Chapter 11. Sg and the "proc" file system

The sg driver provides information about the SCSI subsystem and the current internal state of the sg driver in the /proc/scsi/sg directory. Some sg driver defaults can be changed by super user writing values to these "pseudo" files [1].

The following files which are readable by all:

allow_dio 0 indicates direct IO disable, 1 for enabled debug debug information including active request data def_reserved_size default buffer size reserved for each file descriptor devices one line of numeric data per device device_hdr single line of column names corresponding to 'devices' device_strs one line of vendor, product and rev info per device hosts one line of numeric data per host host_hdr single line of column names corresponding to 'hosts' host_strs one line of host information (string) per host version sg version as a number followed by a string representation

Each line in 'devices' and 'device_strs' corresponds to an sg device. For example the first line corresponds to /dev/sg0. The line number (origin 0) also corresponds to the sg minor device number. This mapping is local to sg and is normally the same as given by th cat /proc/scsi/scsi command which is reported by the SCSI mid level driver. The two mappings may diverge when 'remove-single-device' and 'add-single-device' are used (see the SCSI-2.4-HOWTO for more information).

Each line in 'hosts' and 'host_strs' corresponds to a SCSI host. For example the first line corresponds to the host normally represented as "scsi0". This mapping is invariant across the SCSI sub system. [So these entries could arguably be migrated to the mid level.]

The column headers in 'device_hdr' are given below. If the device is not present (and one is present after it) then a line of "-1" entries is output. Each entry is separated by a whitespace (currently a tab):

host host number (indexes 'hosts' table, origin 0) chan channel number of device id SCSI id of device lun Logical Unit number of device type SCSI type (e.g. 0->disk, 5->cdrom, 6->scanner) opens number of opens (by sd, sr, sr and sg) at this time depth maximum queue depth supported by device busy number of commands being processed by host for this device online 1 indicates device is in normal online state, 0->offline

A SCSI device is set offline by the SCSI mid level when it decides that a device is no longer responding (e.g. the device does not respond to an SCSI INQUIRY command after it has been reset).

The column headers in 'host_hdr' are given below. Each entry is separated by a whitespace (currently a tab):

uid unique id (non-zero if multiple hosts of same type) busy number of commands being processed for this host cpl maximum number of command per lun (may be 0 if "device depth" is given sgat maximum elements of scatter gather the adapter (pseudo) DMA can accommodate isa 0 -> non-ISA adapter, 1 -> ISA adapter. ISA adapters are assumed to have a 24 bit address bus limit (16 MB). emu 0 -> real SCSI adapter, 1 -> emulated SCSI adapter (e.g. ide-scsi device driver)

The 'def_reserved_size' is both readable and writable. It is only writable by root. It is initialized to the value of DEF_RESERVED_SIZE in the "sg.h" file. Values between 0 and 1048576 (which is 2 ** 20) are accepted and can be set from the command line with the following syntax:

$ echo "262144" > /proc/scsi/sg/def_reserved_size

Note that the actual reserved buffer associated with a file descriptor could be less than 'def_reserved_size' if appropriate memory is not available. If the sg driver is compiled into the kernel (but not when it is a module) this value can also be read at /proc/sys/kernel/sg-big-buff . This latter feature is deprecated.

The 'allow_dio' is both readable and writable. It is only writable by root. When it is 0 (default) any request to do direct IO (i.e. by setting SG_FLAG_DIRECT_IO) will be ignored and indirect IO will be done instead.