freebsd/lib/libdevstat/devstat.3

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.Dd May 21, 1998
.Dt DEVSTAT 3
.Os FreeBSD
.Sh NAME
.Nm devstat ,
.Nm getnumdevs ,
.Nm getgeneration ,
.Nm getversion ,
.Nm checkversion ,
.Nm getdevs ,
.Nm selectdevs ,
.Nm buildmatch ,
.Nm compute_stats ,
.Nm compute_etime
.Nd device statistics utility library
.Sh LIBRARY
.Lb libdevstat
.Sh SYNOPSIS
.Fd #include <sys/dkstat.h>
.Fd #include <devstat.h>
.Ft int
.Fn getnumdevs "void"
.Ft long
.Fn getgeneration "void"
.Ft int
.Fn getversion "void"
.Ft int
.Fn checkversion "void"
.Ft int
.Fn getdevs "struct statinfo *stats"
.Ft int
.Fo selectdevs
.Fa "struct device_selection **dev_select"
.Fa "int *num_selected"
.Fa "int *num_selections"
.Fa "long *select_generation"
.Fa "long current_generation"
.Fa "struct devstat *devices"
.Fa "int numdevs"
.Fa "struct devstat_match *matches"
.Fa "int num_matches"
.Fa "char **dev_selections"
.Fa "int num_dev_selections"
.Fa "devstat_select_mode select_mode"
.Fa "int maxshowdevs"
.Fa "int perf_select"
.Fc
.Ft int
.Fo buildmatch
.Fa "char *match_str"
.Fa "struct devstat_match **matches"
.Fa "int *num_matches"
.Fc
.Ft int
.Fo compute_stats
.Fa "struct devstat *current"
.Fa "struct devstat *previous"
.Fa "long double etime"
.Fa "u_int64_t *total_bytes"
.Fa "u_int64_t *total_transfers"
.Fa "u_int64_t *total_blocks"
.Fa "long double *kb_per_transfer"
.Fa "long double *transfers_per_second"
.Fa "long double *mb_per_second"
.Fa "long double *blocks_per_second"
.Fa "long double *ms_per_transaction"
.Fc
.Ft long double
.Fo compute_etime
.Fa "struct timeval cur_time"
.Fa "struct timeval prev_time"
.Fc
.Sh DESCRIPTION
The
.Nm
library is a library of helper functions for dealing with the kernel
.Xr devstat 9
interface, which is accessible to users via
.Xr sysctl 3 .
.Pp
.Fn getnumdevs
returns the number of devices registered with the
.Nm
subsystem in the kernel.
.Pp
.Fn getgeneration
returns the current generation of the
.Nm
list of devices in the kernel.
.Pp
.Fn getversion
returns the current kernel
.Nm
version.
.Pp
.Fn 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 
.Va devstat_errbuf
and returns -1.
.Pp
.Fn getdevs
fetches the current list of devices and statistics into the supplied
.Va statinfo
structure.  The
.Va statinfo
structure can be found in 
.Aq Pa devstat.h :
.Bd -literal -offset indent
struct statinfo {
	long            cp_time[CPUSTATES];
	long            tk_nin;  
	long            tk_nout;
	struct devinfo  *dinfo;
	struct timeval  busy_time;
};
.Ed
.Pp
.Fn getdevs
expects the
.Va statinfo
structure to be allocated, and it also expects the
.Va dinfo
subelement to be allocated and zeroed prior to the first invocation of
.Fn getdevs .
The
.Va dinfo
subelement is used to store state between calls, and should not be modified
after the first call to
.Fn getdevs .
The
.Va dinfo
subelement contains the following elements:
.Bd -literal -offset indent
struct devinfo {
	struct devstat	*devices;
	u_int8_t	*mem_ptr;
	long		generation;
	int		numdevs;
};
.Ed
.Pp
The
.Va kern.devstat.all
.Nm sysctl
variable contains an array of
.Nm
structures, but at the head of the array is the current
.Nm
generation.  The reason the generation is at the head of the buffer is so
that userland software accessing the devstat statistics information can
atomically get both the statistics information and the corresponding
generation number.  If client software were forced to get the generation
number via a separate
.Nm 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.
