rrdcached - Man Page

rrdcached [-a alloc_size] [-b base_dir [-B]] [-F] [-f timeout] [-G group]] [-g] [-j journal_dir] [-L] [-l address] [-m mode] [-O] [-o log_file] [-P permissions] [-p pid_file] [-R] [-s group] [-t write_threads] [-U user]] [-V log_level] [-w timeout] [-z delay]

rrdcached 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 flush 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 IO related 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 “Security Considerations” below before using this daemon! A detailed description of how the daemon operates can be found in the “How It Works” section below.

-l address

Tells the daemon to bind to address and accept incoming TCP connections on that socket. If address begins with unix:, everything following that prefix is interpreted as the path to a UNIX domain socket. Otherwise the address or node name are resolved using getaddrinfo().

For network sockets, a port may be specified by using the form [address]:port. If the address is an IPv4 address or a fully qualified domain name (i. e. the address contains at least one dot (.)), the square brackets can be omitted, resulting in the (simpler) address:port pattern. The default port is 42217. If you specify a network socket, it is mandatory to read the “Security Considerations” section.

The following formats are accepted. Please note that the address of the UNIX domain socket must start with a slash in the second case!

   unix:</path/to/unix.sock>
   /<path/to/unix.sock>
   <hostname-or-ip>
   [<hostname-or-ip>]:<port>
   <hostname-or-ipv4>:<port>

Given a port without a host (e.g. -l :42217) the daemon will listen on that port on all network interfaces.  Use -L to avoid the need to explicitly provide the port if the default port is desired.

If no -l option is not specified the default address, unix:/tmp/rrdcached.sock, will be used. Multiple -l options may be provided.

-L

Tells the daemon to bind to the default TCP port on all available interfaces.  It is equivalent to -l '' without the confusion of the empty string parameter.

-s group_name|gid

Set the group permissions of a UNIX domain socket. The option accepts either a numeric group id or group name. That group will then have both read and write permissions (the socket will have file permissions 0760) for the socket and, therefore, is able to send commands to the daemon. This may be useful in cases where you cannot easily run all RRD processes with the same user privileges (e.g. graph generating CGI scripts that typically run in the permission context of the web server).

This option affects the following UNIX socket addresses (the following -l options) or the default socket (if no -l options have been specified), i.e., you may specify different settings for different sockets.

The default is not to change ownership or permissions of the socket and, thus, use the system default.

-m mode

Set the file permissions of a UNIX domain socket. The option accepts an octal number representing the bit pattern for the mode (see chmod(1) for details).

Please note that not all systems honor this setting. On Linux, read/write permissions are required to connect to a UNIX socket. However, many BSD-derived systems ignore permissions for UNIX sockets. See unix(7) for details.

This option affects the following UNIX socket addresses (the following -l options) or the default socket (if no -l options have been specified), i.e., you may specify different settings for different sockets.

The default is not to change ownership or permissions of the socket and, thus, use the system default.

-P command[,command[,...]]

Specifies the commands accepted via both a network and a UNIX socket. This allows administrators of RRDCacheD to control the actions accepted from various sources.

The arguments given to the -P option is a comma separated list of commands. For example, to allow one the FLUSH and PENDING commands one could specify:

  rrdcached -P FLUSH,PENDING $MORE_ARGUMENTS

The -P option affects the following socket addresses (the following -l options) or the default socket (if no -l options have been specified). In the following example, only the IPv4 network socket (address 10.0.0.1) will be restricted to the FLUSH and PENDING commands:

  rrdcached -l unix:/some/path -P FLUSH,PENDING -l 10.0.0.1

A complete list of available commands can be found in the section “Valid Commands” below. There are two minor special exceptions:

• The HELP and QUIT commands are always allowed.
• If the BATCH command is accepted, the . command will automatically be accepted, too.

Please also read “Security Considerations” below.

-V log_level

rrdcached under load can severely flood the logs. This command line option specifies the maximum log_level to be used, meaning that a message with verbosity higher than log_level is muted (LOG_EMERG being the lowest and LOG_DEBUG highest).

