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ALQ(9) FreeBSD Kernel Developer's Manual ALQ(9)
NAME
alq, alq_open_flags, alq_open, alq_writen, alq_write, alq_flush,
alq_close, alq_getn, alq_get, alq_post_flags, alq_post - Asynchronous
Logging Queues
SYNOPSIS
#include <sys/alq.h>
int
alq_open_flags(struct alq **app, const char *file, struct ucred *cred,
int cmode, int size, int flags);
int
alq_open(struct alq **app, const char *file, struct ucred *cred,
int cmode, int size, int count);
int
alq_writen(struct alq *alq, void *data, int len, int flags);
int
alq_write(struct alq *alq, void *data, int flags);
void
alq_flush(struct alq *alq);
void
alq_close(struct alq *alq);
struct ale *
alq_getn(struct alq *alq, int len, int flags);
struct ale *
alq_get(struct alq *alq, int flags);
void
alq_post_flags(struct alq *alq, struct ale *ale, int flags);
void
alq_post(struct alq *alq, struct ale *ale);
DESCRIPTION
The alq facility provides an asynchronous fixed or variable length
recording mechanism, known as Asynchronous Logging Queues. It can record
to any vnode(9), thus providing the ability to journal logs to character
devices as well as regular files. All functions accept a struct alq
argument, which is an opaque type that maintains state information for an
Asynchronous Logging Queue. The logging facility runs in a separate
kernel thread, which services all log entry requests.
An "asynchronous log entry" is defined as struct ale, which has the
following members:
struct ale {
intptr_t ae_bytesused; /* # bytes written to ALE. */
char *ae_data; /* Write ptr. */
int ae_pad; /* Unused, compat. */
};
FUNCTIONS
The alq_open_flags() function creates a new variable length asynchronous
logging queue. The file argument is the name of the file to open for
logging. If the file does not yet exist, alq_open() will attempt to
create it. The cmode argument will be passed to vn_open() as the
requested creation mode, to be used if the file will be created by
alq_open(). Consumers of this API may wish to pass ALQ_DEFAULT_CMODE, a
default creation mode suitable for most applications. The cred argument
specifies the credentials to use when opening and performing I/O on the
file. The size argument sets the size (in bytes) of the underlying
queue. The ALQ_ORDERED flag may be passed in via flags to indicate that
the ordering of writer threads waiting for a busy alq to free up
resources should be preserved.
The deprecated alq_open() function is implemented as a wrapper around
alq_open_flags() to provide backwards compatibility to consumers that
have not been updated to utilise the newer alq_open_flags() function. It
passes all arguments through to alq_open_flags() untouched except for
size and count, and sets flags to 0. To create a variable length mode
alq, the size argument should be set to the size (in bytes) of the
underlying queue and the count argument should be set to 0. To create a
fixed length mode alq, the size argument should be set to the size (in
bytes) of each write and the count argument should be set to the number
of size byte chunks to reserve capacity for.
The alq_writen() function writes len bytes from data to the designated
variable length mode queue alq. If alq_writen() could not write the
entry immediately and ALQ_WAITOK is set in flags, the function will be
allowed to msleep_spin(9) with the "alqwnord" or "alqwnres" wait message.
A write will automatically schedule the queue alq to be flushed to disk.
This behaviour can be controlled by passing ALQ_NOACTIVATE via flags to
indicate that the write should not schedule alq to be flushed to disk.
The deprecated alq_write() function is implemented as a wrapper around
alq_writen() to provide backwards compatibility to consumers that have
not been updated to utilise variable length mode queues. The function
will write size bytes of data (where size was specified at queue creation
time) from the data buffer to the alq. Note that it is an error to call
alq_write() on a variable length mode queue.
The alq_flush() function is used for flushing alq to the log medium that
was passed to alq_open(). If alq has data to flush and is not already in
the process of being flushed, the function will block doing IO.
Otherwise, the function will return immediately.
The alq_close() function will close the asynchronous logging queue alq
and flush all pending write requests to the log medium. It will free all
resources that were previously allocated.
The alq_getn() function returns an asynchronous log entry from alq,
initialised to point at a buffer capable of receiving len bytes of data.
This function leaves alq in a locked state, until a subsequent alq_post()
or alq_post_flags() call is made. If alq_getn() could not obtain len
bytes of buffer immediately and ALQ_WAITOK is set in flags, the function
will be allowed to msleep_spin(9) with the "alqgnord" or "alqgnres" wait
message. The caller can choose to write less than len bytes of data to
the returned asynchronous log entry by setting the entry's ae_bytesused
field to the number of bytes actually written. This must be done prior
time). Note that it is an error to call alq_get() on a variable length
mode queue.
The alq_post_flags() function schedules the asynchronous log entry ale
(obtained from alq_getn() or alq_get()) for writing to alq. The
ALQ_NOACTIVATE flag may be passed in via flags to indicate that the queue
should not be immediately scheduled to be flushed to disk. This function
leaves alq in an unlocked state.
The alq_post() function is implemented as a wrapper around
alq_post_flags() to provide backwards compatibility to consumers that
have not been updated to utilise the newer alq_post_flags() function. It
simply passes all arguments through to alq_post_flags() untouched, and
sets flags to 0.
IMPLEMENTATION NOTES
The alq_writen() and alq_write() functions both perform a bcopy(3) from
the supplied data buffer into the underlying alq buffer. Performance
critical code paths may wish to consider using alq_getn() (variable
length queues) or alq_get() (fixed length queues) to avoid the extra
memory copy. Note that a queue remains locked between calls to
alq_getn() or alq_get() and alq_post() or alq_post_flags(), so this
method of writing to a queue is unsuitable for situations where the time
between calls may be substantial.
LOCKING
Each asynchronous logging queue is protected by a spin mutex.
Functions alq_flush() and alq_open() may attempt to acquire an internal
sleep mutex, and should consequently not be used in contexts where
sleeping is not allowed.
RETURN VALUES
The alq_open() function returns one of the error codes listed in open(2),
if it fails to open file, or else it returns 0.
The alq_writen() and alq_write() functions return EWOULDBLOCK if
ALQ_NOWAIT was set in flags and either the queue is full or the system is
shutting down.
The alq_getn() and alq_get() functions return NULL if ALQ_NOWAIT was set
in flags and either the queue is full or the system is shutting down.
NOTE: invalid arguments to non-void functions will result in undefined
behaviour.
SEE ALSO
syslog(3), kproc(9), ktr(9), msleep_spin(9), vnode(9)
HISTORY
The Asynchronous Logging Queues (ALQ) facility first appeared in
FreeBSD 5.0.
AUTHORS
The alq facility was written by Jeffrey Roberson <jeff@FreeBSD.org> and
extended by Lawrence Stewart <lstewart@freebsd.org>.
This manual page was written by Hiten Pandya <hmp@FreeBSD.org> and
revised by Lawrence Stewart <lstewart@freebsd.org>.