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LOGARCHIVE(5) File Formats Manual LOGARCHIVE(5)

LOGARCHIVE - Performance Co-Pilot archive log formats

Performance Co-Pilot (PCP) archives store historical values about arbitrary metrics recorded from a single host. Archives are machine independent and self-contained - all metric data and metadata required for off-line or off-site analysis is held within an archive.

The format is stable in order to allow long-term historical storage and processing by PMAPI(3) client tools. However some format variants are supported over time, and currently Versions 2 and 3 are supported. The mandate is that PCP will provide long-term backwards compatibility, so an archive created on any version of PCP can be read on that version of PCP and all subsequent versions of PCP. The exception is Version 1 that was retired in the PCP Version 2.0 release in May 1998.

Archives may be read by most PCP client tools, using the -a/--archive NAME option, or dumped raw by pmdumplog(1). Archives are created primarily by pmlogger(1), however they can also be created using the LOGIMPORT(3) programming interface.

Archives may be merged, analyzed, modified and subsampled using pmlogreduce(1), pmlogsummary(1), pmlogrewrite(1) and pmlogextract(1). In addition, PCP archives may be examined in sets or grouped together into ``archive folios'', which are created and managed by the mkaf(1) and pmafm(1) tools.

An archive consists of several physical files that share a common arbitrary prefix, e.g. myarchive.

myarchive.0, myarchive.1, ...
One or more data volumes containing the metric values and any error codes encountered during metric sampling. Typically the largest of the files and may grow very rapidly, depending on the selection of metrics to be logged by pmlogger(1) and the sampling intervals being used.
Information for PMAPI functions such as pmLookupName(3), pmLookupDesc(3), pmLookupLabels(3) and pmLookupInDom(3). The metadata file may grow sporadically as logged metrics, instance domains and labels vary over time.
A temporal index, mapping timestamps to byte offsets in the other files.

All three types of files have a similar record-based structure, a convention of network byte-order (big-endian) encoding, and 32-bit fields for tagging/padding for those records. Strings are stored as 8-bit characters without assuming a specific encoding, so normally ASCII. See also the __pmLog* types in src/include/pcp/libpcp.h.

The volume and .meta files are divided into self-identifying records.

Offset Length Name
0 4 N, length of record, in bytes, including this field
4 N-8 record payload, usually starting with a 32-bit record type tag
N-4 4 N, length of record (again)

All three types of files begin with an ``archive label'' header, which identifies the host name, starting timestamp and timezone information; all referring to the host that was the source of the performance data (which may be different to the host where pmlogger(1) was running).

The ``archive label'' format differs between Version 2 and Version 3, with the latter providing enhanced timestamps (64-bit encoding of the seconds part and nanosecond precision) and some additional fields.

Version 2
Offset Length Name
0 4 tag, PM_LOG_MAGIC | PM_LOG_VERS02=0x50052602
4 4 process id (PID) of pmlogger process that wrote file
8 4 archive start time, seconds part (past UNIX epoch)
12 4 archive start time, microseconds part
16 4 current archive volume number (or -1=.meta, -2=.index)
20 64 name of collection host
80 40 time zone string for collection host ($TZ environment variable)

Version 3
Offset Length Name
0 4 tag, PM_LOG_MAGIC | PM_LOG_VERS03=0x50052603
4 4 PID of pmlogger process that wrote file
8 8 archive start time, seconds part (past UNIX epoch)
16 4 archive start time, nanoseconds part
20 4 current archive volume number (or -1=.meta, -2=.index)
24 4 archive feature bits
28 4 reserved for future use
32 256 name of collection host
288 256 timezone string for collection host ($TZ environment variable), e.g. AEDT-11
544 256 timezone zoneinfo string for collection host, e.g. :Australia/Melbourne

The ``archive feature bits'' are intended to encode possible future extensions or differences to the on-disk structure or the the archive semantics. At this stage there are no such features, but if they are introduced at some point in the future, there will be associated PM_LOG_FEATURE_XXX macros added to the <pcp/pmapi.h> header file.

All fields, except for the ``current archive volume number'', match for all files in a single PCP archive.

