Each file starts with the following header:
┌───────┬────────┬──────────────────────────┐
│Offset │ Length │ Name │
├───────┼────────┼──────────────────────────┤
│ 0 │ 4 │ tag == "MMV\0" │
├───────┼────────┼──────────────────────────┤
│ 4 │ 4 │ Version (1 or 2) │
├───────┼────────┼──────────────────────────┤
│ 8 │ 8 │ Generation 1 │
├───────┼────────┼──────────────────────────┤
│ 16 │ 8 │ Generation 2 │
├───────┼────────┼──────────────────────────┤
│ 24 │ 4 │ Number of TOC entries │
├───────┼────────┼──────────────────────────┤
│ 28 │ 4 │ Flags │
├───────┼────────┼──────────────────────────┤
│ 32 │ 4 │ Process identifier (PID) │
├───────┼────────┼──────────────────────────┤
│ 36 │ 4 │ Cluster identifier │
└───────┴────────┴──────────────────────────┘
The version number specifies which mapping layout format is in
use. There are two, both are very similar, as described below.
The sole purpose of the MMV version 2 format is to allow the use
of longer metric and instance names. If names longer than
MMV_NAMEMAX are not in use, it is best to use MMV version 1
format as this allows older versions of PCP to also consume the
data. Support for v2 format was added in the pcp-3.11.4 release.
The generation numbers are timestamps at the time of file
creation, and must match for the file to be considered by the MMV
PMDA.
The flags can specify ways in which the client would like the MMV
PMDA to behave - e.g. the MMV_FLAG_PROCESS flag specifies that
only if the process identified by PID is currently running should
those values be exported.
Finally, if set, the cluster identifier is a hint to the MMV PMDA
as to what cluster should be used with this application when
forming the individual metric identifiers. A performance metric
identifier (see PMDA(3)) consists of the PMDA domain number, the
cluster number, and the individual item numbers described in the
Metrics section.
The header is followed by at least 2 TOC sections: one section
for metrics and another for values. The TOC section has the
following format:
┌───────┬────────┬─────────────────────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼─────────────────────────────────────────────┤
│ 0 │ 4 │ Section Type (see mmv_stats.h) │
├───────┼────────┼─────────────────────────────────────────────┤
│ 4 │ 4 │ Number of entries in the section │
├───────┼────────┼─────────────────────────────────────────────┤
│ 8 │ 8 │ Section's offset from the start of the file │
└───────┴────────┴─────────────────────────────────────────────┘
The section types are:
1: Indoms (instance domain definitions)
2: Instances
3: Metrics (metric definitions)
4: Values
5: String
6: Labels
The only mandatory sections are Metrics and Values. Indoms and
Instances sections of either version only appear if there are
metrics with multiple instances. String sections only appear if
there are metrics with string values, when Metrics or Indoms are
defined with help text, or in v2 MMV format where metric names
and instance names are stored in string sections (providing
significantly more space than v1 MMV format for these names).
Label sections only appear if there are metrics annotated with
labels (name/value pairs). Labels are supported in v3 MMV
format.
The entries in the Indoms sections have the following format:
┌───────┬────────┬───────────────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼───────────────────────────────────────┤
│ 0 │ 4 │ Unique serial number for this domain │
├───────┼────────┼───────────────────────────────────────┤
│ 4 │ 4 │ Number of entries in the domain │
├───────┼────────┼───────────────────────────────────────┤
│ 8 │ 8 │ Offset to first Instance or Instance2 │
├───────┼────────┼───────────────────────────────────────┤
│ 16 │ 8 │ Short help text offset │
├───────┼────────┼───────────────────────────────────────┤
│ 24 │ 8 │ Long help text offset │
└───────┴────────┴───────────────────────────────────────┘
The entries in the Instances (v1) section have the following
format:
┌───────┬────────┬───────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼───────────────────────────────┤
│ 0 │ 8 │ Offset into the indom section │
├───────┼────────┼───────────────────────────────┤
│ 8 │ 4 │ Unused padding (zero filled) │
├───────┼────────┼───────────────────────────────┤
│ 12 │ 4 │ Internal instance identifier │
├───────┼────────┼───────────────────────────────┤
│ 16 │ 64 │ External instance identifier │
└───────┴────────┴───────────────────────────────┘
