--- name: "devstat_end_transaction" section: "9" platform: "linux" package: "freebsd-manpages" version: "6.0-5" release: "Debian 4.0 (Etch)" platforms: ["linux", "freebsd"] url: "https://new.carta.tech/devstat_end_transaction" --- # devstat\_end\_transaction(9) kernel interface for keeping device statistics ## NAME **devstat**, **devstat\_add\_entry**, **devstat\_end\_transaction**, **devstat\_end\_transaction\_bio**, **devstat\_remove\_entry**, **devstat\_start\_transaction** – kernel interface for keeping device statistics ## SYNOPSIS **#include \** *void*\ **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*); *void*\ **devstat\_remove\_entry**(*struct devstat \*ds*); *void*\ **devstat\_start\_transaction**(*struct devstat \*ds*); *void*\ **devstat\_end\_transaction**(*struct devstat \*ds*, *u\_int32\_t bytes*, *devstat\_tag\_type tag\_type*, *devstat\_trans\_flags flags*); *void*\ **devstat\_end\_transaction\_bio**(*struct devstat \*ds*, *struct bio \*bp*); ## 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. **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 several 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 decremented. **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 biodone(). 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* 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\_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. - 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 device. - num\_other 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. - 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 transaction. - 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 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. 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; ``` 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)](/systat/1.md), [devstat(3)](/devstat/3.md), [iostat(8)](/iostat/8.md), [rpc.rstatd(8)](/rpc.rstatd/8.md), [vmstat(8)](/vmstat/8.md) ## 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 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 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.