After the archive label record, an archive volume file contains one or more records, each providing metric values corresponding to the pmResult from one pmFetch(3) operation. The record size may vary according to number of metrics being fetched and the number of instances in the associated instance domains.

For Version 2 the file size is limited to 2GiB, due to storage of 32-bit byte offsets within the temporal index. For Version 3 the file size is limited to 8191PiB, due to storage of 62-bit byte offsets within the temporal index.

The pmResult format differs between Version 2 and Version 3, with the latter providing enhanced timestamps (64-bit encoding of the seconds part and nanosecond precision).

Version 2
Offset Length Name
0 4 timestamp, seconds part (past UNIX epoch)
4 4 timestamp, microseconds part
8 4 number of metrics with data following
12 M pmValueSet #0
12+M N pmValueSet #1
12+M+N ... ...
NOP X pmValueBlock #0
NOP+X Y pmValueBlock #1
NOP+X+Y ... ...

Version 3
Offset Length Name
0 8 timestamp, seconds part (past UNIX epoch)
8 4 timestamp, nanoseconds part
12 4 number of metrics with data following
16 M pmValueSet #0
16+M N pmValueSet #1
16+M+N ... ...
NOP X pmValueBlock #0
NOP+X Y pmValueBlock #1
NOP+X+Y ... ...

Records with a ``number of metrics'' equal to zero are ``mark records'', and represent interruptions, missing data, or time discontinuities in logging.

This subrecord represents the values for one metric at one point in time.

Offset Length Name
0 4 Performance Metrics Identifier (PMID)
4 4 number of values
8 4 value format, PM_VAL_INSITU=0 or PM_VAL_DPTR=1
12 M pmValue #0
12+M N pmValue #1
12+M+N ... ...

The metadata describing metrics is found in the .meta file where the entries are not timestamped, as the metadata is assumed to be unchanging throughout an archive.

This subrecord represents one value for one instance of a metric at one point in time. It is a variant type, depending on the parent pmValueSet's value format field. This allows small numbers to be encoded compactly, but retain flexibility for larger or variable length data to be stored later in the pmResult record in a pmValueBlock subrecord.

Offset Length Name
0 4 internal instance identifier (or PM_IN_NULL=-1 for singular metrics)
4 4 value (INSITU) or
offset in pmResult to our pmValueBlock (DPTR)

The metadata describing the instance domain for metrics is found in the .meta file. Since the numeric mappings may change during the lifetime of the logging session, it is important to match up the timestamp of the measurement record with the corresponding instance domain record. That is, the instance domain corresponding to a measurement at time T is the instance domain observation for the metric's instance domain with largest timestamp T' <= T.

Instances of this subrecord are placed at the end of the pmValueSet, after all the pmValue subrecords. If (and only if) needed, they are padded at the end to the next 32-bit boundary.

Offset Length Name
0 1 value type (same as pmDesc.type)
1 3 4 + N, the length of the subrecord
4 N bytes that make up the raw value
4+N 0-3 padding (not included in the 4+N length field)

Note that for PM_TYPE_STRING, the length includes an explicit NULL terminator byte. For PM_TYPE_EVENT, the value byte string is further structured. Refer to PMDAEVENTARRAY(3) for more information about how arrays of event records are packed inside a pmResult container.

After the archive label record, the metadata file contains interleaved metric description records, timestamped instance domain records, timestamped label records (for context, instance domain and metric labels) and (help) text records. Unlike the data volumes, these records are not forced to 32-bit alignment.

For Version 2 the file size is limited to 2GiB, due to storage of 32-bit byte offsets within the temporal index. For Version 3 the file size is limited to 8191PiB, due to storage of 62-bit byte offsets within the temporal index.

See also libpcp/src/logmeta.c.

Instances of this pmDesc) record provide the description or metadata for each metric appearing in the PCP archive. This metadata includes the metric's PMID, data type, data semantics, instance domain identifier (or PM_INDOM_NULL for singular metrics with only one value) and a set of (1 or more) names.

Offset Length Name
0 4 tag, TYPE_DESC=1
4 4 PMID
8 4 data type (PM_TYPE_*)
12 4 instance domain identifier
16 4 metric semantics (PM_SEM_*)
20 4 units: bit-packed pmUnits
4 4 number of alternative names for this PMID
28 4 N: number of bytes in this name
32 N bytes of the name, no NULL terminator nor padding
32+N 4 N2: number of bytes in next name
36+N N2 bytes of the name, no NULL terminator nor padding
... ... ...

