pmdadm — Device Mapper PMDA


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.


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

     # cd $PCP_PMDAS_DIR/dm ($PCP_PMDAS_DIR is usually /var/lib/pcp/pmdas)
     # ./Install

To uninstall, do the following 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.


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).


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.

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


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


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


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


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


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


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


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


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


Nanoseconds spent servicing request per device-mapper device.


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.


Nanoseconds spent servicing reads per device-mapper device.


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.


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.


The number of latency histogram bins configured for the area.


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

# pminfo -f dmstats.histogram.hist_count
   inst [0 or "looptest0"] value 4099

   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



default help text file for the dm metrics


installation script for the pmdadm agent


undo installation script for the pmdadm agent


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).

See Also

pmcd(1), pmstore(1), dmsetup(8), and dmstats(8).

Referenced By


PCP Performance Co-Pilot