Accepted values for “log_level” (lowest to highest verbosity): LOG_EMERG, LOG_ALERT, LOG_CRIT, LOG_ERR, LOG_WARNING, LOG_NOTICE, LOG_INFO, LOG_DEBUG

Default log level when this flag is NOT present: LOG_ERR

See also: syslog.h

-o log_file

Log to the given file instead of syslog.

-w timeout

Data is written to disk every timeout seconds. An optional suffix may be used (e.g. 5m instead of 300 seconds). If this option is not specified the default interval of 300 seconds will be used.

-z delay

If specified, rrdcached will delay writing of each RRD for a random number of seconds in the range [0,delay).  This will avoid too many writes being queued simultaneously.  This value should be no greater than the value specified in -w. An optional suffix may be used (e.g. 3m instead of 180 seconds). By default, there is no delay.

-f timeout

Every timeout 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 3600 seconds, is acceptable in most cases. An optional suffix may be used (e.g. 1h instead of 3600 seconds). This timeout defaults to 3600 seconds.

-p file

Sets the name and location of the PID-file. If not specified, the default, $localstatedir/run/rrdcached.pid will be used.

-t write_threads

Specifies the number of threads used for writing RRD files.  The default is 4.  Increasing this number will allow rrdcached to have more simultaneous I/O requests into the kernel.  This may allow the kernel to re-order disk writes, resulting in better disk throughput.

-j dir

Write updates to a journal in dir.  In the event of a program or system crash, this will allow the daemon to write any updates that were pending at the time of the crash.

On startup, the daemon will check for journal files in this directory.  If found, all updates therein will be read into memory before the daemon starts accepting new connections.

The journal will be rotated with the same frequency as the flush timer given by -f.

When journaling is enabled, the daemon will use a fast shutdown procedure. Rather than flushing all files to disk, it will make sure the journal is properly written and exit immediately.  Although the RRD data files are not fully up-to-date, no information is lost; all pending updates will be replayed from the journal next time the daemon starts up.

To disable fast shutdown, use the -F option.

-F

ALWAYS flush all updates to the RRD data files when the daemon is shut down, regardless of journal setting.

-g

Run in the foreground.  The daemon will not fork().

-b dir

The daemon will change into a specific directory at startup. All files passed to the daemon, that are specified by a relative path, will be interpreted to be relative to this directory. If not given the default, /tmp, 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".

WARNING: The paths up to and including the base directory MUST NOT BE symbolic links.  In other words, if the base directory is specified as:

    -b /base/dir/somewhere

... then NONE of the following should be symbolic links:

    /base
    /base/dir
    /base/dir/somewhere

-B

Only permit writes into the base directory specified in -b (and any sub-directories).  This does NOT detect symbolic links.  Paths containing ../ will also be blocked.

-R

Permit recursive subdirectory creation in the base directory specified in -b (and any sub-directories). Can only be used when -B is also set.

-a alloc_size

Allocate value pointers in chunks of alloc_size.  This may improve CPU utilization on machines with slow realloc() implementations, in exchange for slightly higher memory utilization.  The default is 1. Do not set this more than the -w value divided by your average RRD step size.

-O

Prevent the CREATE command from overwriting existing files, even when it is instructed to do so.  This is for added security.

-G -group

When running as daemon and invoked from a privileged account, reset group privileges to those of group.  The group may be specified as a name or as a group ID.  The daemon will exit with a diagnostic if it cannot successfully transition to the specified group.

-U -user

When running as daemon and invoked from a privileged account, reset user privileges to those of user.  The user may be specified as a name or as a user ID.  The daemon will exit with a diagnostic if it cannot successfully transition to the specified user.

