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DEVSTAT(3)		 BSD Library Functions Manual		    DEVSTAT(3)

NAME
     devstat, getnumdevs, getgeneration, getversion, checkversion, getdevs,
     selectdevs, buildmatch, compute_stats, compute_etime — device statistics
     utility library

LIBRARY
     library “libdevstat”

SYNOPSIS
     #include <devstat.h>

     int
     getnumdevs(void);

     long
     getgeneration(void);

     int
     getversion(void);

     int
     checkversion(void);

     int
     getdevs(struct statinfo *stats);

     int
     selectdevs(struct device_selection **dev_select, int *num_selected,
	 int *num_selections, long *select_generation,
	 long current_generation, struct devstat *devices, int numdevs,
	 struct devstat_match *matches, int num_matches,
	 char **dev_selections, int num_dev_selections,
	 devstat_select_mode select_mode, int maxshowdevs, int perf_select);

     int
     buildmatch(const char *match_str, struct devstat_match **matches,
	 int *num_matches);

     int
     compute_stats(struct devstat *current, struct devstat *previous,
	 long double etime, u_int64_t *total_bytes,
	 u_int64_t *total_transfers, u_int64_t *total_blocks,
	 long double *kb_per_transfer, long double *transfers_per_second,
	 long double *mb_per_second, long double *blocks_per_second,
	 long double *ms_per_transaction);

     long double
     compute_etime(struct timeval cur_time, struct timeval prev_time);

DESCRIPTION
     The devstat library is a library of helper functions for dealing with the
     kernel devstat(9) interface, which is accessible to users via sysctl(3).

     getnumdevs() returns the number of devices registered with the devstat
     subsystem in the kernel.

     getgeneration() returns the current generation of the devstat list of
     devices in the kernel.

     getversion() returns the current kernel devstat version.

     checkversion() checks the userland devstat version against the kernel
     devstat version.  If the two are identical, it returns zero.  Otherwise,
     it prints an appropriate error in devstat_errbuf and returns -1.

     getdevs() fetches the current list of devices and statistics into the
     supplied statinfo structure.  The statinfo structure can be found in
     <devstat.h>:

	   struct statinfo {
		   struct kinfo_cputime	   cp_time;
		   struct devinfo  *dinfo;
		   struct timeval  busy_time;
	   };

     getdevs() expects the statinfo structure to be allocated, and it also
     expects the dinfo subelement to be allocated and zeroed prior to the
     first invocation of getdevs().  The dinfo subelement is used to store
     state between calls, and should not be modified after the first call to
     getdevs().	 The dinfo subelement contains the following elements:

	   struct devinfo {
		   struct devstat  *devices;
		   u_int8_t	   *mem_ptr;
		   long		   generation;
		   int		   numdevs;
	   };

     The kern.devstat.all sysctl variable contains an array of devstat struc‐
     tures, but at the head of the array is the current devstat generation.
     The reason the generation is at the head of the buffer is so that user‐
     land software accessing the devstat statistics information can atomically
     get both the statistics information and the corresponding generation num‐
     ber.  If client software were forced to get the generation number via a
     separate sysctl variable (which is available for convenience), the list
     of devices could change between the time the client gets the generation
     and the time the client gets the device list.

     The mem_ptr subelement of the devinfo structure is a pointer to memory
     that is allocated, and resized if necessary, by getdevs().	 The devices
     subelement of the devinfo structure is basically a pointer to the begin‐
     ning of the array of devstat structures from the kern.devstat.all sysctl
     variable.	The generation subelement of the devinfo structure contains
     the generation number from the kern.devstat.all sysctl variable.  The
     numdevs subelement of the devinfo structure contains the current number
     of devices registered with the kernel devstat subsystem.

     selectdevs() selects devices to display based upon a number of criteria:

     specified devices
	   Specified devices are the first selection priority.	These are gen‐
	   erally devices specified by name by the user e.g. da0, da1, cd0.

     match patterns
	   These are pattern matching expressions generated by buildmatch()
	   from user input.

     performance
	   If performance mode is enabled, devices will be sorted based on the
	   bytes field in the device_selection structure passed in to
	   selectdevs().  The bytes value currently must be maintained by the
	   user.  In the future, this may be done for him in a devstat library
	   routine.  If no devices have been selected by name or by pattern,
	   the performance tracking code will select every device in the sys‐
	   tem, and sort them by performance.  If devices have been selected
	   by name or pattern, the performance tracking code will honor those
	   selections and will only sort among the selected devices.

     order in the devstat list
	   If the selection mode is set to DS_SELECT_ADD, and if there are
	   still less than maxshowdevs devices selected, selectdevs() will
	   automatically select up to maxshowdevs devices.

     selectdevs() performs selections in four different modes:

     DS_SELECT_ADD	In add mode, selectdevs() will select any unselected
			devices specified by name or matching pattern.	It
			will also select more devices, in devstat list order,
			until the number of selected devices is equal to
			maxshowdevs or until all devices are selected.

     DS_SELECT_ONLY	In only mode, selectdevs() will clear all current
			selections, and will only select devices specified by
			name or by matching pattern.

     DS_SELECT_REMOVE	In remove mode, selectdevs() will remove devices spec‐
			ified by name or by matching pattern.  It will not
			select any additional devices.