A set-valued metric is defined over an instance domain, which consists of an instance domain identifier (will have already been mentioned in a prior pmDesc record), a count of the number of instances and a map that defines the association between internal instance identifiers (integers) and external instance names (strings).

Because instance domains can change over time, the instance domain also requires a timestamp, and the same instance domain can occur multiple times within the .meta file. The timestamps are used to search for the temporally correct instance domain when decoding pmResult records from the archive data volumes, or answering metadata queries against the instance domain.

The instance domain format differs markedly between Version 2 and Version 3. Version 3 provides enhanced timestamps (64-bit encoding of the seconds part and nanosecond precision) and introduces a new ``delta'' instance domain format that encodes differences between the previous observation of the instance domain and the current state of the instance domain.

Full Instance Domain - Version 2
Offset Length Name
0 4 tag, TYPE_INDOM_V2=2
4 4 timestamp, seconds part (past UNIX epoch)
8 4 timestamp, microseconds part
12 4 instance domain number
16 4 N: number of instances in domain, normally >0
20 4 first instance number
24 4 second instance number (if appropriate)
... ... ...
20+4*N 4 first offset into string table (see below)
20+4*N+4 4 second offset into string table (etc.)
... ... ...
20+8*N M base of string table, containing
packed, NULL-terminated instance names

Full Instance Domain - Version 3
Offset Length Name
0 4 tag, TYPE_INDOM=5
4 8 timestamp, seconds part (past UNIX epoch)
12 4 timestamp, nanoseconds part
16 4 instance domain number
20 4 N: number of instances in domain, normally >0
24 4 first instance number
28 4 second instance number (if appropriate)
... ... ...
24+4*N 4 first offset into string table (see below)
24+4*N+4 4 second offset into string table (etc.)
... ... ...
24+8*N M base of string table, containing
packed, NULL-terminated instance names

The ``delta'' instance domain record in Version 3 uses the same physical structure as the ``full'' instance domain above with the following differences:

  • The tag is TYPE_INDOM_DELTA=6.
  • The ``number of instances in domain'' field becomes the sum of the number of instances added and the number of instances deleted.
  • Deleted instances are encoded with the string offset set to -1 and there is no corresponding string table entry.
  • Added instances are encoded exactly the same way.

The ``delta'' instance domain format is used to provide a more compact on-disk encoding for instance domains that have a large number of instances and are subject to frequent small changes, e.g. the instance domain of process ids, as exported by pmdaproc(1).

For ``full'' instance domain records the instance domain replace the previous instance domain: prior records are not searched for instance domain metadata queries after this timestamp.

Each instance domain in a Version 3 archive must have an initial ``full'' instance domain record. Subsequent records for the same instance domain can be the `full'' or the ``delta'' variant. Any instance mentioned in the prior observation of an instance domain that is not mentioned in the ``delta'' instance domain record is assumed to continue to exist for the current observation of the instance domain.

Instances of this (pmLogLabelSet) record provide sets of label-name:label-value pairs associated with labels of the context, instance domains and individual performance metrics - refer to pmLookupLabels(3) for further details.

Any instance domain identifier will have already been mentioned in a prior pmDesc record.

As new labels can appear during an archiving session, these records are timestamped and must be searched when decoding pmResult records from the archive data volumes. The pmLogLabelSet format differs between Version 2 and Version 3, with the latter providing enhanced timestamps (64-bit encoding of the seconds part and nanosecond precision).

