SYSTEMD-JOURNALD.SERVICE(8)systemd-journald.serviceSYSTEMD-JOURNALD.SERVICE(8)
NAME
systemd-journald.service, systemd-journald.socket, systemd-journald-
dev-log.socket, systemd-journald-audit.socket, systemd-
journald@.service, systemd-journald@.socket, systemd-journald-
varlink@.socket, systemd-journald - Journal service
SYNOPSIS
systemd-journald.service
systemd-journald.socket
systemd-journald-dev-log.socket
systemd-journald-audit.socket
systemd-journald@.service
systemd-journald@.socket
systemd-journald-varlink@.socket
/lib/systemd/systemd-journald
DESCRIPTION
systemd-journald is a system service that collects and stores logging
data. It creates and maintains structured, indexed journals based on
logging information that is received from a variety of sources:
o Kernel log messages, via kmsg
o Simple system log messages, via the libc syslog(3) call
o Structured system log messages via the native Journal API, see
sd_journal_print(3)
o Standard output and standard error of service units. For further
details see below.
o Audit records, originating from the kernel audit subsystem
The daemon will implicitly collect numerous metadata fields for each
log messages in a secure and unfakeable way. See systemd.journal-
fields(7) for more information about the collected metadata.
Log data collected by the journal is primarily text-based but can also
include binary data where necessary. Individual fields making up a log
record stored in the journal may be up to 2^64-1 bytes in size.
The journal service stores log data either persistently below
/var/log/journal or in a volatile way below /run/log/journal/ (in the
latter case it is lost at reboot). By default, log data is stored
persistently if /var/log/journal/ exists during boot, with an implicit
fallback to volatile storage otherwise. Use Storage= in
journald.conf(5) to configure where log data is placed, independently
of the existence of /var/log/journal/.
On systems where /var/log/journal/ does not exist yet but where
persistent logging is desired (and the default journald.conf is used),
it is sufficient to create the directory, and ensure it has the correct
access modes and ownership:
mkdir -p /var/log/journal
systemd-tmpfiles --create --prefix /var/log/journal
See journald.conf(5) for information about the configuration of this
service.
STREAM LOGGING
The systemd service manager invokes all service processes with standard
output and standard error connected to the journal by default. This
behaviour may be altered via the StandardOutput=/StandardError= unit
file settings, see systemd.exec(5) for details. The journal converts
the log byte stream received this way into individual log records,
splitting the stream at newline ("\n", ASCII 10) and NUL bytes.
If systemd-journald.service is stopped, the stream connections
associated with all services are terminated. Further writes to those
streams by the service will result in EPIPE errors. In order to react
gracefully in this case it is recommended that programs logging to
standard output/error ignore such errors. If the SIGPIPE UNIX signal
handler is not blocked or turned off, such write attempts will also
result in such process signals being generated, see signal(7). To
mitigate this issue, systemd service manager explicitly turns off the
SIGPIPE signal for all invoked processes by default (this may be
changed for each unit individually via the IgnoreSIGPIPE= option, see
systemd.exec(5) for details). After the standard output/standard error
streams have been terminated they may not be recovered until the
services they are associated with are restarted. Note that during
normal operation, systemd-journald.service stores copies of the file
descriptors for those streams in the service manager. If
systemd-journald.service is restarted using systemctl restart or
equivalent operation instead of a pair of separate systemctl stop and
systemctl start commands (or equivalent operations), these stream
connections are not terminated and survive the restart. It is thus safe
to restart systemd-journald.service, but stopping it is not
recommended.
Note that the log record metadata for records transferred via such
standard output/error streams reflect the metadata of the peer the
stream was originally created for. If the stream connection is passed
on to other processes (such as further child processes forked off the
main service process), the log records will not reflect their metadata,
but will continue to describe the original process. This is different
from the other logging transports listed above, which are inherently
record based and where the metadata is always associated with the
individual record.
In addition to the implicit standard output/error logging of services,
stream logging is also available via the systemd-cat(1) command line
tool.
Currently, the number of parallel log streams systemd-journald will
accept is limited to 4096. When this limit is reached further log
streams may be established but will receive EPIPE right from the
beginning.
JOURNAL NAMESPACES
Journal 'namespaces' are both a mechanism for logically isolating the
log stream of projects consisting of one or more services from the rest
of the system and a mechanism for improving performance. Multiple
journal namespaces may exist simultaneously, each defining its own,
independent log stream managed by its own instance of systemd-journald.
Namespaces are independent of each other, both in the data store and in
the IPC interface. By default only a single 'default' namespace exists,
managed by systemd-journald.service (and its associated socket units).
Additional namespaces are created by starting an instance of the
systemd-journald@.service service template. The instance name is the
namespace identifier, which is a short string used for referencing the
journal namespace. Service units may be assigned to a specific journal
namespace through the LogNamespace= unit file setting, see
systemd.exec(5) for details. The --namespace= switch of journalctl(1)
may be used to view the log stream of a specific namespace. If the
switch is not used the log stream of the default namespace is shown,
i.e. log data from other namespaces is not visible.
