.
http://oss.oetiker.ch/rrdtool/doc/rrdlastupdate.en.html
This returns the last entry time and values
Note that 'rrdtool last' by contrast returns the time of the last update.

Steve

Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.ship...@auckland.ac.nz
Ph: +64 9 373 7599 ext 86487


From: rrd-users-bounces+s.shipway=auckland.ac...@lists.oetiker.ch 
[rrd-users-bounces+s.shipway=auckland.ac...@lists.oetiker.ch] on behalf of 
Steven Sim [unixan...@outlook.com]
Sent: Tuesday, 1 July 2014 3:08 p.m.
To: rrd-users@lists.oetiker.ch
Subject: [rrd-users] Fetch the time of the first entry

Hello;

I'm being asked to write a scrip to append data to an rrd and plot daily, 
weekly and monthly graphs of said data.

I already have a script to create an rrd and write data to it.

To append, I'll simply bypass the 'creation' logic of my script and use a user 
specified rrd file.

But to plot daily, weekly and monthly, I need to know the start time of the 
first entry in the rrd file.

Is there a way to determine the time of the first entry of an rrd file?

Deepest Regards
Steven Sim

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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Steven Sim  wrote:

> I'm being asked to write a scrip to append data to an rrd and plot daily, 
> weekly and monthly graphs of said data.
> 
> I already have a script to create an rrd and write data to it.
> 
> To append, I'll simply bypass the 'creation' logic of my script and use a 
> user specified rrd file.

It may be just the way you write, but the wording makes me wonder if you 
properly understand how RRD works. Because in RRD, there is no such function as 
appending data - only updating. You specify the number of consolidated data 
points to keep at the point of creation, after that the size never changes and 
you only update values.

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Re: [rrd-users] Fetch the time of the first entry

[Steven Sim]

Simon;

Thanks for pointing this out!

I intend to process a data file each day but create the rrd database on the
FIRST day of the month.

The creation shall ensure sufficient data points in the first day to
contain the entire month but plot daily graphs until the end of the month,
whereby it will plot the entire month.

Say for example,

If i had a step interval of 15 minutes (900 seconds),

rrdtool create $RRDB --step 900 --start $STARTIME \
DS:..
DS:..
.
.
.
DS:..
DS:..
RRA:AVERAGE:0.5:1:2880

2880 = 24 hours x 4 data points per hour x 30 (thirty days in a month)

4 data points since one hour has 4 data points (15 minutes step).

Would the above be correct?

Deepest Regards
Steven Sim



On Tue, Jul 1, 2014 at 3:11 PM, Simon Hobson  wrote:

> Steven Sim  wrote:
>
> > I'm being asked to write a scrip to append data to an rrd and plot
> daily, weekly and monthly graphs of said data.
> >
> > I already have a script to create an rrd and write data to it.
> >
> > To append, I'll simply bypass the 'creation' logic of my script and use
> a user specified rrd file.
>
> It may be just the way you write, but the wording makes me wonder if you
> properly understand how RRD works. Because in RRD, there is no such
> function as appending data - only updating. You specify the number of
> consolidated data points to keep at the point of creation, after that the
> size never changes and you only update values.
>
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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Steven Sim  wrote:

> I intend to process a data file each day but create the rrd database on the 
> FIRST day of the month.
> 
> The creation shall ensure sufficient data points in the first day to contain 
> the entire month but plot daily graphs until the end of the month, whereby it 
> will plot the entire month.
> 
> Say for example,
> 
> If i had a step interval of 15 minutes (900 seconds), 
> 
> rrdtool create $RRDB --step 900 --start $STARTIME \
> DS:..
> DS:..
> .
> .
> .
> DS:..
> DS:..
> RRA:AVERAGE:0.5:1:2880
> 
> 2880 = 24 hours x 4 data points per hour x 30 (thirty days in a month)
> 
> 4 data points since one hour has 4 data points (15 minutes step).
> 
> Would the above be correct?

Yes, your numbers are correct, but I wonder about your methodology. It might 
help if you said what you are trying to achieve, because it's a rather unusual 
way of using RRD. Normally, you simply create one RRD that collects, stores, 
and consolidates the data you want. Typically this means keeping high 
resolution data for a shortish time, and keeping progressively lower resolution 
for progressively longer times - eg most applications don't need to keep (say) 
5 minute resolution traffic data for 2 years ago. That's not to say you can't 
keep several years worth of high-resolution data if you want to.

