[rrd-users] Re: understanding RRD's; 5 questions

BAARDA, Don don.baarda at baesystems.com
Sun Feb 25 23:32:40 MET 2001


> -----Original Message-----
> From:	Dave Bodenstab [SMTP:imdave at mcs.net]
> Sent:	Saturday, February 24, 2001 5:21 AM
> To:	rrd-users at list.ee.ethz.ch
> Cc:	alex at slot.hollandcasino.nl
> Subject:	[rrd-users] Re: understanding RRD's; 5 questions

> [
>   As an aside, although rrdtutorial mentions the time
>   values being UTC, it doesn't make clear how the rrd
>   is organized with respect to local or UTC time.
>   Furthermore, the use of the absolute numbers obscures what
>   is going on -- who can make sense out of a plain 9-digit
>   number?
> ]
	This is a unix-ism... generally all human readable times are in
localtime by default, unless they have a timezone specifier on the end (ie
GMT). All numeric times are always in GMT... due to oddities like
daylight-savings and leap years/seconds etc in localtime, GMT is the only
numeric time-stamp that can uniquely identify any particular time.

> In addition, the fact that an AT_STYLE time spec is taken to
> be local time just adds to the confusion.  Being able to use
> --start AT_TIME for rrdcreate makes one believe that one can
> set the alignment of the time slots.  Couldn't the time spec
> optionally accept "GMT" or "UTC" to avoid the local time
> conversion?

	It would be possible to modify rrd so that the --start time
specifies an alignment point. This could be useful for people with large
steps that want them aligned in a particular way. However, it would make the
code a little more complex, and would require adding storage for the start
time/alignment point in the rrd and dump files.

> With respect to my question #2 (allowing an AT_STYLE time
> spec with rrdupdate), is there no reason why this couldn't
> be done?  Having to compute and manipulate the absolute
> seconds number (and take into account localtime) is a real
> pain when it's not available directly from a particular
> data source.

	Probably another unix-ism... There are plenty of unix utilities for
converting between different time formats (try "man date"). I know windoze
is starved for decent commandline utilities; I can recommend Cygwin to help
solve the problem (if you are actually using windoze).


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