path: root/share/man/man9/devstat.9

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.Dd May 22, 1998
.Dt DEVSTAT 9
.Os FreeBSD 3.0
.Sh NAME
.Nm devstat
.Nd kernel interface for keeping device statistics
.Sh SYNOPSIS
.Fd #include <sys/devicestat.h>
.Ft void
.Fo devstat_add_entry
.Fa "struct devstat *ds"
.Fa "const char *dev_name"
.Fa "int unit_number"
.Fa "u_int32_t block_size"
.Fa "devstat_support_flags flags"
.Fa "devstat_type_flags device_type"
.Fa "devstat_priority priority"
.Fc
.Ft void
.Fn devstat_remove_entry "struct devstat *ds"
.Ft void
.Fn devstat_start_transaction "struct devstat *ds"
.Ft void
.Fo devstat_end_transaction
.Fa "struct devstat *ds"
.Fa "u_int32_t bytes"
.Fa "devstat_tag_type tag_type"
.Fa "devstat_trans_flags flags"
.Fc
.Sh DESCRIPTION
The devstat subsystem is an interface for recording device
statistics, as its name implies.  The idea is to keep reasonably detailed
statistics while utilizing a minimum amount of CPU time to record them.
Thus, no statistical calculations are actually performed in the kernel
portion of the
.Nm
code.  Instead, that is left for user programs to handle.
.Pp
.Fn devstat_add_entry
registers a device with the
.Nm
subsystem.  The caller is expected to have already allocated \fBand zeroed\fR
the devstat structure before calling this function.
.Fn devstat_add_entry
takes several arguments:
.Bl -tag -width device_type
.It ds
The
.Va devstat
structure, allocated and zeroed by the client.
.It dev_name
The device name. e.g. da, cd, sa.
.It unit_number
Device unit number.
.It block_size
Block size of the device, if supported.  If the device does not support a
block size, or if the blocksize is unknown at the time the device is added
to the
.Nm
list, it should be set to 0.
.It flags
Flags indicating operations supported or not supported by the device.  See
below for details.
.It device_type
The device type.  This is broken into three sections:  base device type
(e.g. direct access, CDROM, sequential access), interface type (IDE, SCSI
or other) and a passthrough flag to indicate pasthrough devices.  See below
for a complete list of types.
.It priority
The device priority.  The priority is used to determine how devices are
sorted within 
.Nm devstat's
list of devices.  Devices are sorted first by priority (highest to lowest),
and then by attach order.  See below for a complete list of available
priorities.
.El
.Pp
.Fn devstat_remove_entry
removes a device from the
.Nm
subsystem.  It takes the devstat structure for the device in question as
an argument.  The
.Nm
generation number is incremented and the number of devices is decremented.
.Pp
.Fn devstat_start_transaction
registers the start of a transaction with the
.Nm
subsystem.  The busy count is incremented with each transaction start.
When a device goes from idle to busy, the system uptime is recorded in the
.Va start_time
field of the
.Va devstat
structure.
.Pp
.Fn devstat_end_transaction
registers the end of a transaction with the
.Nm
subsystem.  It takes four arguments:
.Bl -tag -width tag_type
.It ds
The
.Va devstat
structure for the device in question.
.It bytes
The number of bytes transferred in this transaction.
.It tag_type
Transaction tag type.  See below for tag types.
.It flags
Transaction flags indicating whether the transaction was a read, write, or
whether no data was transferred.
.El
.Pp
The
.Va devstat
structure is composed of the following fields:
.Bl -tag -width dev_creation_time
.It dev_links
Each
.Va devstat
structure is placed in a linked list when it is registered.  The
.Va dev_links
field contains a pointer to the next entry in the list of
.Va devstat
structures.
.It device_number
The device number is a unique identifier for each device.  The device
number is incremented for each new device that is registered.  The device
number is currently only a 32-bit integer, but it could be enlarged if
someone has a system with more than four billion device arrival events.
.It device_name
The device name is a text string given by the registering driver to
identify itself.  (e.g. 
.Dq da ,
.Dq cd ,
.Dq sa ,
etc.)
.It unit_number
The unit number identifies the particular instance of the peripheral driver
in question.
