Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
CAM(4)			 BSD Kernel Interfaces Manual			CAM(4)

     CAM -- Common Access Method Storage subsystem

     device scbus
     device ada
     device cd
     device ch
     device da
     device pass
     device pt
     device sa
     options CAMDEBUG
     options CAM_DEBUG_BUS=-1
     options CAM_DEBUG_TARGET=-1
     options CAM_DEBUG_LUN=-1
     options CAM_MAX_HIGHPOWER=4
     options SCSI_NO_OP_STRINGS
     options SCSI_DELAY=8000

     The CAM subsystem provides	a uniform and modular system for the implemen-
     tation of drivers to control various SCSI,	ATA, NMVe, and MMC / SD	de-
     vices, and	to utilize different SCSI, ATA,	NVMe, and MMC /	SD host
     adapters through host adapter drivers.  When the system probes buses, it
     attaches any devices it finds to the appropriate drivers.	The pass(4)
     driver, if	it is configured in the	kernel,	will attach to all devices.

     There are a number	of generic kernel configuration	options	for the	CAM

     CAM_BOOT_DELAY	    Additional time to wait after the static parts of
			    the	kernel have run	to allow for discovery of ad-
			    ditional devices which may take time to connect,
			    such as USB	attached storage.

     CAM_IOSCHED_DYNAMIC    Enable dynamic decisions in	the I/O	scheduler
			    based on hints and the current performance of the
			    storage devices.

     CAM_IO_STATS	    Enable collection of statistics for	periph de-

     CAM_TEST_FAILURE	    Enable ability to simulate I/O failures.

     CAMDEBUG		    This option	compiles in all	the CAM	debugging
			    printf code.  This will not	actually cause any de-
			    bugging information	to be printed out when in-
			    cluded by itself.  See below for details.

     CAM_MAX_HIGHPOWER=4    This sets the maximum allowable number of concur-
			    rent "high power" commands.	 A "high power"	com-
			    mand is a command that takes more electrical power
			    than most to complete.  An example of this is the
			    SCSI START UNIT command.  Starting a disk often
			    takes significantly	more electrical	power than
			    normal operation.  This option allows the user to
			    specify how	many concurrent	high power commands
			    may	be outstanding without overloading the power
			    supply on his computer.

     SCSI_NO_SENSE_STRINGS  This eliminates text descriptions of each SCSI Ad-
			    ditional Sense Code	and Additional Sense Code
			    Qualifier pair.  Since this	is a fairly large text
			    database, eliminating it reduces the size of the
			    kernel somewhat.  This is primarily	necessary for
			    boot floppies and other low	disk space or low mem-
			    ory	space environments.  In	most cases, though,
			    this should	be enabled, since it speeds the	inter-
			    pretation of SCSI error messages.  Do not let the
			    "kernel bloat" zealots get to you -- leave the
			    sense descriptions in your kernel!

     SCSI_NO_OP_STRINGS	    This disables text descriptions of each SCSI op-
			    code.  This	option,	like the sense string option
			    above, is primarily	useful for environments	like a
			    boot floppy	where kernel size is critical.	En-
			    abling this	option for normal use is not recom-
			    mended, since it slows debugging of	SCSI problems.

     SCSI_DELAY=8000	    This is the	SCSI "bus settle delay."  In CAM, it
			    is specified in milliseconds, not seconds like the
			    old	SCSI layer used	to do.	When the kernel	boots,
			    it sends a bus reset to each SCSI bus to tell each
			    device to reset itself to a	default	set of trans-
			    fer	negotiations and other settings.  Most SCSI
			    devices need some amount of	time to	recover	from a
			    bus	reset.	Newer disks may	need as	little as
			    100ms, while old, slow devices may need much
			    longer.  If	the SCSI_DELAY is not specified, it
			    defaults to	2 seconds.  The	minimum	allowable
			    value for SCSI_DELAY is "100", or 100ms.  One spe-
			    cial case is that if the SCSI_DELAY	is set to 0,
			    that will be taken to mean the "lowest possible
			    value."  In	that case, the SCSI_DELAY will be re-
			    set	to 100ms.

     All devices and buses support dynamic allocation so that an upper number
     of	devices	and controllers	does not need to be configured;	device da will
     suffice for any number of disk drivers.

