NAME¶
rrdfetch - Fetch data from an RRD.
SYNOPSIS¶
rrdtool fetch filename CF [
--resolution|
-r resolution] [
--start|
-s start] [
--end|
-e end] [
--daemon address]
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).
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.
- --daemon 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
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
- 1.
- both start and end time are a multiple of 900
- 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"'
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 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 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)).
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>