.Pp
The
.Va mem_ptr
subelement of the
.Va devinfo
structure is a pointer to memory that is allocated, and resized if
necessary, by
.Fn getdevs .
The devices subelement of the
.Va devinfo
structure is basically a pointer to the beginning of the array of devstat
structures from the
.Va kern.devstat.all
.Nm sysctl
variable.  The generation subelement of the
.Va devinfo
structure contains the generation number from the
.Va kern.devstat.all
.Nm sysctl
variable.  
The
.Va numdevs
subelement of the 
.Va devinfo
structure contains the current
number of devices registered with the kernel
.Nm
subsystem.
.Pp
.Fn selectdevs
selects devices to display based upon a number of criteria:
.Bl -tag -width flag
.It specified devices
Specified devices are the first selection priority.  These are generally
devices specified by name by the user.  e.g. da0, da1, cd0.
.It match patterns
These are pattern matching expressions generated by
.Fn buildmatch
from user input.
.It performance
If performance mode is enabled, devices will be sorted based on the
.Va bytes
field in the 
.Va device_selection
structure passed in to
.Fn selectdevs .
The 
.Va bytes
value currently must be maintained by the user.  In the future,
this may be done for him in a
.Nm
library routine.
If no devices have been selected by name or by pattern, the performance
tracking code will select every device in the system, 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.
.It order in the devstat list
If the selection mode is set to DS_SELECT_ADD, and if there are still less
than 
.Va maxshowdevs
devices selected,
.Fn selectdevs
will automatically select up to 
.Va maxshowdevs
devices.
.El
.Pp
.Fn selectdevs
performs selections in four different modes:
.Bl -tag -width DS_SELECT_ADDONLY
.It DS_SELECT_ADD
In add mode,
.Fn 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 
.Va maxshowdevs
or until all devices are
selected.
.It DS_SELECT_ONLY
In only mode,
.Fn selectdevs
will clear all current selections, and will only select devices specified
by name or by matching pattern.
.It DS_SELECT_REMOVE
In remove mode,
.Fn selectdevs
will remove devices specified by name or by matching pattern.  It will not
select any additional devices.
.It DS_SELECT_ADDONLY
In add only mode,
.Fn selectdevs
will select any unselected 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.
.El
.Pp
In all selection modes,
.Fn selectdevs
will not select any more than 
.Va maxshowdevs
devices.  One exception to
this is when you are in
.Dq top
mode and no devices have been selected.  In
this case,
.Fn 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.
.Pp
.Fn selectdevs
handles allocation and resizing of the 
.Va dev_select
structure passed in
by the client.
.Fn selectdevs
uses the 
.Va numdevs
and 
.Va current_generation
fields to track the
current
.Nm
generation and number of devices.  If 
.Va num_selections
is not the same
as 
.Va numdevs
or if 
.Va select_generation
is not the same as 
.Va current_generation ,
.Fn selectdevs
will resize the selection list as necessary, and re-initialize the
selection array.
.Pp
.Fn buildmatch
takes a comma separated match string and compiles it into a
\fBdevstat_match\fR structure that is understood by
.Fn selectdevs .
Match strings have the following format:
.Pp
.Bd -literal -offset indent
device,type,if
.Ed
.Pp
.Fn buildmatch
takes care of allocating and reallocating the match list as necessary.
Currently known match types include:
.Pp
.Bl -tag -width indent -compact
.It device type:
.Bl -tag -width 9n -compact
.It da 
Direct Access devices
.It sa
Sequential Access devices
.It printer
Printers
.It proc
Processor devices
.It worm
Write Once Read Multiple devices
.It cd
CD devices
.It scanner
Scanner devices
.It optical
Optical Memory devices
.It changer
Medium Changer devices
.It comm
Communication devices
.It array
Storage Array devices
.It enclosure
Enclosure Services devices
.It floppy
Floppy devices
.El
.Pp
.It interface:
.Bl -tag -width 9n -compact
.It IDE
Integrated Drive Electronics devices
.It SCSI
Small Computer System Interface devices
.It other
Any other device interface
.El
.Pp
.It passthrough:
.Bl -tag -width 9n -compact
.It pass
Passthrough devices
.El
.El
.Pp
.Fn compute_stats
provides an easy way to obtain various device statistics.  Only two
arguments are mandatory:
.Va current
and
.Va etime .