     The devices are either wired so they appear as a particular device	unit
     or	counted	so that	they appear as the next	available unused unit.

     Units are wired down by setting kernel environment	hints.	This is	usu-
     ally done either interactively from the loader(8),	or automatically via
     the /boot/device.hints file.  The basic syntax is:"value"

     Individual	CAM bus	numbers	can be wired down to specific controllers with
     a config line similar to the following:"ahd1"

     This assigns CAM bus number 0 to the ahd1 driver instance.	 For con-
     trollers supporting more than one bus, a particular bus can be assigned
     as	follows:"ahc1"

     This assigns CAM bus 0 to the bus 1 instance on ahc1.  Peripheral drivers
     can be wired to a specific	bus, target, and lun as	so:"scbus0""0"

     This assigns da0 to target	0, unit	(lun) 0	of scbus 0.  Omitting the tar-
     get or unit hints will instruct CAM to treat them as wildcards and	use
     the first respective counted instances.  These examples can be combined
     together to allow a peripheral device to be wired to any particular con-
     troller, bus, target, and/or unit instance.

     This also works with nvme(4) drives as well."pci7:0:0""nvme4""scbus10""1"

     This assigns the NVMe card	living at PCI bus 7 to scbus 10	(in PCIe, slot
     and function are rarely used and usually 0).  The target for nda(4) de-
     vices is always 1.	 The unit is the namespace identifier from the drive.
     The namespace id 1	is exported as nda10 and namespace id 2	is exported as

     When you have a mixture of	wired down and counted devices then the	count-
     ing begins	with the first non-wired down unit for a particular type.
     That is, if you have a disk wired down as device da1, then	the first non-
     wired disk	shall come on line as da2.

     The system	allows common device drivers to	work through many different
     types of adapters.	 The adapters take requests from the upper layers and
     do	all IO between the SCSI, ATA, NVMe, or MMC / SD	bus and	the system.
     The maximum size of a transfer is governed	by the adapter.	 Most adapters
     can transfer 64KB in a single operation, however many can transfer	larger

     Some adapters support target mode in which	the system is capable of oper-
     ating as a	device,	responding to operations initiated by another system.
     Target mode is supported for some adapters, but is	not yet	complete for
     this version of the CAM SCSI subsystem.

     The CAM subsystem glues together the upper	layers of the system to	the
     storage devices.  PERIPH devices accept storage requests from GEOM	and
     other upper layers	of the system and translates them into protocol	re-
     quests.  XPT (transport) dispatches these protocol	requests to a SIM
     driver.  A	SIM driver takes protocol requests and translates them into
     hardware commands the host	adapter	understands to transfer	the protocol
     requests, and data	(if any) to the	storage	device.	 The CCB transports
     these requests around as messages.

     The Common	Access Method was a standard defined in	the 1990s to talk to
     disk drives.  FreeBSD is one of the few operating systems to fully	imple-
     ment this model.  The interface between different parts of	CAM is the CCB
     (or CAM Control Block).  Each CCB has a standard header, which contains
     the type of request and dispatch information, and a command specific por-
     tion.  A CAM Periph generates requests.  The XPT layer dispatches these
     requests to the appropriate SIM.  Some CCBs are sent directly to the SIM
     for immediate processing, while others are	queued and complete when the
     I/O has finished.	A SIM takes CCBs and translates	them into hardware
     specific commands to push the SCSI	CDB or other protocol control block to
     the peripheral, along with	setting	up the DMA for the associated data.

   Periph Devices
     A periph driver knows how to translate standard requests into protocol
     messages that a SIM can deliver to	hardware.  These requests can come
     from any upper layer source, but primarily	come in	via GEOM as a bio re-
     quest.  They can also come	in directly from character device requests for
     tapes and pass through commands.

     Disk devices, or direct access (da) in CAM, are one type of peripheral.
     These devices present themselves to the kernel a device ending in "da".
     Each protocol has a unique	device name:

       SCSI or SAS device, or devices that accept SCSI CDBs for	I/O.

       ATA or SATA device

       NVME device

       An SD or	MMC block storage device.