By keeping separate RRD files for each 'month', you'll find that it's hard work 
if you later want to do a graph for a year ! You'd also need to lock your 
"filled" data files otherwise one update to the wrong file could wipe out all 
the data.

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Re: [rrd-users] Fetch the time of the first entry

[Steven Sim]

Simon;

Thanks for your very swift reply.

I'm still digesting your reply but I do have a small query.

my rrdtool first  command keeps returning different timing after
each rrdtool update.

Why is that so?

Shouldn't it ALWAYS return the Unix time stamp for the first entry?

Deepest Regards
Steven Sim

On Tue, Jul 1, 2014 at 7:43 PM, Simon Hobson  wrote:

> Steven Sim  wrote:
>
> > I intend to process a data file each day but create the rrd database on
> the FIRST day of the month.
> >
> > The creation shall ensure sufficient data points in the first day to
> contain the entire month but plot daily graphs until the end of the month,
> whereby it will plot the entire month.
> >
> > Say for example,
> >
> > If i had a step interval of 15 minutes (900 seconds),
> >
> > rrdtool create $RRDB --step 900 --start $STARTIME \
> > DS:..
> > DS:..
> > .
> > .
> > .
> > DS:..
> > DS:..
> > RRA:AVERAGE:0.5:1:2880
> >
> > 2880 = 24 hours x 4 data points per hour x 30 (thirty days in a month)
> >
> > 4 data points since one hour has 4 data points (15 minutes step).
> >
> > Would the above be correct?
>
> Yes, your numbers are correct, but I wonder about your methodology. It
> might help if you said what you are trying to achieve, because it's a
> rather unusual way of using RRD. Normally, you simply create one RRD that
> collects, stores, and consolidates the data you want. Typically this means
> keeping high resolution data for a shortish time, and keeping progressively
> lower resolution for progressively longer times - eg most applications
> don't need to keep (say) 5 minute resolution traffic data for 2 years ago.
> That's not to say you can't keep several years worth of high-resolution
> data if you want to.
>
> By keeping separate RRD files for each 'month', you'll find that it's hard
> work if you later want to do a graph for a year ! You'd also need to lock
> your "filled" data files otherwise one update to the wrong file could wipe
> out all the data.
>
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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Steven Sim  wrote:

> my rrdtool first  command keeps returning different timing after 
> each rrdtool update.
> 
> Why is that so?
> 
> Shouldn't it ALWAYS return the Unix time stamp for the first entry?

It's not a function I've used ...

The docs say it should return the first value entered. Once you've "filled" a 
data set, then it's going to give you the timestamp of the oldest available 
value - I'm not sure what it gives you for an RRD file that hasn't been 
"filled".
Here I'm using the term "filled" to mean you done enough updates that the whose 
database (or at least data series) has been filled with data and you are now 
losing older data to make way for the new data.

Have you looked at how the value is changing ? Does it by any chance advance at 
the same rate as the timestamp of your updates ?

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Re: [rrd-users] Fetch the time of the first entry

[Steven Sim]

Simon;

Something is seriously wrong and I don't know what it is.

My Perl script parses the data file just fine.

It plots just fine.

The legends are correct

The dates are correct.

BUT when I use rrdtool first  in an attempt to get the time
of the first entry, I get a Unix time stamp which is one entire month
EARLIER than the first entry in the file.

And each time I do update, rrdtool first  returns a DIFFERENT
number.

The rrd database is created with an RRA sufficient to contain an entire
month with readings taking every 15 minutes (900 seconds).

I would appreciate any suggestions.

Deepest Regards
Steven Sim

On Tue, Jul 1, 2014 at 8:23 PM, Simon Hobson  wrote:

> Steven Sim  wrote:
>
> > my rrdtool first  command keeps returning different timing
> after each rrdtool update.
> >
> > Why is that so?
> >
> > Shouldn't it ALWAYS return the Unix time stamp for the first entry?
>
> It's not a function I've used ...
>
> The docs say it should return the first value entered. Once you've
> "filled" a data set, then it's going to give you the timestamp of the
> oldest available value - I'm not sure what it gives you for an RRD file
> that hasn't been "filled".
> Here I'm using the term "filled" to mean you done enough updates that the
> whose database (or at least data series) has been filled with data and you
> are now losing older data to make way for the new data.
>
> Have you looked at how the value is changing ? Does it by any chance
> advance at the same rate as the timestamp of your updates ?
>
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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Steven Sim  wrote:

> BUT when I use rrdtool first  in an attempt to get the time of 
> the first entry, I get a Unix time stamp which is one entire month EARLIER 
> than the first entry in the file.
> 
> And each time I do update, rrdtool first  returns a DIFFERENT 
> number.