Every other argument is optional.  For most applications, the user will
want to supply both
.Va current
and
.Va 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
.Va previous
argument.  In that case,
.Fn compute_stats
will use the total stats in the
.Va current
structure to calculate statistics over
.Va etime .
The various statistics that may be calculated by
.Fn 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:
.Bl -tag -width transfers_per_second
.It total_bytes
This is the total number of bytes transferred on the given device, both
reads and writes, between the acquisition of
.Va previous
and the acquisition of
.Va current .
If
.Va previous
is NULL, the result will be the total reads and writes given in
.Va current .
.It total_transfers
This is the total number of transfers completed between the
acquisition of
.Va previous
and the acquisition of
.Va current .
If
.Va previous
is NULL, the result will be the total number of transactions listed in
.Va current .
.It total_blocks
This is basically
.Va total_bytes
divided by the device blocksize.  If the device blocksize is listed as 
.Sq 0 ,
the device blocksize will default to 512 bytes.
.It kb_per_transfer
This is the average number of kilobytes per transfer during the measurement
period.
.It transfers_per_second
This is the average number of transfers per second.
.It mb_per_second
This is average megabytes per second.
.It blocks_per_second
This is average blocks per second.  If the device blocksize is
.Sq 0 ,
a default blocksize of 512 bytes will be used instead.
.It ms_per_transaction
The average number of milliseconds per transaction.
.El
.Pp
.Fn compute_etime
provides an easy way to find the difference in seconds between two
.Va timeval
structures.  This is most commonly used in conjunction with the time
recorded by the
.Fn getdevs
function (in struct 
.Va statinfo )
each time it fetches the current
.Nm
list.
.Sh RETURN VALUES
.Fn getnumdevs ,
.Fn getgeneration ,
and
.Fn getversion
return the indicated \fBsysctl\fR variable, or -1 if there is an error
fetching the variable.
.Pp
.Fn checkversion
returns 0 if the kernel and userland
.Nm
versions match.  If they do not match, it returns -1.
.Pp
.Fn getdevs
and
.Fn 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
.Fn getdevs
is usually a hint to re-run
.Fn selectdevs
because the device list has changed.
.Pp
.Fn buildmatch
returns -1 for error, and 0 if there is no error.
.Pp
.Fn compute_stats
returns -1 for error, and 0 for success.
.Pp
.Fn compute_etime
returns the computed elapsed time.
.Pp
If an error is returned from one of the
.Nm
library functions, the reason for the error is generally printed in
the global string
.Va devstat_errbuf
which is
.Dv DEVSTAT_ERRBUF_SIZE
characters long.
.Sh SEE ALSO
.Xr systat 1 ,
.Xr iostat 8 ,
.Xr rpc.rstatd 8 ,
.Xr vmstat 8 ,
.Xr devstat 9
.Sh HISTORY
The
.Nm
statistics system first appeared in
.Fx 3.0 .
.Sh AUTHORS
.An Kenneth Merry Aq ken@FreeBSD.org
.Sh BUGS
There should probably be an interface to de-allocate memory allocated by
.Fn getdevs ,
.Fn selectdevs ,
and
.Fn buildmatch .
.Pp
.Fn selectdevs
should probably not select more than
.Va maxshowdevs
devices in
.Dq top
mode when no devices have been selected previously.
.Pp
There should probably be functions to perform the statistics buffer
swapping that goes on in most of the clients of this library.
.Pp
The
.Va statinfo
and
.Va devinfo
structures should probably be cleaned up and thought out a little more.