[rrd-users] Re: A doubt on RRD database

Dick Davies rasputnik at hellooperator.net
Wed Jan 26 15:50:54 MET 2005

* nero freak <nero_god at mail.pt> [0140 13:40]:
> Hi all!
> I'm interested in installing RRDTool on my FreeBSD box.
> I searched the web and found a nice tutorial:
> Using SNMP and RRDTool on FreeBSD
> But there's a section where I have doubts:
> ( ... ) 
> Your database can only hold a fixed amount of data. Once this database has
> been "filled" with new data, the oldest data is overwritten. In this way,
> the data entry goes in a round robin, and ultimately there is no "start" or
> "end" to the database. It is just one loop.
> ( ... )
> What type of database does RRD create? If the database is filled with new
> data and the oldest is overwritten then how can RRD generate a timeline
> graph? What's this "loop" thing?

man rrdtutorial for a good intro, read the below first for an overview.

Think of an RRD database like a clock face.
As the hand sweeps around the RRD, it writes values at points on the face.

When the hand comes around again, the data is overwritten.
(so RRDs never grow in size, you have a fixed number of slots
for data and just reuse them).

So a one day rrd with five minute slots has 12 slots per hour and 288 (12 x 24)
slots in all. each slot holds a value and a timestamp.

when you do the graphing you pull all the values out and draw them, ending with
the current slot.

now go and read the tutorial.

'Everybody I know who is right always agrees with ME.'
		-- Rev Lady Mal
Rasputin :: Jack of All Trades - Master of Nuns

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