.It bytes_written
This is the number of bytes that have been written to the device.  This
number is currently an unsigned 64 bit integer.  This will hopefully
eliminate the counter wrap that would come very quickly on some systems if
32 bit integers were used.
.It bytes_read
This is the number of bytes that have been read from the device.
.It num_reads
This is the number of reads from the device.
.It num_writes
This is the number of writes to the device.
.It num_other
This is the number of transactions to the device which are neither reads or
writes.  For instance,
.Tn SCSI
drivers often send a test unit ready command to
.Tn SCSI
devices.  The test unit ready command does not read or write any data.  It
merely causes the device to return its status.
.It busy_count
This is the current number of outstanding transactions for the device.
This should never go below zero, and on an idle device it should be zero.
If either one of these conditions is not true, it indicates a problem in
the way
.Fn devstat_start_transaction
and
.Fn devstat_end_transaction
are being called in client code.  There should be one and only one
transaction start event and one transaction end event for each transaction.
.It block_size
This is the block size of the device, if the device has a block size.
.It tag_types
This is an array of counters to record the number of various tag types that
are sent to a device.  See below for a list of tag types.
.It dev_creation_time
This is the time, as reported by
.Fn getmicrotime
that the device was registered.
.It busy_time
This is the amount of time that the device busy count has been greater than
zero.  This is only updated when the busy count returns to zero.
.It start_time
This is the time, as reported by
.Fn getmicrouptime
that the device busy count went from zero to one.
.It last_comp_time
This is the time as reported by
.Fn getmicrouptime
that a transaction last completed.  It is used along with
.Va start_time
to calculate the device busy time.
.It flags
These flags indicate which statistics measurements are supported by a
particular device.  These flags are primarily intended to serve as an aid
to userland programs that decipher the statistics.
.It device_type
This is the device type.  It consists of three parts:  the device type
(e.g. direct access, CDROM, sequential access, etc.), the interface (IDE,
SCSI or other) and whether or not the device in question is a passthrough
driver.  See below for a complete list of device types.
.It priority
This is the priority.  This is the first parameter used to determine where
to insert a device in the
.Nm devstat
list.  The second parameter is attach order.  See below for a list of
available priorities.
.El
.Pp
Each device is given a device type.  Passthrough devices have the same
underlying device type and interface as the device they provide an
interface for, but they also have the passthrough flag set.  The base
device types are identical to the
.Tn SCSI
device type numbers, so with
.Tn SCSI
peripherals, the device type returned from an inquiry is usually ORed with
the
.Tn SCSI
interface type and the passthrough flag if appropriate.  The device type
flags are as follows:
.Bd -literal -offset indent
typedef enum {
	DEVSTAT_TYPE_DIRECT	= 0x000,
	DEVSTAT_TYPE_SEQUENTIAL	= 0x001,
	DEVSTAT_TYPE_PRINTER	= 0x002,
	DEVSTAT_TYPE_PROCESSOR	= 0x003,
	DEVSTAT_TYPE_WORM	= 0x004,
	DEVSTAT_TYPE_CDROM	= 0x005,
	DEVSTAT_TYPE_SCANNER	= 0x006,
	DEVSTAT_TYPE_OPTICAL	= 0x007,
	DEVSTAT_TYPE_CHANGER	= 0x008,
	DEVSTAT_TYPE_COMM	= 0x009,
	DEVSTAT_TYPE_ASC0	= 0x00a,
	DEVSTAT_TYPE_ASC1	= 0x00b,
	DEVSTAT_TYPE_STORARRAY	= 0x00c,
