systemd-journald - Journal service
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:
•Kernel log messages, via kmsg
•Simple system log messages, via the
libc syslog(3) call
•Structured system log messages via the
native Journal API, see sd_journal_print(4)
•Standard output and standard error of
service units. For further details see below.
•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
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. All objects stored in the journal can be up to
2^64-1 bytes in size.
By default, the journal stores log data in /run/log/journal/. Since /run/ is
volatile, log data is lost at reboot. To make the data persistent, it is
sufficient to create /var/log/journal/ where systemd-journald will then store
mkdir -p /var/log/journal
systemd-tmpfiles --create --prefix /var/log/journal
(5) for information about the configuration of this
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=
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
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 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
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
equivalent operation instead of a pair of separate systemctl stop
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 the implicit standard output/error logging of services,
stream logging is also available via the systemd-cat
(1) command line
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 receieve EPIPE
right from the beginning.
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. The journalctl --flush
command uses this signal to request flushing of the journal files, and then
waits for the operation to complete. See journalctl(1) for
Request immediate rotation of the journal
files. The journalctl --rotate command uses this signal to request
journal file rotation.
Request that all unwritten log data is written
to disk. The journalctl --sync command uses this signal to trigger
journal synchronization, and then waits for the operation to complete.
A few configuration parameters from journald.conf may be overridden on the
kernel command line:
Enables/disables forwarding of collected log
messages to syslog, the kernel log buffer, the system console or wall.
(5) for information about these settings.
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 her/him to read the journal files.
By default, each logged in user will get her/his own set of journal files in
/var/log/journal/. These 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.
Configure systemd-journald behavior.
/*.journal, /var/log/journal/ machine-id
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)
/dev/kmsg, /dev/log, /run/systemd/journal/dev-log, /run/systemd/journal/socket,
Sockets and other paths that
systemd-journald will listen on that are visible in the file system. In
addition to these, journald can listen for audit events using netlink.