[rrd-users] invalid data / spikes FIXED

[Tobias Oetiker]

Wednesday you sent me mail regarding [rrd-users] Re: followup on invalid...:

*> I have been able to shorten it considerably.
*> Here is the new sequence (using csh, or tcsh):
*> 
*> 
*> mon1:~ rrdtool create my.rrd -b 946843500 -s 300 "DS:load:GAUGE:300:0:5000" "DS:out_oct:COUNTER:300:0:1250000" "DS:drop_oct:COUNTER:300:0:U"
*> "DS:out_pkts:COUNTER:300:0:25000" "DS:drop_pkts:COUNTER:300:0:U" "RRA:AVERAGE:0.5:1:864" "RRA:AVERAGE:0.5:3:1344" "RRA:AVERAGE:0.5:12:744"
*> "RRA:AVERAGE:0.5:288:91"
*> mon1:~  rrdtool update my.rrd 947097000:0.00:107015782:0:156363:0
*> mon1:~ rrdtool dump my.rrd > db.out
*> mon1:~ rrdtool restore db.out my.rrd
*> mon1:~  rrdtool update my.rrd 947097300:0.01:107015782:0:156363:0
*> mon1:~ rrdtool dump my.rrd > db.out
*> mon1:~ rrdtool restore -r db.out db.xml
*> out of range found [ds: 3], [value : 1.9991590274e+37]
*> 

Folks! Thanks to Bill Fenner <fenner at research.att.com> the mistery is solved
... rrd_update is in violation of the ANSY C standard (7.9.5.3) by issuing an
fwrite immediately after fread some times ... here is the patch ... 

-------------------------------------------------------------------------------
--- rrd_update.old       Sat Dec 25 21:10:02 1999
+++ rrd_update.c        Fri Jan  7 07:36:30 2000
@@ -109,7 +109,18 @@
        return -1;
     }
     rra_current = rra_start = rra_begin = ftell(rrd_file);
+    /* This is defined in the ANSI C standard, section 7.9.5.3:
 
+        When a file is opened with udpate mode ('+' as the second
+        or third character in the ... list of mode argument
+        variables), both input and ouptut may be performed on the
+        associated stream.  However, ...  input may not be directly
+        followed by output without an intervening call to a file
+        positioning function, unless the input oepration encounters
+        end-of-file. */
+    fseek(rrd_file, 0, SEEK_CUR);
+
+    
     /* get exclusive lock to whole file.
      * lock gets removed when we close the file.
      */
-------------------------------------------------------------------------------

it seems that the most Unix C libraries do not care for this, and work all
the same ... thats why it was so difficult to catch this ... 

cheers
tobi

-- 
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/_  __/_  / /  (_) Oetiker, Timelord & SysMgr @ EE-Dept ETH-Zurich
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/_/ \.__/_.__/_/ oetiker at ee.ethz.ch http://ee-staff.ethz.ch/~oetiker

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