LOGARCHIVE man page

LOGARCHIVE — performance metrics archive format

Description

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

The format is stable in order to allow long-term historical storage and processing by PMAPI client tools.

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.

Archives consist 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 pmlogger sampling interval(s) being used.

myarchive.meta

Information for PMAPI functions such as pmLookupDesc(3), pmLookupLabels(3) and pmLookupInDom(3). The metadata file may grow sporadically as logged metrics, instance domains and labels vary over time.

myarchive.index

A temporal index, mapping timestamps to offsets in the other files.

Common Features

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 libpcp.h.

Record Framing

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 tag
N-4 4 N, length of record (again)

Archive Log Label

All three types of files begin with a "log label" header, which identifies the host name, the time interval covered, and a time zone.

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

All fields, except for the current log volume number field, match for all archive-related files produced by a single run of the tool.

Archive Volume (.0, .1, ...) Records

pmResult

After the archive log label record, an archive volume file contains metric values corresponding to the pmResult set of one pmFetch operation, which is almost identical to the form on disk. The record size may vary according to number of PMIDs being fetched, the number of instances for their domains. File size is limited to 2GiB, due to storage of 32-bit offsets within the temporal index.

Offset Length Name
0 4 timestamp, seconds part (past UNIX epoch)
4 4 timestamp, microseconds part
8 4 number of PMIDs 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 ... ...

Records with a number-of-PMIDs equal to zero are "mark records", and represent interruptions, missing data, or time discontinuities in logging.

pmValueSet

This subrecord represents the measurements for one metric.

Offset Length Name
0 4 PMID
4 4 number of values
8 4 storage mode, PM_VAL_INSITU=0 or PM_VAL_DPTR=1
12 M pmValue #0
12+M N pmValue #1
12+M+N ... ...

The metric-description metadata for PMIDs is found in the .meta files. These entries are not timestamped, so the metadata is assumed to be unchanging throughout the archiving session.

pmValue

This subrecord represents one measurement for one instance of the metric. 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.

Offset Length Name
0 4 number in instance-domain (or PM_IN_NULL=-1)
4 4 value (INSITU) or
offset in pmResult to our pmValueBlock (DPTR)

The instance-domain metadata for PMIDs is found in the .meta files. 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 are the records with largest timestamps T' <= T.

pmValueBlock

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-higher 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 bytestring is further structured.

METADATA FILE (.meta) RECORDS

After the archive log label record, the metadata file contains interleaved metric-description and timestamped instance-domain descriptors. File size is limited to 2GiB, due to storage of 32-bit offsets within the temporal index. Unlike the data volumes, these records are not forced to 32-bit alignment. See also libpcp/logmeta.c.

pmDesc

Instances of this record represent the metric description, giving a name, type, instance-domain identifier, and a set of names to each PMID used in the archive volume.

Offset Length Name
0 4 tag, TYPE_DESC=1
4 4 PMID
8 4 type (PM_TYPE_*)
12 4 instance domain number
16 4 semantics of value (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

pmLogIndom

Instances of this record represent the number-string mapping table of an instance domain. The instance domain number will have already been mentioned in a prior pmDesc record. As new instances may appear over a long archiving run these records are timestamped, and must be searched when decoding pmResult records from the archive data volumes. Instance names may be reused between instance numbers, so an offset-based string table is used that facilitates sharing.

Offset Length Name
0 4 tag, TYPE_INDOM=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

Records of this form replace the existing instance-domain: prior records are not searched for resolving instance numbers in measurements after this timestamp.

pmLogLabelSet

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

Any instance domain number 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.

Offset Length Name
0 4 tag, TYPE_LABEL=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

Records of this form replace the existing labels for a given 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)

pmLogText

This 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 number 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

INDEX FILE (.index) RECORDS

After the archive log label record, the temporal index file contains a plainly concatenated, unframed group of tuples, which relate timestamps to 32-bit seek 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. See also libpcp/logutil.c.

Offset Length Name
0 4 event time, seconds part (past UNIX epoch)
4 4 event time, microseconds part
8 4 archive volume number (0...N)
12 4 byte offset in .meta file of pmDesc or pmLogIndom
16 4 byte offset in archive volume file of pmResult

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 byte-offset in the index.

Files

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

$PCP_LOG_DIR/pmmgr

default location for the PCP daemon manager pmmgr(1)

See Also

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

Referenced By

pcp2elasticsearch(1), pcp2graphite(1), pcp2influxdb(1), pcp2json(1), pcp2xml(1), pcp2zabbix(1), PCPIntro(1), pmrep(1).

Performance Co-Pilot