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><H1
><A
NAME="PROC">Chapter 11. Sg and the "proc" file system</H1
><P
>The sg driver provides information about the SCSI subsystem and the current
internal state of the sg driver in the <TT
CLASS="FILENAME"
>/proc/scsi/sg</TT
>
directory. Some sg driver defaults can be changed by super user writing values
to these "pseudo" files
<A
NAME="AEN730"
HREF="#FTN.AEN730"
>[1]</A
>.</P
><P
>The following files which are readable by all:
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><PRE
CLASS="PROGRAMLISTING"
>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</PRE
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></P
><P
>Each line in 'devices' and 'device_strs' corresponds to an sg device. For
example the first line corresponds to <TT
CLASS="FILENAME"
>/dev/sg0</TT
>. 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
<B
CLASS="COMMAND"
>cat /proc/scsi/scsi</B
> 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 <A
HREF="refs.html#SCSI_HOWTO"
>SCSI-2.4-HOWTO</A
> for more information).</P
><P
>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.]</P
><P
>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):
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>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</PRE
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>
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).</P
><P
>The column headers in 'host_hdr' are given below. Each entry is separated
by a whitespace (currently a tab):
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CLASS="PROGRAMLISTING"
>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)</PRE
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><P
>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:
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CLASS="PROGRAMLISTING"
>$ echo "262144" > /proc/scsi/sg/def_reserved_size</PRE
></FONT
></TD
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></TABLE
>
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.</P
><P
>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.</P
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><H3
CLASS="FOOTNOTES"
>Notes</H3
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><A
NAME="FTN.AEN730"
HREF="proc.html#AEN730"
>[1]</A
></TD
><TD
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WIDTH="95%"
><P
>One strange quirk is that the <TT
CLASS="FILENAME"
>/proc/scsi/sg</TT
> directory
will not appear if there are no SCSI devices (or pseudo devices such as
USB mass storage) attached to the system. The reason for this is that
in the absence of SCSI devices, the SCSI mid level does not initialize
the sg driver (even if it has been loaded as a module).
When the sg driver is a module and the <B
CLASS="COMMAND"
>rmmod sg</B
> is
successfully executed then the <TT
CLASS="FILENAME"
>/proc/scsi/sg</TT
> directory
and its contents are removed.</P
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