     Tape devices are called serial access (sa(4)) in CAM.  They interface to
     the system	via a character	device and provide ioctl(2) control for	tape

     The pass(4) device	will pass through CCB requests from userland to	the
     SIM directly.  The	device is used to send commands	other than read,
     write, trim or flush to a device.	The camcontrol(8) command uses this

   XPT drivers
     The transport driver connects the periph to the SIM.  It is not config-
     ured separately.  It is also responsible for device discovery for those
     SIM drivers that do not enumerate themselves.

   SIM driver
     SIM used to stand for SCSI	Interface Module.  Now it is just SIM because
     it	understands protocols other than SCSI.	There are two types of SIM
     drivers: virtual and physical.  Physical SIMs are typically called	host
     bus adapters (HBA), but not universally.  Virtual SIM drivers are for
     communicating with	virtual	machine	hosts.

     see other CAM device entries.

     An	XPT_DEBUG CCB can be used to enable various amounts of tracing infor-
     mation on any specific bus/device from the	list of	options	compiled into
     the kernel.  There	are currently seven debugging flags that may be	com-
     piled in and used:

     CAM_DEBUG_INFO	 This flag enables general informational printfs for
			 the device or devices in question.

     CAM_DEBUG_TRACE	 This flag enables function-level command flow tracing
			 i.e., kernel printfs will happen at the entrance and
			 exit of various functions.

     CAM_DEBUG_SUBTRACE	 This flag enables debugging output internal to	vari-
			 ous functions.

     CAM_DEBUG_CDB	 This flag will	cause the kernel to print out all ATA
			 and SCSI commands sent	to a particular	device or de-

     CAM_DEBUG_XPT	 This flag will	enable command scheduler tracing.

     CAM_DEBUG_PERIPH	 This flag will	enable peripheral drivers messages.

     CAM_DEBUG_PROBE	 This flag will	enable devices probe process tracing.

     Some of these flags, most notably CAM_DEBUG_TRACE and CAM_DEBUG_SUBTRACE,
     will produce kernel printfs in EXTREME numbers.

     Users can enable debugging	from their kernel config file, by using	the
     following kernel config options:

     CAMDEBUG		This builds into the kernel all	possible CAM debug-

     CAM_DEBUG_COMPILE	This specifies support for which debugging flags de-
			scribed	above should be	built into the kernel.	Flags
			may be ORed together if	the user wishes	to see printfs
			for multiple debugging levels.

     CAM_DEBUG_FLAGS	This sets the various debugging	flags from a kernel
			config file.

     CAM_DEBUG_BUS	Specify	a bus to debug.	 To debug all buses, set this
			to -1.

     CAM_DEBUG_TARGET	Specify	a target to debug.  To debug all targets, set
			this to	-1.

     CAM_DEBUG_LUN	Specify	a lun to debug.	 To debug all luns, set	this
			to -1.

     Users may also enable debugging on	the fly	by using the camcontrol(8)
     utility, if wanted	options	built into the kernel.	See camcontrol(8) for

	 camcontrol(8),	camdd(8)


     Periph Drivers:
	 ada(4), da(4),	nda(4),	pass(4), sa(4)

     SIM Devices:
	 aac(4), aacraid(4), ahc(4), ahci(4), ata(4), aw_mmc(4), ciss(4),
	 hv_storvsc(4),	isci(4), iscsi(4), isp(4), mpr(4), mps(4), mpt(4),
	 mrsas(4), mvs(4), nvme(4), pms(4), pvscsi(4), sdhci(4), smartpqi(4),
	 sym(4), tws(4), umass(4), virtio_scsi(4)

     Deprecated	or Poorly Supported SIM	Devices:
	 ahd(4), amr(4), arcmsr(4), esp(4), hpt27xx(4),	hptiop(4), hptmv(4),
	 hptnr(4), iir(4) mfi(4), sbp(4), twa(4)

     The CAM SCSI subsystem first appeared in FreeBSD 3.0.  The	CAM ATA	sup-
     port was added in FreeBSD 8.0.

     The CAM SCSI subsystem was	written	by Justin Gibbs	and Kenneth Merry.
     The CAM ATA support was added by Alexander	Motin <>.  The
     CAM NVMe support was added	by Warner Losh <>.

BSD				 June 18, 2020				   BSD


Want to link to this manual page? Use this URL:

home | help