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DEVSTAT(9)		 BSD Kernel Developer's Manual		    DEVSTAT(9)

     devstat, devstat_add_entry, devstat_end_transaction,
     devstat_end_transaction_bio, devstat_remove_entry,
     devstat_start_transaction — kernel interface for keeping device statis‐

     #include <sys/devicestat.h>

     devstat_add_entry(struct devstat *ds, const char *dev_name,
	 int unit_number, u_int32_t block_size, devstat_support_flags flags,
	 devstat_type_flags device_type, devstat_priority priority);

     devstat_remove_entry(struct devstat *ds);

     devstat_start_transaction(struct devstat *ds);

     devstat_end_transaction(struct devstat *ds, u_int32_t bytes,
	 devstat_tag_type tag_type, devstat_trans_flags flags);

     devstat_end_transaction_bio(struct devstat *ds, struct bio *bp);

     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 devstat code.	Instead, that is left for user programs to handle.

     devstat_add_entry() registers a device with the devstat subsystem.	 The
     caller is expected to have already allocated and zeroed the devstat
     structure before calling this function.  devstat_add_entry() takes sev‐
     eral arguments:

     ds		  The devstat structure, allocated and zeroed by the client.

     dev_name	  The device name, e.g. da, cd, sa.

     unit_number  Device unit number.

     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 devstat list, it should
		  be set to 0.

     flags	  Flags indicating operations supported or not supported by
		  the device.  See below for details.

     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 pass-through flag
		  to indicate pas-through devices.  See below for a complete
		  list of types.

     priority	  The device priority.	The priority is used to determine how
		  devices are sorted within 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.

     devstat_remove_entry() removes a device from the devstat subsystem.  It
     takes the devstat structure for the device in question as an argument.
     The devstat generation number is incremented and the number of devices is

     devstat_start_transaction() registers the start of a transaction with the
     devstat 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 start_time field of the devstat structure.

     devstat_end_transaction() registers the end of a transaction with the
     devstat subsystem.	 It takes four arguments:

     ds	       The devstat structure for the device in question.

     bytes     The number of bytes transferred in this transaction.

     tag_type  Transaction tag type.  See below for tag types.

     flags     Transaction flags indicating whether the transaction was a
	       read, write, or whether no data was transferred.

     devstat_end_transaction_bio() is a wrapper for devstat_end_transaction()
     which pulls all the information from a struct bio which is ready for

     The devstat structure is composed of the following fields:

     dev_links		Each devstat structure is placed in a linked list when
			it is registered.  The dev_links field contains a
			pointer to the next entry in the list of devstat

     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 cur‐
			rently only a 32-bit integer, but it could be enlarged
			if someone has a system with more than four billion
			device arrival events.

     device_name	The device name is a text string given by the regis‐
			tering driver to identify itself.  (e.g. “da”, “cd”,
			“sa”, etc.)

     unit_number	The unit number identifies the particular instance of
			the peripheral driver in question.

     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 sys‐
			tems if 32 bit integers were used.

     bytes_read		This is the number of bytes that have been read from
			the device.

     bytes_freed	This is the number of bytes that have been
			freed/erased on the device.

     num_reads		This is the number of reads from the device.

     num_writes		This is the number of writes to the device.

     num_frees		This is the number of free/erase operations on the

     num_other		This is the number of transactions to the device which
			are neither reads or writes.  For instance, SCSI driv‐
			ers often send a test unit ready command to SCSI
			devices.  The test unit ready command does not read or
			write any data.	 It merely causes the device to return
			its status.

     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 devstat_start_transaction() and
			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

     block_size		This is the block size of the device, if the device
			has a block size.

     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.

     dev_creation_time	This is the time, as reported by getmicrotime() that
			the device was registered.

     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.

     start_time		This is the time, as reported by getmicrouptime() that
			the device busy count went from zero to one.

     last_comp_time	This is the time as reported by getmicrouptime() that
			a transaction last completed.  It is used along with
			start_time to calculate the device busy time.