With each update, does the FIRST value update to be one month (or 30 days) 
before the timestamp of the update ?

If so then I think I know what's happening. When you create teh RRD file, it is 
created in it's entirety - in your case with buckets for 2880 consolidated 
values. These exist regardless of what updates you do or do not do, and the 
timespan of them is determined by your step and consolidation values.
What I suspect is happening, and what I alluded to earlier, is that even though 
you haven't done any updates for those historical buckets, they are still there 
- and FIRST is merely reporting the timestamp of the oldest bucket. Since you 
are setting the start time of the RRD when you create it, then the timestamp of 
the oldest bucket will be 30 days prior to that. As you perform updates, you 
overwrite the oldest buckets and the value of FIRST will advance to be 30 days 
prior to the last update.

Internally to RRD, there is no storage for whether a bucket actually had any 
updates - only for it's value after applying the consolidation rules specified. 
Thus there is no way to know if the oldest bucket ever had an update if it's 
value is NaN - you don't know if it was created with NaN and has never been 
overwritten, or if it was updated but the calculated value was NaN. You would 
have to search back through the database to find the oldest bucket with an 
actual value and infer that this bucket was *probably* the one with the oldest 
data.

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Re: [rrd-users] Fetch the time of the first entry

[Alex van den Bogaerdt]

> Simon;
>
> Something is seriously wrong and I don't know what it is.
>
> My Perl script parses the data file just fine.
>
> It plots just fine.
>
> The legends are correct
>
> The dates are correct.
>
> BUT when I use rrdtool first  in an attempt to get the time
> of the first entry, I get a Unix time stamp which is one entire month
> EARLIER than the first entry in the file.

As already explained, "rrdtool first" gives you the first available slot.

Maybe you want to look at the data provided by the VDEF function "FIRST".
"

Return the last/first non-nan or infinite value for the selected data
stream, including its timestamp.

Example: VDEF:first=mydata,FIRST"

If you "rrdtool graph" without actually using any graphing elements, and
the PRINT (not GPRINT) this value's time component, you can use it in a
script.

HTH
Alex


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Re: [rrd-users] Fetch the time of the first entry

[Steven Sim]

Simon;

Thanks deeply pal.

So what you and Alex van den Bogaerdt is saying is that FIRST does NOT give
the first actual 'update' but the actual first available slot, which is 30
days prior to the last update.

And  yes, the FIRST value changes in step with each of my actual 'update',
so it really gives credence to your explanation.

One questions;

When I create the RRD, I am forced to give the --start  as one step
BEHIND my actual update.

So let's say my data starts at midnight 1st July, I am forced to create a
RRD with --start time at 11:45 30th of June.

data starts -> 1st July 00:00:00 <-- first data entry.
Step = 15 minutes (900 seconds)
RRD has to be created with --start 11:45 30th of June (one step behind).

if I do not create it 1 step behind, I get an error with my updates.

Why is the above so?

Secondly, Thanks to Alex can den Bogaerdt, I should use the VDEF function
and use rrdtool graph without graphing and PRINT to parse the value to a
Perl script variable.

Deepest Regards
Steven Sim

On Tue, Jul 1, 2014 at 9:53 PM, Simon Hobson  wrote:

