NAME
CAM
—
Common Access Method Storage
subsystem
SYNOPSIS
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_DEBUG_COMPILE=CAM_DEBUG_INFO|CAM_DEBUG_CDB|CAM_DEBUG_PROBE
options CAM_DEBUG_FLAGS=CAM_DEBUG_INFO|CAM_DEBUG_CDB
options CAM_MAX_HIGHPOWER=4
options SCSI_NO_SENSE_STRINGS
options SCSI_NO_OP_STRINGS
options SCSI_DELAY=8000
DESCRIPTION
The CAM
subsystem provides a uniform and
modular system for the implementation of drivers to control various SCSI,
ATA, NMVe, and MMC / SD devices, 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.
KERNEL CONFIGURATION
There are a number of generic kernel configuration options for the
CAM
subsystem:
CAM_BOOT_DELAY
- Additional time to wait after the static parts of the kernel have run to allow for discovery of additional 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 devices.
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 debugging information to be printed out when included by itself. See below for details. CAM_MAX_HIGHPOWER=4
- This sets the maximum allowable number of concurrent "high power" commands. A "high power" command 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 Additional 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 memory space environments. In most cases, though, this should be enabled, since it speeds the interpretation 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 opcode. This option, like the sense string option above, is primarily useful for environments like a boot floppy where kernel size is critical. Enabling this option for normal use is not recommended, 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 transfer 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 theSCSI_DELAY
is not specified, it defaults to 2 seconds. The minimum allowable value forSCSI_DELAY
is "100", or 100ms. One special case is that if theSCSI_DELAY
is set to 0, that will be taken to mean the "lowest possible value." In that case, theSCSI_DELAY
will be reset 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 usually done either interactively from the loader(8), or automatically via the /boot/device.hints file. The basic syntax is:
hint.device.unit.property="value"
Individual CAM
bus numbers can be wired
down to specific controllers with a config line similar to the
following:
hint.scbus.0.at="ahd1"
This assigns CAM
bus number 0 to
the ahd1 driver
instance. For controllers supporting more than one bus, a particular bus can
be assigned as follows:
hint.scbus.0.at="ahc1" hint.scbus.0.bus="1"
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:
hint.da.0.at="scbus0" hint.da.0.target="0" hint.da.0.unit="0"
This assigns
da0 to target 0, unit
(lun) 0 of scbus 0. Omitting the target 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 controller, bus,
target, and/or unit instance.
This also works with nvme(4) drives as well.
hint.nvme.4.at="pci7:0:0" hint.scbus.10.at="nvme4" hint.nda.10.at="scbus10" hint.nda.10.target="1" hint.nda.10.unit="12" hint.nda.11.at="scbus10" hint.nda.11.target="1" hint.nda.11.unit="2"
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) devices 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 nda11.
When you have a mixture of wired down and counted devices then the counting 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.
ADAPTERS
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 amounts.
TARGET MODE
Some adapters support
target
mode in which the system is capable of operating 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.
ARCHITECTURE
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 requests. 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.
CAM
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 implement 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 portion. 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 request. 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:
- da(4)
- SCSI or SAS device, or devices that accept SCSI CDBs for I/O.
- ada(4)
- ATA or SATA device
- nda(4)
- NVME device
- sdda(4)
- 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 drives.
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 device.
XPT drivers
The transport driver connects the periph to the SIM. It is not configured 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.
FILES
see other CAM
device entries.
DIAGNOSTICS
An XPT_DEBUG CCB can be used to enable various amounts of tracing information on any specific bus/device from the list of options compiled into the kernel. There are currently seven debugging flags that may be compiled 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 various functions.
CAM_DEBUG_CDB
- This flag will cause the kernel to print out all ATA and SCSI commands sent to a particular device or devices.
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
debugging. CAM_DEBUG_COMPILE
- This specifies support for which debugging flags described 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 details.
SEE ALSO
- Commands:
- camcontrol(8), camdd(8)
- Libraries:
- cam(3)
- 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)
HISTORY
The CAM
SCSI subsystem first appeared in
FreeBSD 3.0. The CAM
ATA
support was added in FreeBSD 8.0.
AUTHORS
The CAM
SCSI subsystem was written by
Justin Gibbs and Kenneth
Merry. The CAM
ATA support was added by
Alexander Motin
<mav@FreeBSD.org>. The
CAM
NVMe support was added by
Warner Losh
<imp@FreeBSD.org>.