     DS_SELECT_ADDONLY	In add only mode, selectdevs() will select any unse‐
			lected devices specified by name or matching pattern.
			In this respect it is identical to add mode.  It will
			not, however, select any devices other than those
			specified.

     In all selection modes, selectdevs() will not select any more than
     maxshowdevs devices.  One exception to this is when you are in “top” mode
     and no devices have been selected.	 In this case, selectdevs() will
     select every device in the system.	 Client programs must pay attention to
     selection order when deciding whether to pay attention to a particular
     device.  This may be the wrong behavior, and probably requires additional
     thought.

     selectdevs() handles allocation and resizing of the dev_select structure
     passed in by the client.  selectdevs() uses the numdevs and
     current_generation fields to track the current devstat generation and
     number of devices.	 If num_selections is not the same as numdevs or if
     select_generation is not the same as current_generation, selectdevs()
     will resize the selection list as necessary, and re-initialize the selec‐
     tion array.

     buildmatch() takes a comma separated match string and compiles it into a
     devstat_match structure that is understood by selectdevs().  Match
     strings have the following format:

	   device,type,if

     buildmatch() takes care of allocating and reallocating the match list as
     necessary.	 Currently known match types include:

     device type:
	     da		Direct Access devices
	     sa		Sequential Access devices
	     printer	Printers
	     proc	Processor devices
	     worm	Write Once Read Multiple devices
	     cd		CD devices
	     scanner	Scanner devices
	     optical	Optical Memory devices
	     changer	Medium Changer devices
	     comm	Communication devices
	     array	Storage Array devices
	     enclosure	Enclosure Services devices
	     floppy	Floppy devices

     interface:
	     IDE	Integrated Drive Electronics devices
	     SCSI	Small Computer System Interface devices
	     other	Any other device interface

     passthrough:
	     pass	Passthrough devices

     compute_stats() provides an easy way to obtain various device statistics.
     Only two arguments are mandatory: current and etime.  Every other argu‐
     ment is optional.	For most applications, the user will want to supply
     both current and previous devstat structures so that statistics may be
     calculated over a given period of time.  In some instances, for instance
     when calculating statistics since system boot, the user may pass in a
     NULL pointer for the previous argument.  In that case, compute_stats()
     will use the total stats in the current structure to calculate statistics
     over etime.  The various statistics that may be calculated by
     compute_stats() should be mostly explained by the function declaration
     itself, but for completeness here is a list of variable names and the
     statistics that will be put in them:

     total_bytes	   This is the total number of bytes transferred on
			   the given device, both reads and writes, between
			   the acquisition of previous and the acquisition of
			   current.  If previous is NULL, the result will be
			   the total reads and writes given in current.

     total_transfers	   This is the total number of transfers completed
			   between the acquisition of previous and the acqui‐
			   sition of current.  If previous is NULL, the result
			   will be the total number of transactions listed in
			   current.

     total_blocks	   This is basically total_bytes divided by the device
			   blocksize.  If the device blocksize is listed as
			   ‘0’, the device blocksize will default to 512
			   bytes.

     kb_per_transfer	   This is the average number of kilobytes per trans‐
			   fer during the measurement period.

     transfers_per_second  This is the average number of transfers per second.

     mb_per_second	   This is average megabytes per second.

     blocks_per_second	   This is average blocks per second.  If the device
			   blocksize is ‘0’, a default blocksize of 512 bytes
			   will be used instead.

     ms_per_transaction	   The average number of milliseconds per transaction.

     compute_etime() provides an easy way to find the difference in seconds
     between two timeval structures.  This is most commonly used in conjunc‐
     tion with the time recorded by the getdevs() function (in struct
     statinfo) each time it fetches the current devstat list.

RETURN VALUES
     getnumdevs(), getgeneration(), and getversion() return the indicated
     sysctl variable, or -1 if there is an error fetching the variable.

     checkversion() returns 0 if the kernel and userland devstat versions
     match.  If they do not match, it returns -1.

     getdevs() and selectdevs() return -1 in case of an error, 0 if there is
     no error and 1 if the device list or selected devices have changed.  A
     return value of 1 from getdevs() is usually a hint to re-run selectdevs()
     because the device list has changed.

     buildmatch() returns -1 for error, and 0 if there is no error.

     compute_stats() returns -1 for error, and 0 for success.

     compute_etime() returns the computed elapsed time.

     If an error is returned from one of the devstat library functions, the
     reason for the error is generally printed in the global string
     devstat_errbuf which is DEVSTAT_ERRBUF_SIZE characters long.

SEE ALSO
     systat(1), iostat(8), rpc.rstatd(8), vmstat(8), devstat(9)

HISTORY
     The devstat statistics system first appeared in FreeBSD 3.0.

AUTHORS
     Kenneth Merry ⟨ken@FreeBSD.org⟩

BUGS
     There should probably be an interface to de-allocate memory allocated by
     getdevs(), selectdevs(), and buildmatch().

     selectdevs() should probably not select more than maxshowdevs devices in
     “top” mode when no devices have been selected previously.

     There should probably be functions to perform the statistics buffer swap‐
     ping that goes on in most of the clients of this library.

     The statinfo and devinfo structures should probably be cleaned up and
     thought out a little more.

BSD				 May 21, 1998				   BSD
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