=pod =head1 NAME rrdcached - Data caching daemon for rrdtool =head1 SYNOPSIS B [B<-l> I
] [B<-w> I] [B<-f> I] =head1 DESCRIPTION B is a daemon that receives updates to existing RRD files, accumulates them and, if enough have been received or a defined time has passed, writes the updates to the RRD file. A I command may be used to force writing of values to disk, so that graphing facilities and similar can work with up-to-date data. The daemon was written with big setups in mind. Those setups usually run into IOErelated problems sooner or later for reasons that are beyond the scope of this document. Check the wiki at the RRDTool homepage for details. Also check L below before using this daemon! A detailed description of how the daemon operates can be found in the L section below. =head1 OPTIONS =over 4 =item B<-l> I
Tells the daemon to bind to I
and accept incoming connections on that socket. If I
begins with C, everything following that prefix is interpreted as the path to a UNIX domain socket. Otherwise the address or node name are resolved using L. If the B<-l> option is not specified the default address, C, will be used. =item B<-w> I Data is written to disk every I seconds. If this option is not specified the default interval of 300Eseconds will be used. =item B<-f> I Every I seconds the entire cache is searched for old values which are written to disk. This only concerns files to which updates have stopped, so setting this to a high value, such as 3600Eseconds, is acceptable in most cases. This timeout defaults to 3600Eseconds. =item B<-p> I Sets the name and location of the PID-file. If not specified, the default, C/run/rrdcached.pid> will be used. =item B<-b> I The daemon will change into a specific directory at startup. All files passed to the daemon, that are specified by a B path, will be interpreted to be relative to this directory. If not given the default, C, will be used. +------------------------+------------------------+ ! Command line ! File updated ! +------------------------+------------------------+ ! foo.rrd ! /tmp/foo.rrd ! ! foo/bar.rrd ! /tmp/foo/bar.rrd ! ! /var/lib/rrd/foo.rrd ! /var/lib/rrd/foo.rrd ! +------------------------+------------------------+ Paths given on the command line and paths actually updated by the daemon, assuming the base directory "/tmp". =back =head1 HOW IT WORKS When receiving an update, B does not write to disk but looks for an entry for that file in its internal tree. If not found, an entry is created including the current time (called "First" in the diagram below). This time is B the time specified on the command line but the time the operating system considers to be "now". The value and time of the value (called "Time" in the diagram below) are appended to the tree node. When appending a value to a tree node, it is checked whether it's time to write the values to disk. Values are written to disk if S= timeout>>, where C is the timeout specified using the B<-w> option, see L. If the values are "old enough" they will be enqueued in the "update queue", i.Ee. they will be appended to the linked list shown below. Because the tree nodes and the elements of the linked list are the same data structures in memory, any update to a file that has already been enqueued will be written with the next write to the RRD file, too. A separate "update thread" constantly dequeues the first element in the update queue and writes all its values to the appropriate file. So as long as the update queue is not empty files are written at the highest possible rate. Since the timeout of files is checked only when new values are added to the file, "dead" files, i.Ee. files that are not updated anymore, would never be written to disk. Therefore, every now and then, controlled by the B<-f> option, the entire tree is walked and all "old" values are enqueued. Since this only affects "dead" files and walking the tree is relatively expensive, you should set the "flush interval" to a reasonably high value. The default is 3600Eseconds (one hour). The downside of caching values is that they won't show up in graphs generated from the RRDEfiles. To get around this, the daemon provides the "flush command" to flush specific files. This means that the file is inserted at the B of the update queue or moved there if it is already enqueued. The flush command will return after the update thread has dequeued the file, so there is a good chance that the file has been updated by the time the client receives the response from the daemon, but there is no guarantee. +------+ +------+ +------+ ! head ! ! root ! ! tail ! +---+--+ +---+--+ +---+--+ ! /\ ! ! / \ ! ! /\ /\ ! ! /\/\ \ `----------------- ... --------, ! V / `-------, ! V +---+----+---+ +------+-----+ +---+----+---+ ! File: foo ! ! File: bar ! ! File: qux ! ! First: 101 ! ! First: 119 ! ! First: 180 ! ! Next: ---+--->! Next: ---+---> ... --->! Next: - ! +============+ +============+ +============+ ! Time: 100 ! ! Time: 120 ! ! Time: 180 ! ! Value: 10 ! ! Value: 0.1 ! ! Value: 2,2 ! +------------+ +------------+ +------------+ ! Time: 110 ! ! Time: 130 ! ! Time: 190 ! ! Value: 26 ! ! Value: 0.1 ! ! Value: 7,3 ! +------------+ +------------+ +------------+ : : : : : : +------------+ +------------+ +------------+ ! Time: 230 ! ! Time: 250 ! ! Time: 310 ! ! Value: 42 ! ! Value: 0.2 ! ! Value: 1,2 ! +------------+ +------------+ +------------+ The above diagram demonstrates: =over 4 =item Files/values are stored in a (balanced) tree. =item Tree nodes and entries in the update queue are the same data structure. =item The local time ("First") and the time specified in updates ("Time") may differ. =item Timed out values are inserted at the "tail". =item Explicitely flushed values are inserted at the "head". =item ASCII art rocks. =back =head1 SECURITY CONSIDERATIONS This daemon is meant to improve IOEperformance for setups with thousands of RRDEfile to be updated. So security measures built into the daemon can be summarized easily: B There is no authentication and authorization, so B will have to take care that only authorized clients can talk to the daemon. Since we assume that graph collection is done on a dedicated machine, i.Ee. the box doesn't do anything else and especially does not have any interactive logins other than root, a UNIX domain socket should take care of that. If you (want to) use the network capability, i.Ee. let the daemon bind to an IPv4 or IPv6 socket, it is B job to install a packet filter or similar mechanism to prevent unauthorized connections. Unless you have a dedicated VLAN or VPN for this, using the network option is probably a bad idea! The daemon will blindly write to any file it gets told, so you really should create a separate user just for this daemon. Also it does not do any sanity checks, so if it gets told to write values for a time far in the future, your files will be messed up good! You have been warned. =head1 BUGS =over 4 =item Tree nodes are never deleted. =back =head1 SEE ALSO L, L =head1 AUHOR B and this manual page have been written by Florian Forster EoctoEatEverplant.orgE.