The following commands may be made aware of the rrdcached using the command line argument --daemon or the environment variable RRDCACHED_ADDRESS:

• dump
• fetch
• flush
• graph
• graphv
• info
• first
• last
• lastupdate
• update
• xport
• create
• list

The update command can send values to the daemon instead of writing them to the disk itself. All other commands can send a FLUSH command (see below) to the daemon before accessing the files, so they work with up-to-date data even if the cache timeout is large.

The daemon reports errors in one of two ways: During startup, error messages are printed to STDERR. One of the steps when starting up is to fork to the background and closing STDERR - after this writing directly to the user is no longer possible. Once this has happened, the daemon will send log messages to the system logging daemon using syslog(3). The facility used is LOG_DAEMON.

When receiving an update, rrdcached 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 not 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 now() - First >= timeout, where timeout is the timeout specified using the -w option, see “Options”. If the values are “old enough” they will be enqueued in the “update queue”, i. e. 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. e. files that are not updated anymore, would never be written to disk. Therefore, every now and then, controlled by the -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 3600 seconds (one hour).

The downside of caching values is that they won't show up in graphs generated from the RRD files. To get around this, the daemon provides the “flush command” to flush specific files. This means that the file is inserted at the head of the update queue or moved there if it is already enqueued. The flush command will return only after the file's pending updates have been written to disk.

 +------+   +------+                               +------+
 ! head !   ! root !                               ! tail !
 +---+--+   +---+--+                               +---+--+
     !         /\                                      !
     !        /  \                                     !
     !       /\  /\                                    !
     !      /\/\ \ `----------------- ... --------,    !
     V     /      `-------,                       !    V
 +---+----+---+    +------+-----+             +---+----+---+
 ! File:  foo !    ! File:  bar !             ! File:  qux !
 ! First: 101 !    ! First: 119 !             ! First: 180 !
 ! Next:&bar -+--->! Next:&... -+---> ... --->! Next:NULL  !
 | Prev:NULL  !<---+-Prev:&foo  !<--- ... ----+-Prev: &... !
 +============+    +============+             +============+
 ! 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:

• Files/values are stored in a (balanced) tree.
• Tree nodes and entries in the update queue are the same data structure.
• The local time (“First”) and the time specified in updates (“Time”) may differ.
• Timed out values are inserted at the “tail”.
• Explicitly flushed values are inserted at the “head”.
• ASCII art rocks.

The daemon communicates with clients using a line based ASCII protocol which is easy to read and easy to type. This makes it easy for scripts to implement the protocol and possible for users to use telnet to connect to the daemon and test stuff “by hand”.

The protocol is line based, this means that each record consists of one or more lines. A line is terminated by the line feed character 0x0A, commonly written as \n. In the examples below, this character will be written as <LF> (“line feed”).

After the connection has been established, the client is expected to send a “command”. A command consists of the command keyword, possibly some arguments, and a terminating newline character. For a list of commands, see “Valid Commands” below.

Example:

  FLUSH /tmp/foo.rrd<LF>

The daemon answers with a line consisting of a status code and a short status message, separated by one or more space characters. A negative status code signals an error, a positive status code or zero signal success. If the status code is greater than zero, it indicates the number of lines that follow the status line.

Examples:

 0 Success<LF>

 2 Two lines follow<LF>
 This is the first line<LF>
 And this is the second line<LF>

SIGINT and SIGTERM

The daemon exits normally on receipt of either of these signals.  Pending updates are handled in accordance with the -j and -F options.

SIGUSR1

The daemon exits AFTER flushing all updates out to disk.  This may take a while.

SIGUSR2

The daemon exits immediately, without flushing updates out to disk. Pending updates will be replayed from the journal when the daemon starts up again.  WARNING: if journaling (-j) is NOT enabled, any pending updates WILL BE LOST.

No known bugs at the moment.

rrdtool, rrdgraph

Florian Forster <octo at verplant.org>

Both rrdcached and this manual page have been written by Florian.

kevin brintnall <kbrint@rufus.net> Steve Shipway <steve@steveshipway.org> Martin Sperl <rrdtool@martin.sperl.org>

collectd.conf(5).