	DEVSTAT_TYPE_ENCLOSURE	= 0x00d,
	DEVSTAT_TYPE_FLOPPY	= 0x00e,
	DEVSTAT_TYPE_MASK	= 0x00f,
	DEVSTAT_TYPE_IF_SCSI	= 0x010,
	DEVSTAT_TYPE_IF_IDE	= 0x020,
	DEVSTAT_TYPE_IF_OTHER	= 0x030,
	DEVSTAT_TYPE_IF_MASK	= 0x0f0,
	DEVSTAT_TYPE_PASS	= 0x100
} devstat_type_flags;
.Ed
.Pp
Devices have a priority associated with them, which controls roughly where
they are placed in the
.Nm devstat
list.  The priorities are as follows:
.Bd -literal -offset indent
typedef enum {
	DEVSTAT_PRIORITY_MIN	= 0x000,
	DEVSTAT_PRIORITY_OTHER	= 0x020,
	DEVSTAT_PRIORITY_PASS	= 0x030,
	DEVSTAT_PRIORITY_FD	= 0x040,
	DEVSTAT_PRIORITY_WFD	= 0x050,
	DEVSTAT_PRIORITY_SA	= 0x060,
	DEVSTAT_PRIORITY_OCD	= 0x070,
	DEVSTAT_PRIORITY_WCD	= 0x080,
	DEVSTAT_PRIORITY_CD	= 0x090,
	DEVSTAT_PRIORITY_WD	= 0x100,
	DEVSTAT_PRIORITY_DA	= 0x110,
	DEVSTAT_PRIORITY_CCD	= 0x120,
	DEVSTAT_PRIORITY_MAX	= 0xfff
} devstat_priority;
.Ed
.Pp
Each device has associated with it flags to indicate what operations are
supported or not supported.  The
.Va devstat_support_flags
values are as follows:
.Bl -tag -width DEVSTAT_NO_ORDERED_TAGS
.It DEVSTAT_ALL_SUPPORTED
Every statistic type is supported by the device.
.It DEVSTAT_NO_BLOCKSIZE
This device does not have a blocksize.
.It DEVSTAT_NO_ORDERED_TAGS
This device does not support ordered tags.
.It DEVSTAT_BS_UNAVAILABLE
This device supports a blocksize, but it is currently unavailable.  This
flag is most often used with removable media drives.
.El
.Pp
Transactions to a device fall into one of three categories, which are
represented in the
.Va flags
passed into
.Fn devstat_end_transaction .
The transaction types are as follows:
.Bd -literal -offset indent
typedef enum { 
	DEVSTAT_NO_DATA	= 0x00,
	DEVSTAT_READ	= 0x01,
	DEVSTAT_WRITE	= 0x02
} devstat_trans_flags;
.Ed
.Pp
There are four possible values for the
.Va tag_type
argument to
.Fn devstat_end_transaction :
.Bl -tag -width DEVSTAT_TAG_ORDERED
.It DEVSTAT_TAG_SIMPLE
The transaction had a simple tag.
.It DEVSTAT_TAG_HEAD
The transaction had a head of queue tag.
.It DEVSTAT_TAG_ORDERED
The transaction had an ordered tag.
.It DEVSTAT_TAG_NONE
The device doesn't support tags.
.El
.Pp
The tag type values correspond to the lower four bits of the
.Tn SCSI
tag definitions.  In CAM, for instance, the
.Va tag_action
from the CCB is ORed with 0xf to determine the tag type to pass in to
.Fn devstat_end_transaction .
.Pp
There is a macro,
.Dv DEVSTAT_VERSION
that is defined in
.Aq sys/devicestat.h .
This is the current version of the
.Nm
subsystem, and it should be incremented each time a change is made that
would require recompilation of userland programs that access
.Nm
statistics.  Userland programs use this version, via the
.Va kern.devstat.version
.Nm sysctl
variable to determine whether they are in sync with the kernel
.Nm
structures.
.Sh SEE ALSO
.Xr systat 1 ,
.Xr devstat 3 ,
.Xr iostat 8 ,
.Xr rpc.rstatd 8 ,
.Xr vmstat 8
.Sh HISTORY
The
.Nm
statistics system appeared in
.Fx 3.0 .
.Sh AUTHORS
.An Kenneth Merry Aq ken@FreeBSD.org
.Sh BUGS
There may be a need for
.Fn spl
protection around some of the
.Nm
list manipulation code to insure, for example, that the list of devices
is not changed while someone is fetching the
.Va kern.devstat.all
.Nm sysctl
variable.
.Pp
It is impossible with the current
.Nm
architecture to accurately measure time per transaction.  The only feasible
way to accurately measure time per transaction would be to record a
timestamp for every transaction.  This measurement is probably not
worthwhile for most people as it would adversely affect the performance of
the system and cost space to store the timestamps for individual
transactions.