The entries in the Instances (v2) section have the following
format:
┌───────┬────────┬────────────────────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼────────────────────────────────────────────┤
│ 0 │ 8 │ Offset into the indom section │
├───────┼────────┼────────────────────────────────────────────┤
│ 8 │ 4 │ Unused padding (zero filled) │
├───────┼────────┼────────────────────────────────────────────┤
│ 12 │ 4 │ Internal instance identifier │
├───────┼────────┼────────────────────────────────────────────┤
│ 16 │ 8 │ External instance identifier string offset │
└───────┴────────┴────────────────────────────────────────────┘
The entries in the Metrics (v1) section have the following
format:
┌───────┬────────┬───────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼───────────────────────────────┤
│ 0 │ 64 │ Metric Name │
├───────┼────────┼───────────────────────────────┤
│ 64 │ 4 │ Metric Item (see PMDA(3)) │
├───────┼────────┼───────────────────────────────┤
│ 68 │ 4 │ Metric Type (see mmv_stats.h) │
├───────┼────────┼───────────────────────────────┤
│ 72 │ 4 │ Semantics (see PMAPI(3)) │
├───────┼────────┼───────────────────────────────┤
│ 76 │ 4 │ Dimensions (see PMAPI(3)) │
├───────┼────────┼───────────────────────────────┤
│ 80 │ 4 │ Instance Domain ID │
├───────┼────────┼───────────────────────────────┤
│ 84 │ 4 │ Unused padding (zero filled) │
├───────┼────────┼───────────────────────────────┤
│ 88 │ 8 │ Short help text offset │
├───────┼────────┼───────────────────────────────┤
│ 96 │ 8 │ Long help text offset │
└───────┴────────┴───────────────────────────────┘
The entries in the Metrics (v2) section have the following
format:
┌───────┬────────┬───────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼───────────────────────────────┤
│ 0 │ 8 │ Metric Name string offset │
├───────┼────────┼───────────────────────────────┤
│ 8 │ 4 │ Metric Item (see PMDA(3)) │
├───────┼────────┼───────────────────────────────┤
│ 12 │ 4 │ Metric Type (see mmv_stats.h) │
├───────┼────────┼───────────────────────────────┤
│ 16 │ 4 │ Semantics (see PMAPI(3)) │
├───────┼────────┼───────────────────────────────┤
│ 20 │ 4 │ Dimensions (see PMAPI(3)) │
├───────┼────────┼───────────────────────────────┤
│ 24 │ 4 │ Instance Domain ID │
├───────┼────────┼───────────────────────────────┤
│ 28 │ 4 │ Unused padding (zero filled) │
├───────┼────────┼───────────────────────────────┤
│ 32 │ 8 │ Short help text offset │
├───────┼────────┼───────────────────────────────┤
│ 40 │ 8 │ Long help text offset │
└───────┴────────┴───────────────────────────────┘
The entries in the Values section have the following format:
┌───────┬────────┬────────────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼────────────────────────────────────┤
│ 0 │ 8 │ pmAtomValue (see PMAPI(3)) │
├───────┼────────┼────────────────────────────────────┤
│ 8 │ 8 │ Extra space for STRING and ELAPSED │
├───────┼────────┼────────────────────────────────────┤
│ 16 │ 8 │ Offset into the Metrics section │
├───────┼────────┼────────────────────────────────────┤
│ 24 │ 8 │ Offset into the Instances section │
└───────┴────────┴────────────────────────────────────┘
Each entry in the strings section is a 256 byte character array,
containing a single NULL-terminated character string. So each
string has a maximum length of 256 bytes, which includes the
terminating NULL.
The entries in the Labels (v3) section have the following format:
┌───────┬────────┬──────────────────────────────────────────────────────────┐
│Offset │ Length │ Value │
├───────┼────────┼──────────────────────────────────────────────────────────┤
│ 0 │ 4 │ Flags (PM_LABEL_[CLUSTER|ITEM|INDOM|INSTANCES]|OPTIONAL) │
├───────┼────────┼──────────────────────────────────────────────────────────┤
│ 4 │ 4 │ Identifier for given type (indom, cluster or item) │
├───────┼────────┼──────────────────────────────────────────────────────────┤
│ 8 │ 4 │ Internal Instance or PM_IN_NULL │
├───────┼────────┼──────────────────────────────────────────────────────────┤
│ 12 │ 244 │ Payload (Name and Value JSONB String) │
└───────┴────────┴──────────────────────────────────────────────────────────┘
Each entry in the payload is a 244 byte (maximum) character
array, containing a single NULL-terminated name:value pair in
JSON format. Insignificant whitespace must not be present.
Label names consist only of alphanumeric characters or
underscores, and must begin with an alphabetic. Upper and lower
case characters are considered distinct.
