[rrd-users] Re: illegal attempt to update Problem.

[Alex van den Bogaerdt]

On Mon, Sep 16, 2002 at 10:24:49AM -0600, Terry Poperszky wrote:
> 
[snip: after NTP server problem]

> When I do a rrdtool dump, it shows a large gap of data where the NTP
> server malfunctioned, the data collection for a few days afterwards,
> then no data collection after I reset the date. Is there any way to fix
> this?

Sure.  Restore a backup.

If you didn't backup your database, you can probably(!) change
the database but make sure you understand the round robin mechanism.

1) visualize each RRA as a circle and a hand.  The hand points to the
   next slot to be filled.  The entire amount of time in this RRA is
   the amount of time per row times the amount of time per entry
2) from that position, go back in time one row at a time.  For each
   entry encountered, decrease the timestamp with the amount of time
   in that RRA.  This means you're canceling that update
3) alter the last update time

This only covered the basics.  There's much more happening in reality.
For each RRA there's also an entry where the next row is prepared as
each PDP is inserted in that RRA.

Most likely the best way to cope with this (short of restoring your
backup!) is to go back in time so that each RRA has no partial updates
happening (I think this is called the least common denominator?).

Oh, and don't forget to set each row that was "rolled back" to NaN.
I'm talking about step 2 here. If you don't, you will have altered
the past.

HTH
Alex

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