Services associated with a specific log namespace may log via syslog,
the native logging protocol of the journal and via stdout/stderr; the
logging from all three transports is associated with the namespace.
By default only the default namespace will collect kernel and audit log
messages.
The systemd-journald instance of the default namespace is configured
through /etc/systemd/journald.conf (see below), while the other
instances are configured through /etc/systemd/journald@NAMESPACE.conf.
The journal log data for the default namespace is placed in
/var/log/journal/MACHINE_ID (see below) while the data for the other
namespaces is located in /var/log/journal/MACHINE_ID.NAMESPACE.
SIGNALS
SIGUSR1
Request that journal data from /run/ is flushed to /var/ in order
to make it persistent (if this is enabled). This must be used after
/var/ is mounted, as otherwise log data from /run is never flushed
to /var regardless of the configuration. Use the journalctl --flush
command to request flushing of the journal files, and wait for the
operation to complete. See journalctl(1) for details.
SIGUSR2
Request immediate rotation of the journal files. Use the journalctl
--rotate command to request journal file rotation, and wait for the
operation to complete.
SIGRTMIN+1
Request that all unwritten log data is written to disk. Use the
journalctl --sync command to trigger journal synchronization, and
wait for the operation to complete.
KERNEL COMMAND LINE
A few configuration parameters from journald.conf may be overridden on
the kernel command line:
systemd.journald.forward_to_syslog=, systemd.journald.forward_to_kmsg=,
systemd.journald.forward_to_console=, systemd.journald.forward_to_wall=
Enables/disables forwarding of collected log messages to syslog,
the kernel log buffer, the system console or wall.
See journald.conf(5) for information about these settings.
Note that these kernel command line options are only honoured by the
default namespace, see above.
ACCESS CONTROL
Journal files are, by default, owned and readable by the
"systemd-journal" system group but are not writable. Adding a user to
this group thus enables them to read the journal files.
By default, each user, with a UID outside the range of system users,
dynamic service users, and the nobody user, will get their own set of
journal files in /var/log/journal/. See Users, Groups, UIDs and GIDs on
systemd systems[1] for more details about UID ranges. These journal
files will not be owned by the user, however, in order to avoid that
the user can write to them directly. Instead, file system ACLs are used
to ensure the user gets read access only.
Additional users and groups may be granted access to journal files via
file system access control lists (ACL). Distributions and
administrators may choose to grant read access to all members of the
"wheel" and "adm" system groups with a command such as the following:
# setfacl -Rnm g:wheel:rx,d:g:wheel:rx,g:adm:rx,d:g:adm:rx /var/log/journal/
Note that this command will update the ACLs both for existing journal
files and for future journal files created in the /var/log/journal/
directory.
FILES
/etc/systemd/journald.conf
Configure systemd-journald behavior. See journald.conf(5).
/run/log/journal/machine-id/*.journal,
/run/log/journal/machine-id/*.journal~,
/var/log/journal/machine-id/*.journal,
/var/log/journal/machine-id/*.journal~
systemd-journald writes entries to files in
/run/log/journal/machine-id/ or /var/log/journal/machine-id/ with
the ".journal" suffix. If the daemon is stopped uncleanly, or if
the files are found to be corrupted, they are renamed using the
".journal~" suffix, and systemd-journald starts writing to a new
file. /run is used when /var/log/journal is not available, or when
Storage=volatile is set in the journald.conf(5) configuration file.
When systemd-journald ceases writing to a journal file, it will be
renamed to "original-name@suffix.journal" (or
"original-name@suffix.journal~"). Such files are "archived" and
will not be written to any more.
In general, it is safe to read or copy any journal file (active or
archived). journalctl(1) and the functions in the sd-journal(3)
library should be able to read all entries that have been fully
written.
systemd-journald will automatically remove the oldest archived
journal files to limit disk use. See SystemMaxUse= and related
settings in journald.conf(5).
/dev/kmsg, /dev/log, /run/systemd/journal/dev-log,
/run/systemd/journal/socket, /run/systemd/journal/stdout
Sockets and other file node paths that systemd-journald will listen
on and are visible in the file system. In addition to these,
systemd-journald can listen for audit events using netlink(7).
If journal namespacing is used these paths are slightly altered to
include a namespace identifier, see above.
SEE ALSO
systemd(1), journalctl(1), journald.conf(5), systemd.journal-fields(7),
sd-journal(3), systemd-coredump(8), setfacl(1), sd_journal_print(3),
pydoc systemd.journal
NOTES
1. Users, Groups, UIDs and GIDs on systemd systems
https://systemd.io/UIDS-GIDS
systemd 245 SYSTEMD-JOURNALD.SERVICE(8)