Ubuntu Manpage: pmdadm - Device Mapper PMDA

NAME

       pmdadm - Device Mapper PMDA

DESCRIPTION

       pmdadm  is a Performance Metrics Domain Agent (PMDA) which exports metric values for Device Mapper on the
       local system.

       This PMDA collects its data through the dmsetup(8) utility and the dmstats  API  and  requires  that  the
       program  is  installed in order to function.  In addition, at least one statistics region must be created
       using the dmstats(8) utility in order to get basic counter values.  See below for examples.

       Note that device-mapper statistics collection is not enabled by default  and  is  not  persistent  across
       reboots  (unless a system administrator has configured something to run in rc.local or equivalent).  This
       is because there are overheads associated with statistics collection.  In addition, the pmdadm PMDA  does
       not automatically enable any statistics collection because it may not be the only consumer of the data.

INSTALLATION

       Install the DM PMDA by using the Install script as root:

           # cd $PCP_PMDAS_DIR/dm
           # ./Install

       To uninstall, the following must be done as root:

           # cd $PCP_PMDAS_DIR/dm
           # ./Remove

       pmdadm  is  launched  by  pmcd(1)  and should never be executed directly.  The Install and Remove scripts
       notify pmcd when the agent is installed or removed.

CONFIGURATION

       This PMDA uses the dmstats API (the userspace device-mapper support  library)  to  capture  device-mapper
       performance data.
       Using  this PMDA, users need the userspace device-mapper support library (libdevmapper) and the userspace
       command line tool dmstats(8).  Before device-mapper metrics will be available for PCP, statistics regions
       have to be created.  For example, to enable basic statistics for all local device-mapper logical devices,
       use the following command:

            # dmstats create --alldevices

       To create statistics regions with specified histogram boundaries, use the following:

            # dmstats create --alldevices --bounds histogram_boundaries

       Specify the boundaries of a latency histogram to be tracked for the region as a comma separated  list  of
       latency  values. Latency values are given in nanoseconds. An optional unit suffix of ns, us, ms, or s may
       be given after each  value  to  specify  units  of  nanoseconds,  microseconds,  miliseconds  or  seconds
       respectively.  For example,

            # dmstats create --alldevices --bounds 10us,30us,50us,70us,90us

       Further details and more complex examples can be found in dmstats(8).

METRICS

Basic Counters
Basic counters provide access to the raw counter data from the kernel, allowing further processing to
be carried out by another program.
The Kernel provides thirteen separate counters for each statistics area. The first eleven of these
match the counters provided in /proc/diskstats or /sys/block/*/*/stat. The final pair provide
separate counters for read and write time.

dmstats.read
Count of reads completed this interval per device-mapper device.

dmstats.reads_merged
Count of reads merged this interval per device-mapper device.

dmstats.read_bytes
Count of kbytes read this interval per device-mapper device.

dmstats.reads_time
Accumulated duration of all read requests per device-mapper device.

dmstats.write
Count of writes completed this interval per device-mapper device.

dmstats.writes_merged
Count of writes completed this interval per device-mapper device.

dmstats.write_bytes
Count of kbytes write this interval per device-mapper device.

dmstats.writes_time
Accumulated duration of all write requests per device-mapper device.

dmstats.in_progress
Count of requests currently in progress per device-mapper device.

dmstats.io_ticks
Nanoseconds spent servicing request per device-mapper device.

dmstats.queue_ticks
This field is incremented at each I/O start, I/O completion, I/O merge, or read of these stats by
the number of I/Os in progress multiplied by the number of nanoseconds spent doing I/O since the
last update of this field. This can provide an easy measure of both I/O completion time and the
backlog that may be accumulating.

dmstats.read_ticks
Nanoseconds spent servicing reads per device-mapper device.

dmstats.write_ticks
Nanoseconds spent servicing writes per device-mapper device.

Histogram fields
Histograms measure the frequency distribution of user specified I/O latency intervals. Histogram bin
boundaries are specified when a region is created.
Instance name represents devicename, region id and bin boundaries.

dmstats.histogram.hist_count
A list of the histogram counts for the current statistics area in order of ascending latency
value. Each value represents the number of I/Os with latency times falling into that bin's time
range during the sample period.

dmstats.histogram.hist.bins
The number of latency histogram bins configured for the area.

EXAMPLES

       # dmstats create looptest0 --bounds 10us,30us,50us
       looptest0: Created new region with 1 area(s) as region ID 0

       # pminfo -f dmstats.read dmstats.histogram.hist_count

       dmstats.read
           inst [0 or "looptest0"] value 4099

       dmstats.histogram.hist_count
           inst [0 or "looptest0:0:0s"] value 1
           inst [1 or "looptest0:0:10us"] value 3752
           inst [2 or "looptest0:0:30us"] value 250
           inst [3 or "looptest0:0:50us"] value 96

FILES

       $PCP_PMDAS_DIR/dm/help
           default help text file for the dm metrics

       $PCP_PMDAS_DIR/dm/Install
           installation script for the pmdadm agent

       $PCP_PMDAS_DIR/dm/Remove
           undo installation script for the pmdadm agent

       $PCP_LOG_DIR/pmcd/dm.log
           default log file for error messages from pmdadm

PCP ENVIRONMENT

       Environment  variables with the prefix PCP_ are used to parameterize the file and directory names used by
       PCP. On each installation, the file /etc/pcp.conf contains the local values  for  these  variables.   The
       $PCP_CONF variable may be used to specify an alternative configuration file, as described in pcp.conf(5).