NAME
rrdfetch - Fetch data from an RRD.
SYNOPSIS
rrdtool
fetch
filename
CF
[
--resolution
|
-r
resolution
]
[
--start
|
-s
start
]
[
--end
|
-e
end
]
DESCRIPTION
The
fetch
function is normally used internally by the graph
function to get data from
RRD
s.
fetch
will analyze the
RRD
and try to retrieve the data in the resolution requested.
The data fetched is printed to stdout.
*UNKNOWN*
data is often
represented by the string ``NaN'' depending on your OS's printf
function.
filename
the name of the
RRD
you want to fetch the data from.
CF
the consolidation function that is applied to the data you
want to fetch (AVERAGE,MIN,MAX,LAST)
--resolution
|
-r
resolution
(default is the highest resolution)
the interval you want the values to have (seconds per
value).
rrdfetch
will try to match your request, but it will return
data even if no absolute match is possible.
NB.
See note below.
--start
|
-s
start
(default end-1day)
start of the time series. A time in seconds since epoch (1970-01-01)
is required. Negative numbers are relative to the current time. By default,
one day worth of data will be fetched. See also AT-STYLE TIME SPECIFICATION
section for a detailed explanation on  ways to specify the start time.
--end
|
-e
end
(default now)
the end of the time series in seconds since epoch. See also AT-STYLE
TIME SPECIFICATION section for a detailed explanation of how to
specify the end time.
RESOLUTION INTERVAL
In order to get RRDtool to fetch anything other than the finest resolution RRA
both
the start and end time must be specified on boundaries that are
multiples of the desired resolution. Consider the following example:
rrdtool create subdata.rrd -s 10 DS:ds0:GAUGE:300:0:U \
RRA:AVERAGE:0.5:30:3600 \
RRA:AVERAGE:0.5:90:1200 \
RRA:AVERAGE:0.5:360:1200 \
RRA:MAX:0.5:360:1200 \
RRA:AVERAGE:0.5:8640:600 \
RRA:MAX:0.5:8640:600
This RRD collects data every 10 seconds and stores its averages over 5
minutes, 15 minutes, 1 hour, and 1 day, as well as the maxima for 1 hour
and 1 day.
Consider now that you want to fetch the 15 minute average data for the
last hour.  You might try
rrdtool fetch subdata.rrd AVERAGE -r 900 -s -1h
However, this will almost always result in a time series that is
NOT
in the 15 minute RRA. Therefore, the highest resolution RRA,
i.e. 5 minute averages, will be chosen which in this case is not
what you want.
Hence, make sure that
both start and end time are a multiple of 900
both start and end time are within the desired RRA
So, if time now is called ``t'', do
end time == int(t/900)*900,
start time == end time - 1hour,
resolution == 900.
Using the bash shell, this could look be:
TIME=$(date +%s)
RRDRES=900
rrdtool fetch subdata.rrd AVERAGE -r $RRDRES \
-e $(echo $(($TIME/$RRDRES*$RRDRES))) -s e-1h
Or in Perl:
perl -e '$ctime = time; $rrdres = 900; \
system "rrdtool fetch subdata.rrd AVERAGE \
-r $rrdres -e @{[int($ctime/$rrdres)*$rrdres]} -s e-1h"'
AT-STYLE TIME SPECIFICATION
Apart from the traditional
Seconds since epoch
, RRDtool does also
understand at-style time specification. The specification is called
``at-style'' after the Unix command
at(1)
that has moderately complex
ways to specify time to run your job at a certain date and time. The
at-style specification consists of two parts: the
TIME REFERENCE
specification and the
TIME OFFSET
specification.
TIME REFERENCE SPECIFICATION
The time reference specification is used, well, to establish a reference
moment in time (to which the time offset is then applied to). When present,
it should come first, when omitted, it defaults to
now
. On its own part,
time reference consists of a
time-of-day
reference (which should come
first, if present) and a
day
reference.
The
time-of-day
can be specified as
HH:MM
,
HH.MM
,
or just
HH
. You can suffix it with
am
or
pm
or use
24-hours clock. Some special times of day are understood as well,
including
midnight
(00:00),
noon
(12:00) and British
teatime
(16:00).
The
day
can be specified as
month-name
day-of-the-month
and
optional a 2- or 4-digit
year
number (e.g. March 8 1999). Alternatively,
you can use
day-of-week-name
(e.g. Monday), or one of the words:
yesterday
,
today
,
tomorrow
. You can also specify the
day
as a
full date in several numerical formats, including
MM/DD/[YY]YY
,
DD.MM.[YY]YY
, or
YYYYMMDD
.
NOTE1
: this is different from the original
at(1)
behavior, where a
single-number date is interpreted as MMDD[YY]YY.
NOTE2
: if you specify the
day
in this way, the
time-of-day
is
REQUIRED as well.