Version 2
Offset Length Name
0 4 tag, TYPE_LABEL_V2=3
4 4 timestamp, seconds part (past UNIX epoch)
8 4 timestamp, microseconds part
12 4 label type (PM_LABEL_* type macros.)
16 4 numeric identifier - domain, PMID, etc or PM_IN_NULL=-1 for context labels
20 4 N: number of label sets in this record, usually 1 except in the case of instances
24 4 offset to the start of the JSONB labels string
28 L1 first labelset array entry (see below)
... ... ...
28+L1 LN N-th labelset array entry (see below)
... ... ...
28+L1+...LN M concatenated JSONB strings for all labelsets

Version 3
Offset Length Name
0 4 tag, TYPE_LABEL=7
4 8 timestamp, seconds part (past UNIX epoch)
12 4 timestamp, nanoseconds part
16 4 label type (PM_LABEL_* type macros.)
20 4 numeric identifier - domain, PMID, etc or PM_IN_NULL=-1 for context labels
24 4 N: number of label sets in this record, usually 1 except in the case of instances
28 4 offset to the start of the JSONB labels string
32 L1 first labelset array entry (see below)
... ... ...
32+L1 LN N-th labelset array entry (see below)
... ... ...
32+L1+...LN M concatenated JSONB strings for all labelsets

Records of this form replace the existing labels for a given label type: prior records are not searched for resolving that class of label in measurements after this timestamp.

The individual labelset array entries are variable length, depending on the number of labels present within that set. These entries contain the instance identifiers (in the case of type PM_LABEL_INSTANCES labels), lengths and offsets of each label name and value, and also any flags set for each label.

Offset Length Name
0 4 instance identifier (or PM_IN_NULL=-1)
4 4 length of JSONB label string
8 4 N: number of labels in this labelset
12 2 first label name offset
14 1 first label name length
15 1 first label flags (e.g. optionality)
16 2 first label value offset
18 2 first label value length
20 2 second label name offset (if appropriate)
... ... ...

This (pmLogText) record stores help text associated with a metric or an instance domain - as provided by pmLookupText(3) and pmLookupInDomText(3).

The metric identifier and instance domain identifier will have already been mentioned in a prior pmDesc record.

Offset Length Name
0 4 tag, TYPE_TEXT=4
4 4 text and identifier type (PM_TEXT_* macros.)
8 4 numeric identifier - PMID or instance domain
12 M help text string, arbitrary text

After the archive label record, the temporal index file contains a plainly concatenated, unframed group of tuples, which relate timestamps to the byte offsets in the volume and .meta files. These records are fixed size, fixed format, and are not enclosed in the standard length/payload/length wrapper: they take up the entire remainder of the .index file after the archive label record.

The temporal index file provides a rapid way of seeking to a particular point of time within an archive for both the performance metric values and the associated metadata.

See also libpcp/src/logutil.c.

The index format differs between Version 2 and Version 3, with the latter providing enhanced timestamps (64-bit encoding of the seconds part and nanosecond precision) and 64-bit byte offsets.

Version 2
Offset Length Name
0 4 timestamp, seconds part (past UNIX epoch)
4 4 timestamp, microseconds part
8 4 archive volume number (0...N)
12 4 byte offset in .meta file
16 4 byte offset in archive volume file

Version 3
Offset Length Name
0 8 timestamp, seconds part (past UNIX epoch)
8 4 timestamp, nanoseconds part
12 4 archive volume number (0...N)
16 8 byte offset in .meta file
24 8 byte offset in archive volume file

Since the temporal index is optional, and exists only to speed up time-based random access to metrics and their metadata, the index records are emitted only intermittently. An archive reader program should not presume any particular rate of data flow into the index. However, common events that may trigger a new temporal index record include changes in instance domains, switching over to a new archive volume, and starting or stopping logging. One reliable invariant however is that, for each index entry, there are to be no meta or archive volume records with a timestamp after that in the index, but physically before the associated byte offset in the index.

Several PCP tools create archives in standard locations:

$HOME/.pcp/pmlogger
default location for the interactive chart recording mode in pmchart(1)
$PCP_LOG_DIR/pmlogger
default location for pmlogger_daily(1) and pmlogger_check(1) scripts

mkaf(1), PCPIntro(1), pmafm(1), pmchart(1), pmdaproc(1), pmdumplog(1), pmlogger(1), pmlogger_check(1), pmlogger_daily(1), pmlogreduce(1), pmlogrewrite(1), pmlogsummary(1), LOGIMPORT(3), PMAPI(3), pmLookupDesc(3), pmLookupInDom(3), pmLookupInDomText(3), pmLookupLabels(3), pmLookupName(3), pmLookupText(3), pcp.conf(5) and pcp.env(5).

Performance Co-Pilot