DEVSTAT(9)

💅 devstat — kernel interface for keeping device statistics 💅

okay so bestie, welcome to DEVSTAT(9) from the Kernel Developer's Manual. This is the page for the kernel interface for keeping device statistics, and it is lowkey a whole vibe. 💖

✨ 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 💖

okay so the devstat subsystem is literally an interface for recording device statistics, as its name implies. The whole idea is to keep reasonably detailed statistics while using a minimum amount of CPU time to record them. So no statistical calculations actually happen in the kernel portion of the devstat code — that is left for user programs to handle. slay division of labor. ✨

Now the history tea: 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 transaction routines. periodt. 🫶

devstat_new_entry() allocates and initializes a 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 pass-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 is 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:

okay so the device-type flags are literally this whole enum, and every value matters, bestie — no skipping, no cap:

             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_NVME    = 0x040,
                     DEVSTAT_TYPE_IF_MASK    = 0x0f0,
                     DEVSTAT_TYPE_PASS       = 0x100
             } devstat_type_flags;

Devices have a priority associated with them, and that priority controls roughly where they get placed in the devstat list. The priorities are giving:

             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 also has flags attached to it that say what operations are supported or not supported. The devstat_support_flags values are:

• DEVSTAT_ALL_SUPPORTED — every statistic type is supported by the device. iconic behavior, honestly.
• 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.

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Transactions to a device fall into one of three categories — yes bestie, the page says three — and those categories are represented in the flags passed into devstat_end_transaction(). The transaction types are:

             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 transaction to the device that is neither a read nor a write. 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. lowkey iconic.

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(). fr fr, that bit math is the moment.

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. vibe check via sysctl — love that for them.

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🔍 SEE ALSO

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

📜 HISTORY

The devstat statistics system appeared in FreeBSD 3.0. a legend was born ✨

👑 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. race-condition vibes: not the cute kind. 🙈

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this iconic little manual page is from Debian, dated July 15, 2020, and it is literally DEVSTAT(9) — periodt. 💅🫶