DEVSTAT(9) FreeBSD Kernel Developer's Manual DEVSTAT(9)
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.
FreeBSD 13.1-RELEASE-p6 July 15, 2020 FreeBSD 13.1-RELEASE-p6
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