     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 pro‐
			grams that decipher the statistics.

     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 pass-
			through driver.	 See below for a complete list of
			device types.

     priority		This is the priority.  This is the first parameter
			used to determine where to insert a device in the
			devstat list.  The second parameter is attach order.
			See below for a list of available priorities.

     Each device is given a device type.  Pass-through devices have the same
     underlying device type and interface as the device they provide an inter‐
     face for, but they also have the pass-through flag set.  The base device
     types are identical to the SCSI device type numbers, so with SCSI periph‐
     erals, the device type returned from an inquiry is usually ORed with the
     SCSI interface type and the pass-through flag if appropriate.  The device
     type flags are as follows:

	   typedef enum {
		   DEVSTAT_TYPE_DIRECT	   = 0x000,
		   DEVSTAT_TYPE_PRINTER	   = 0x002,
		   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_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;

     Devices have a priority associated with them, which controls roughly
     where they are placed in the devstat list.	 The priorities are as fol‐

	   typedef enum {
		   DEVSTAT_PRIORITY_MIN	   = 0x000,
		   DEVSTAT_PRIORITY_FD	   = 0x040,
		   DEVSTAT_PRIORITY_WFD	   = 0x050,
		   DEVSTAT_PRIORITY_CD	   = 0x090,
		   DEVSTAT_PRIORITY_MAX	   = 0xfff
	   } devstat_priority;

     Each device has associated with it flags to indicate what operations are
     supported or not supported.  The devstat_support_flags values are as fol‐

     DEVSTAT_ALL_SUPPORTED    Every statistic type is supported by the device.

     DEVSTAT_NO_BLOCKSIZE     This device does not have a blocksize.

     DEVSTAT_NO_ORDERED_TAGS  This device does not support ordered tags.

     DEVSTAT_BS_UNAVAILABLE   This device supports a blocksize, but it is cur‐
			      rently unavailable.  This flag is most often
			      used with removable media drives.

     Transactions to a device fall into one of three categories, which are
     represented in the flags passed into devstat_end_transaction().  The
     transaction types are as follows:

	   typedef enum {
		   DEVSTAT_NO_DATA = 0x00,
		   DEVSTAT_READ	   = 0x01,
		   DEVSTAT_WRITE   = 0x02,
		   DEVSTAT_FREE	   = 0x03
	   } devstat_trans_flags;

     There are four possible values for the tag_type argument to

     DEVSTAT_TAG_SIMPLE	  The transaction had a simple tag.

     DEVSTAT_TAG_HEAD	  The transaction had a head of queue tag.

     DEVSTAT_TAG_ORDERED  The transaction had an ordered tag.

     DEVSTAT_TAG_NONE	  The device does not support tags.

     The tag type values correspond to the lower four bits of the SCSI tag
     definitions.  In CAM, for instance, the tag_action from the CCB is ORed
     with 0xf to determine the tag type to pass in to

     There is a macro, DEVSTAT_VERSION that is defined in <sys/devicestat.h>.
     This is the current version of the devstat subsystem, and it should be
     incremented each time a change is made that would require recompilation
     of userland programs that access devstat statistics.  Userland programs
     use this version, via the kern.devstat.version sysctl variable to deter‐
     mine whether they are in sync with the kernel devstat structures.

     systat(1), devstat(3), iostat(8), rpc.rstatd(8), vmstat(8)

     The devstat statistics system appeared in FreeBSD 3.0.

     Kenneth Merry ⟨⟩

     There may be a need for spl() protection around some of the devstat list
     manipulation code to insure, for example, that the list of devices is not
     changed while someone is fetching the kern.devstat.all sysctl variable.

     It is impossible with the current devstat architecture to accurately mea‐
     sure time per transaction.	 The only feasible way to accurately measure
     time per transaction would be to record a timestamp for every transac‐
     tion.  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.

BSD				 May 22, 1998				   BSD

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