> Steven Sim  wrote:
>
> > BUT when I use rrdtool first  in an attempt to get the
> time of the first entry, I get a Unix time stamp which is one entire month
> EARLIER than the first entry in the file.
> >
> > And each time I do update, rrdtool first  returns a
> DIFFERENT number.
>
> With each update, does the FIRST value update to be one month (or 30 days)
> before the timestamp of the update ?
>
> If so then I think I know what's happening. When you create teh RRD file,
> it is created in it's entirety - in your case with buckets for 2880
> consolidated values. These exist regardless of what updates you do or do
> not do, and the timespan of them is determined by your step and
> consolidation values.
> What I suspect is happening, and what I alluded to earlier, is that even
> though you haven't done any updates for those historical buckets, they are
> still there - and FIRST is merely reporting the timestamp of the oldest
> bucket. Since you are setting the start time of the RRD when you create it,
> then the timestamp of the oldest bucket will be 30 days prior to that. As
> you perform updates, you overwrite the oldest buckets and the value of
> FIRST will advance to be 30 days prior to the last update.
>
> Internally to RRD, there is no storage for whether a bucket actually had
> any updates - only for it's value after applying the consolidation rules
> specified. Thus there is no way to know if the oldest bucket ever had an
> update if it's value is NaN - you don't know if it was created with NaN and
> has never been overwritten, or if it was updated but the calculated value
> was NaN. You would have to search back through the database to find the
> oldest bucket with an actual value and infer that this bucket was
> *probably* the one with the oldest data.
>
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Re: [rrd-users] Fetch the time of the first entry

[Steven Sim]

Alex;

Er ... I cannot seem to get the syntax correct..

Can you advice?

rrdtool graph - VDEF:FIRST:myrrd.rrd:CPUBusy:FIRST

where

CPUBusy is the defined DS
myrrd.rrd is the RRD database file itself..

The above keeps

$ rrdtool graph - VDEF:FIRST=myrrd.rrd:CPUBusy:FIRST
ERROR: Cannot parse line 'VDEF:FIRST=myrrd.rrd.rrd:CPUBusy:FIRST'

Deepest Regards
Steven Sim


On Tue, Jul 1, 2014 at 10:08 PM, Alex van den Bogaerdt <
a...@vandenbogaerdt.nl> wrote:

> > Simon;
> >
> > Something is seriously wrong and I don't know what it is.
> >
> > My Perl script parses the data file just fine.
> >
> > It plots just fine.
> >
> > The legends are correct
> >
> > The dates are correct.
> >
> > BUT when I use rrdtool first  in an attempt to get the time
> > of the first entry, I get a Unix time stamp which is one entire month
> > EARLIER than the first entry in the file.
>
> As already explained, "rrdtool first" gives you the first available slot.
>
> Maybe you want to look at the data provided by the VDEF function "FIRST".
> "
>
> Return the last/first non-nan or infinite value for the selected data
> stream, including its timestamp.
>
> Example: VDEF:first=mydata,FIRST"
>
> If you "rrdtool graph" without actually using any graphing elements, and
> the PRINT (not GPRINT) this value's time component, you can use it in a
> script.
>
> HTH
> Alex
>
>
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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Steven Sim  wrote:

> When I create the RRD, I am forced to give the --start  as one step 
> BEHIND my actual update.
> 
> So let's say my data starts at midnight 1st July, I am forced to create a RRD 
> with --start time at 11:45 30th of June.
> 
> data starts -> 1st July 00:00:00 <-- first data entry.
> Step = 15 minutes (900 seconds)
> RRD has to be created with --start 11:45 30th of June (one step behind).
> 
> if I do not create it 1 step behind, I get an error with my updates.

This came up recently. In effect, the START parameter sets the last update time 
for the RRD, as you can't have two updates with the same time, you need to set 
the start parameter to before the timestamp of your first update.
In principal the actual value doesn't matter - it could even be 0. However, if 
it's "a long time ago" then on your first update, RRD will have to roll forward 
and process all the non-existant updates to get up to date. How this came up 
was someone found that if they had no updates for a few days (machine down) 
then they found the first update took a lot longer than normal.
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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Alex van den Bogaerdt  wrote:

> As already explained, "rrdtool first" gives you the first available slot.

Looks like the documentation is misleading then :

http://oss.oetiker.ch/rrdtool/doc/rrdfirst.en.html says :
> The first function returns the UNIX timestamp of the first data sample 
> entered into the specified RRA of the RRD file.

Can't just come up with the right words though. It's the timestamp of the 
oldest bucket, or is CDP slot a better term ?

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Re: [rrd-users] Fetch the time of the first entry

[Steve Shipway]

You seem to be misunderstanding how RRDTool works.

Remember that, after every update of an RRA, the oldest value is thrown away to 
make room for the new one.  So, the 'first' time (the timestamp of the oldest 
bucket in the RRA) will increase every time you update, as the last bucket is 
thrown away.

Also, the 'first' value is the oldest bucket, *whether or not it has been 
filled*.  So, if the RRA is 1 month long, then 'first' will be 1 month before 
the last update time, even if you have only updated once, since the buckets are 
implicitly created.

