Provided by: freebsd-manpages_12.2-1_all 
NAME
devstat, devstat_end_transaction, devstat_end_transaction_bio,
devstat_end_transaction_bio_bt, devstat_new_entry, devstat_remove_entry,
devstat_start_transaction, devstat_start_transaction_bio — kernel interface for keeping
device statistics
SYNOPSIS
#include <sys/devicestat.h>
struct devstat *
devstat_new_entry(const void *dev_name, int unit_number, uint32_t block_size,
devstat_support_flags flags, devstat_type_flags device_type, devstat_priority priority);
void
devstat_remove_entry(struct devstat *ds);
void
devstat_start_transaction(struct devstat *ds, const struct bintime *now);
void
devstat_start_transaction_bio(struct devstat *ds, struct bio *bp);
void
devstat_end_transaction(struct devstat *ds, uint32_t bytes, devstat_tag_type tag_type,
devstat_trans_flags flags, const struct bintime *now, const struct bintime *then);
void
devstat_end_transaction_bio(struct devstat *ds, const struct bio *bp);
void
devstat_end_transaction_bio_bt(struct devstat *ds, const struct bio *bp,
const struct bintime *now);
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 devstat code. Instead, that is left for user programs to handle.
The historical and antiquated devstat model assumed a single active IO operation per device,
which is not accurate for most disk-like drivers in the 2000s and beyond. New consumers of
the interface should almost certainly use only the "bio" variants of the start and end
transacation routines.
devstat_new_entry() allocates and initializes devstat structure and returns a pointer to it.
devstat_new_entry() takes several arguments:
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 decremented.
devstat_start_transaction() registers the start of a transaction with the devstat subsystem.
Optionally, if the caller already has a binuptime() value available, it may be passed in
*now. Usually the caller can just pass NULL for now, and the routine will gather the
current binuptime() itself. The busy count is incremented with each transaction start.
When a device goes from idle to busy, the system uptime is recorded in the busy_from field
of the devstat structure.
devstat_start_transaction_bio() records the binuptime() in the provided bio's bio_t0 and
then invokes devstat_start_transaction().
devstat_end_transaction() registers the end of a transaction with the devstat subsystem. It
takes six 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.
now The binuptime() at the end of the transaction, or NULL.
then The binuptime() at the beginning of the transaction, or NULL.
If now is NULL, it collects the current time from binuptime(). If then is NULL, the
operation is not tracked in the devstat duration table.
devstat_end_transaction_bio() is a thin wrapper for devstat_end_transaction_bio_bt() with a
NULL now parameter.
devstat_end_transaction_bio_bt() is a wrapper for devstat_end_transaction() which pulls all
needed information from a struct bio prepared by devstat_start_transaction_bio(). The bio
must be ready for biodone() (i.e., bio_bcount and bio_resid must be correctly initialized).
The devstat structure is composed of the following fields:
sequence0,
sequence1 An implementation detail used to gather consistent snapshots of device
statistics.
start_count Number of operations started.
end_count Number of operations completed. The “busy_count” can be calculated by
subtracting end_count from start_count. (sequence0 and sequence1 are
used to get a consistent snapshot.) 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.
There should be one and only one transaction start event and one
transaction end event for each transaction.
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 structures.
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.
device_name The device name is a text string given by the registering 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[4] This array contains the number of bytes that have been read (index
DEVSTAT_READ), written (index DEVSTAT_WRITE), freed or erased (index
DEVSTAT_FREE), or other (index DEVSTAT_NO_DATA). All values are unsigned
64-bit integers.
operations[4] This array contains the number of operations of a given type that have
been performed. The indices are identical to those for bytes above.
DEVSTAT_NO_DATA or "other" represents the number of transactions to the
device which are neither reads, writes, nor frees. For instance, SCSI
drivers 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.
duration[4] This array contains the total bintime corresponding to completed
operations of a given type. The indices are identical to those for bytes
above. (Operations that complete using the historical
devstat_end_transaction() API and do not provide a non-NULL then are not
accounted for.)
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.
creation_time This is the time, as reported by getmicrotime() that the device was
registered.
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.
busy_from If the device is not busy, this was the time that a transaction last
completed. If the device is busy, this the most recent of either the
time that the device became busy, or the time that the last transaction
completed.
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.
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.
id Identification for GEOM nodes.
Each device is given a device type. Pass-through devices have the same underlying device
type and interface as the device they provide an interface 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 peripherals, 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_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;
Devices have a priority associated with them, which controls roughly where they are placed
in the devstat list. The priorities are as follows:
typedef enum {
DEVSTAT_PRIORITY_MIN = 0x000,
DEVSTAT_PRIORITY_OTHER = 0x020,
DEVSTAT_PRIORITY_PASS = 0x030,
DEVSTAT_PRIORITY_FD = 0x040,
DEVSTAT_PRIORITY_WFD = 0x050,
DEVSTAT_PRIORITY_TAPE = 0x060,
DEVSTAT_PRIORITY_CD = 0x090,
DEVSTAT_PRIORITY_DISK = 0x110,
DEVSTAT_PRIORITY_ARRAY = 0x120,
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 follows:
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 currently 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;
#define DEVSTAT_N_TRANS_FLAGS 4
DEVSTAT_NO_DATA is a type of transactions to the device which are neither reads or writes.
For instance, SCSI drivers 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.
There are four possible values for the tag_type argument to devstat_end_transaction():
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 devstat_end_transaction().
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
determine whether they are in sync with the kernel devstat structures.
SEE ALSO
systat(1), devstat(3), iostat(8), rpc.rstatd(8), vmstat(8)
HISTORY
The devstat statistics system appeared in FreeBSD 3.0.
AUTHORS
Kenneth Merry <ken@FreeBSD.org>
BUGS
There may be a need for spl() protection around some of the devstat list manipulation code
to ensure, for example, that the list of devices is not changed while someone is fetching
the kern.devstat.all sysctl variable.