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. */
    };

An alq can be created in either fixed or variable length mode. A variable length alq accommodates writes of varying length using alq_writen() and alq_getn(). A fixed length alq accommodates a fixed number of writes using alq_write() and alq_get(), each of fixed size (set at queue creation time). Fixed length mode is deprecated in favour of variable length mode.

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 to calling alq_post().

The deprecated alq_get() function is implemented as a wrapper around alq_getn() to provide backwards compatibility to consumers that have not been updated to utilise variable length mode queues. The asynchronous log entry returned will be initialised to point at a buffer capable of receiving size bytes of data (where size was specified at queue creation 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
kproc(9), ktr(9), msleep_spin(9), syslog(3), vnode(9)

HISTORY
The Asynchronous Logging Queues (ALQ) facility first appeared in FreeBSD 5.0.

AUTHORS
-nosplit 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>.