Finally, you can use the words
now
,
start
, or
end
as your time
reference.
Now
refers to the current moment (and is also the default
time reference).
Start
(
end
) can be used to specify a time
relative to the start (end) time for those tools that use these
categories (
rrdfetch
,
././rrdgraph.html
the rrdgraph manpage
).
Month and day of the week names can be used in their naturally
abbreviated form (e.g., Dec for December, Sun for Sunday, etc.). The
words
now
,
start
,
end
can be abbreviated as
n
,
s
,
e
.
TIME OFFSET SPECIFICATION
The time offset specification is used to add/subtract certain time
intervals to/from the time reference moment. It consists of a
sign
(
+
or
-
) and an
amount
. The following time units can be
used to specify the
amount
:
years
,
months
,
weeks
,
days
,
hours
,
minutes
, or
seconds
. These units can be used in
singular or plural form, and abbreviated naturally or to a single
letter (e.g. +3days, -1wk, -3y). Several time units can be combined
(e.g., -5mon1w2d) or concatenated (e.g., -5h45min = -5h-45min =
-6h+15min = -7h+1h30m-15min, etc.)
NOTE3
: If you specify time offset in days, weeks, months, or years,
you will end with the time offset that may vary depending on your time
reference, because all those time units have no single well defined
time interval value (1 year contains either 365 or 366 days, 1 month
is 28 to 31 days long, and even 1 day may be not equal to 24 hours
twice a year, when DST-related clock adjustments take place).
To cope with this, when you use days, weeks, months, or years
as your time offset units your time reference date is adjusted
accordingly without too much further effort to ensure anything
about it (in the hope that
mktime(3)
will take care of this later).
This may lead to some surprising (or even invalid!) results,
e.g. 'May 31 -1month' = 'Apr 31' (meaningless) = 'May 1'
(after
mktime(3)
normalization); in the EET timezone
'3:30am Mar 29 1999 -1 day' yields '3:30am Mar 28 1999' (Sunday)
which is an invalid time/date combination (because of 3am -> 4am DST
forward clock adjustment, see the below example).
In contrast, hours, minutes, and seconds are well defined time
intervals, and these are guaranteed to always produce time offsets
exactly as specified (e.g. for EET timezone, '8:00 Mar 27 1999 +2
days' = '8:00 Mar 29 1999', but since there is 1-hour DST forward
clock adjustment that occurs around 3:00 Mar 28 1999, the actual
time interval between 8:00 Mar 27 1999 and 8:00 Mar 29 1999
equals 47 hours; on the other hand, '8:00 Mar 27 1999 +48 hours' =
'9:00 Mar 29 1999', as expected)
NOTE4
: The single-letter abbreviation for both
months
and
minutes
is
m
. To disambiguate them, the parser tries to read your mind :)
by applying the following two heuristics:
If
m
is used in context of (i.e. right after the) years,
months, weeks, or days it is assumed to mean
months
, while
in the context of hours, minutes, and seconds it means minutes.
(e.g., in -1y6m or +3w1m
m
is interpreted as
months
, while in
-3h20m or +5s2m
m
the parser decides for
minutes
).
Out of context (i.e. right after the
+
or
-
sign) the
meaning of
m
is guessed from the number it directly follows.
Currently, if the number's absolute value is below 25 it is assumed
that
m
means
months
, otherwise it is treated as
minutes
.
(e.g., -25m == -25 minutes, while +24m == +24 months)
Final NOTES
: Time specification is case-insensitive.
Whitespace can be inserted freely or omitted altogether.
There are, however, cases when whitespace is required
(e.g., 'midnight Thu'). In this case you should either quote the
whole phrase to prevent it from being taken apart by your shell or use
'_' (underscore) or ',' (comma) which also count as whitespace
(e.g., midnight_Thu or midnight,Thu).
TIME SPECIFICATION EXAMPLES
Oct 12
-- October 12 this year
-1month
or
-1m
-- current time of day, only a month before
(may yield surprises, see NOTE3 above).
noon yesterday -3hours
-- yesterday morning; can also be specified
as
9am-1day
.
23:59 31.12.1999
-- 1 minute to the year 2000.
12/31/99 11:59pm
-- 1 minute to the year 2000 for imperialists.
12am 01/01/01
-- start of the new millennium
end-3weeks
or
e-3w
-- 3 weeks before end time
(may be used as start time specification).
start+6hours
or
s+6h
-- 6 hours after start time
(may be used as end time specification).
931225537
-- 18:45  July 5th, 1999
(yes, seconds since 1970 are valid as well).
19970703 12:45
-- 12:45  July 3th, 1997
(my favorite, and its even got an ISO number (8601)).
AUTHOR
Tobias Oetiker <
mailto:oetiker@ee.ethz.ch
oetiker@ee.ethz.ch
>
