devstat(9) devstat_end_transaction_bio

Other Alias

devstat_add_entry, devstat_end_transaction


In sys/devicestat.h Ft void Fo devstat_add_entry Fa struct devstat *ds Fa const char *dev_name Fa int unit_number Fa uint32_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 uint32_t bytes Fa devstat_tag_type tag_type Fa devstat_trans_flags flags Fc Ft void Fo devstat_end_transaction_bio Fa struct devstat *ds Fa struct bio *bp Fc


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

Fn devstat_add_entry registers a device with the subsystem. The caller is expected to have already allocated and zeroed the devstat structure before calling this function. Fn devstat_add_entry takes several arguments:

The devstat structure, allocated and zeroed by the client.
The device name, e.g. da, cd, sa.
Device unit number.
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 list, it should be set to 0.
Flags indicating operations supported or not supported by the device. See below for details.
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.
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.

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

Fn devstat_start_transaction registers the start of a transaction with the 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.

Fn devstat_end_transaction registers the end of a transaction with the subsystem. It takes four arguments:

The devstat structure for the device in question.
The number of bytes transferred in this transaction.
Transaction tag type. See below for tag types.
Transaction flags indicating whether the transaction was a read, write, or whether no data was transferred.

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

The devstat structure is composed of the following fields:

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.
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.
The device name is a text string given by the registering driver to identify itself. (e.g. ``da'' ``cd'' ``sa'' etc.)
The unit number identifies the particular instance of the peripheral driver in question.
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.
This is the number of bytes that have been read from the device.
This is the number of bytes that have been freed/erased on the device.
This is the number of reads from the device.
This is the number of writes to the device.
This is the number of free/erase operations on the device.
This is the number 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.
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.
This is the block size of the device, if the device has a block size.
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.
This is the time, as reported by Fn getmicrotime that the device was registered.
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.
This is the time, as reported by Fn getmicrouptime that the device busy count went from zero to one.
This is the time as reported by Fn getmicrouptime that a transaction last completed. It is used along with start_time to calculate the device busy time.
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.
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.
This is the priority. This is the first parameter used to determine where to insert a device in the 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 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_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 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:

Every statistic type is supported by the device.
This device does not have a blocksize.
This device does not support ordered tags.
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 Fn 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 Fn devstat_end_transaction :

The transaction had a simple tag.
The transaction had a head of queue tag.
The transaction had an ordered tag.
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 Fn devstat_end_transaction .

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


The statistics system appeared in Fx 3.0 .


An Kenneth Merry Aq [email protected]


There may be a need for Fn spl protection around some of the 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.

It is impossible with the current 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.