Tribblix: manual page: rrdfetch.1

RRDFETCH(1) rrdtool RRDFETCH(1)

NAME

rrdfetch - Fetch data from an RRD.

SYNOPSIS

DESCRIPTION

The fetch function is normally used internally by the graph function
to get data from RRDs. 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).
An optional suffix may be used (e.g. "5m" instead of 300
seconds). rrdfetch will try to match your request, but it
will return data even if no absolute match is possible. See
"RESOLUTION INTERVAL".

--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" 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" for a detailed explanation of
how to specify the end time.

--align-start|-a
Automatically adjust the start time down to be aligned with
the resolution. The end-time is adjusted by the same amount.
This avoids the need for external calculations described in
RESOLUTION INTERVAL, though if a specific RRA is desired this
will not ensure the start and end fall within its bounds.

--daemon|-d address
Address of the rrdcached daemon. If specified, a "flush"
command is sent to the server before reading the RRD files.
This allows rrdtool to return fresh data even if the daemon
is configured to cache values for a long time. For a list of
accepted formats, see the -l option in the rrdcached manual.

rrdtool fetch --daemon unix:/var/run/rrdcached.sock /var/lib/rrd/foo.rrd AVERAGE

Please note that due to thread-safety reasons, the time
specified with -s and -e cannot use the complex forms
described in "AT-STYLE TIME SPECIFICATION". The only accepted
arguments are "simple integers". Positive values are
interpreted as seconds since epoch, negative values (and
zero) are interpreted as relative to now. So "1272535035"
refers to "09:57:15 (UTC), April 29th 2010" and "-3600" means
"one hour ago".

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:5m:0:U \
RRA:AVERAGE:0.5:5m:300h \
RRA:AVERAGE:0.5:15m:300h \
RRA:AVERAGE:0.5:1h:50d \
RRA:MAX:0.5:1h:50d \
RRA:AVERAGE:0.5:1d:600d \
RRA:MAX:0.5:1d:600d

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 15m -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

1. both start and end time are a multiple of 900 ("15m")

2. 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 $(($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"'

Or using the --align-start flag:

rrdtool fetch subdata.rrd AVERAGE -a -r 15m -s -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, end or epoch 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) and epoch indicates the *IX epoch (*IX timestamp
0 = 1970-01-01 00:00:00 UTC). epoch is useful to disambiguate between
a timestamp value and some forms of abbreviated date/time
specifications, because it allows one to use time offset
specifications using units, eg. epoch+19711205s unambiguously denotes
timestamp 19711205 and not 1971-12-05 00:00:00 UTC.

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:

1. 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).

2. 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 6 it is assumed that m means
months, otherwise it is treated as minutes. (e.g., -6m == -6m
minutes, while +5m == +5 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).

931200300 -- 18:45 (UTC), July 5th, 1999 (yes, seconds since 1970 are
valid as well).

19970703 12:45 -- 12:45 July 3th, 1997 (my favorite, and it has even
got an ISO number (8601)).

ENVIRONMENT VARIABLES

The following environment variables may be used to change the
behavior of "rrdtool fetch":

RRDCACHED_ADDRESS
If this environment variable is set it will have the same effect
as specifying the "--daemon" option on the command line. If both
are present, the command line argument takes precedence.

AUTHOR

Tobias Oetiker <tobi@oetiker.ch>

1.8.0 2022-03-14 RRDFETCH(1)