Remember RRDTool is not like an Oracle database -- it is not an 
every-increasing list of updates that starts at size 0 and gets constantly 
bigger.

Also, this is how to use a VDEF to get the value of the first 'foo' item in the 
foo.rrd, using the most appropriate Average RRA for the timewindow:

DEF:foods:foo.rrd:foo:AVERAGE
VDEF:firstfoo:foods,FIRST
PRINT:foo


Steve


Steve Shipway
University of Auckland ITS
UNIX Systems Design Lead
s.ship...@auckland.ac.nz
Ph: +64 9 373 7599 ext 86487

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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Steve Shipway  wrote:

> You seem to be misunderstanding how RRDTool works.
> 
> Remember that, after every update of an RRA, the oldest value is thrown away 
> to make room for the new one.  So, the 'first' time (the timestamp of the 
> oldest bucket in the RRA) will increase every time you update, as the last 
> bucket is thrown away.
> 
> Also, the 'first' value is the oldest bucket, *whether or not it has been 
> filled*.

As I pointed out, it looks like the documentation is a little misleading on 
this. Where teh database has been filled and old data thrown away then the 
documentation is correct - but in the OPs case, it's misleading by talking 
about the "first data sample entered" into the RRA.

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Re: [rrd-users] Fetch the time of the first entry

[Alex van den Bogaerdt]

> Alex;
>
> Er ... I cannot seem to get the syntax correct..
>
> Can you advice?

I suggest you spend some time working your way through the various
tutorials. It should teach you the basics of how RRDtool works.

http://rrdtool.vandenbogaerdt.nl/tutorial/rrdtutorial.php

more info here: http://rrdtool.vandenbogaerdt.nl/
and here: http://oss.oetiker.ch/rrdtool/tut/index.en.html

After this, if you still need help at all, please ask.

HTH
Alex

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Re: [rrd-users] Fetch the time of the first entry

[Alex van den Bogaerdt]

> Steve Shipway  wrote:
>
>> You seem to be misunderstanding how RRDTool works.
>>
>> Remember that, after every update of an RRA, the oldest value is thrown
>> away to make room for the new one.  So, the 'first' time (the timestamp
>> of the oldest bucket in the RRA) will increase every time you update, as
>> the last bucket is thrown away.
>>
>> Also, the 'first' value is the oldest bucket, *whether or not it has
>> been filled*.
>
> As I pointed out, it looks like the documentation is a little misleading
> on this. Where teh database has been filled and old data thrown away then
> the documentation is correct - but in the OPs case, it's misleading by
> talking about the "first data sample entered" into the RRA.

I briefly skimmed the source and there does not seem to be code there to
skip NaN values at the beginning. This would probably be easy to add.

It would however introduce an incompatibility for those currently using
"rrdtool first" and depending on its current behaviour.

The other option is to modify the documentation to match the source.

Burton Strauss, please chime in.


For the topic starter: "rrdtool first" and the graph command
"VDEF:datasource,FIRST" have nothing to do with eachother and are not
implemented by the same person.


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Re: [rrd-users] Fetch the time of the first entry

[Simon Hobson]

Alex van den Bogaerdt  wrote:

> The other option is to modify the documentation to match the source.

How about changing it to "first value in the RRA", plus a note along the lines 
of :
"The value returned will always be the timestamp of the last completed entry in 
the RRA minus the timespan covered by the RRA*. For RRAs which cover a timespan 
longer than the periods covered by updates, the returned value will predate the 
first update."

There are probably better words to express it, but they aren't flowing through 
my mind at the moment :(
* Is this the correct description ?

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Re: [rrd-users] Fetch the time of the first entry

[Alex van den Bogaerdt]

> Alex van den Bogaerdt  wrote:
>
>> The other option is to modify the documentation to match the source.
>
> How about changing it to "first value in the RRA", plus a note along the
> lines of :
> "The value returned will always be the timestamp of the last completed
> entry in the RRA minus the timespan covered by the RRA*. For RRAs which
> cover a timespan longer than the periods covered by updates, the returned
> value will predate the first update."
>
> There are probably better words to express it, but they aren't flowing
> through my mind at the moment :(
> * Is this the correct description ?

I don't know the intentions of the author. What is the actual number
returned? Is it the timestamp of the first available entry (NaN or not) or
is it that time minus the size of the interval?

This is a small but significant difference.



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