Provided by: sg3-utils_1.17-2_i386
sgm_dd - copies data to and from files and devices. Specialized for
devices that understand the SCSI command set and does memory mapped
transfers from sg devices.
sgm_dd [bpt=<n>] [bs=<n>] [cdbsz=6|10|12|16] [count=<n>] [dio=0|1]
[fua=0|1|2|3] [ibs=<n>] [if=<ifile>] [obs=<n>] [of=<ofile>] [seek=<n>]
[skip=<n>] [sync=0|1] [time=0|1] [verbose=<n>] [--version]
Copy data to and from any files. Specialized for "files" that are Linux
SCSI generic (sg) devices and raw devices. Uses memory mapped transfers
on sg devices. Similar syntax and semantics to dd(1) but does not
perform any conversions.
Will only perform memory mapped transfers when <ifile> or <ofile> are
SCSI generic (sg) devices. If both <ifile> and <ofile> are sg devices
then memory mapped transfers are only performed on <ifile>.
each IO transaction will be made using this number of blocks (or
less if near the end of count). Default is 128.
this must be the block size of the physical device. Note that
this differs from dd(1) which permits "bs" to be an integral
multiple. Default is 512 which is usually correct for disks but
incorrect for cdroms (which normally have 2048 byte blocks).
cdbsz=6 | 10 | 12 | 16
size of SCSI READ and/or WRITE commands issued on sg device
names (or block devices when "blk_sgio=1" is given). Default is
10 byte SCSI command blocks (unless calculations indicate that a
4 byte block number may be exceeded, in which case it defaults
to 16 byte SCSI commands).
copy this number of blocks from ’if’ to ’of’. Default is the
minimum (of ’if’ and ’of’) number of blocks that sg devices
return from READ CAPACITY SCSI commands or that block devices
(or their partitions) report. Normal files are not probed for
their size. If ’skip’ or ’seek’ are given and the count is
derived (i.e. not explicitly given) then the derived count is
scaled back so that the copy will not overrun the device. If the
file name is a block device partition and count is not given
then the size of the partition rather than the size of the whole
device is used. If count is not given and cannot be derived then
an error message is issued and no copy takes place.
dio=0 | 1
permits direct IO to be selected on the write-side (i.e. "of").
Only allowed when the read-side (i.e. "if") is a sg device. When
1 there may be a "zero copy" copy (i.e. mmap-ed transfer on the
read into the user space and direct IO from there on the write,
potentially two DMAs and no data copying from the CPU). Default
fua=0 | 1 | 2 | 3
force unit access bit. When 3, fua is set on both "if" and "of",
when 2, fua is set on "if", when 1, fua is set on "of", when 0
(the default), fua is cleared on both. 6 byte SCSI READ and
WRITE commands (cdbsz=6) do not support the fua bit. Only active
for sg device file names.
if given must be the same as bs
read from FILE instead of stdin which is the default. A file
name of "-" is taken to be stdin. Starts reading at the
beginning of FILE unless "skip" is given.
if given must be the same as bs
write to FILE instead of stdout. A file name of - is taken to be
stdout. If FILE is /dev/null then no actual writes are
performed. If FILE is . (period) then it is treated the same
way as /dev/null (this is a shorthand notation)
start writing BLOCKS bs-sized blocks from the start of the
output file. Default is block 0 (i.e. start of file).
start reading BLOCKS bs-sized blocks from the start of input
file. Default is block 0 (i.e. start of file).
sync=0 | 1
when 1, does SYNCHRONIZE CACHE command on "of" at the end of the
transfer. Only active when "of" is a sg device file name
time=0 | 1
when 1, times transfer and does throughput calculation,
outputting the results (to stderr) at completion. When 0
(default) doesn’t perform timing
as <n> increases so does the amount of debug output sent to
stderr. Default value is zero which yields the minimum amount
of debug output. A value of 1 reports extra information that is
not repetitive. A value 2 reports cdbs and responses for SCSI
commands that are not repetitive (i.e. other that READ and
WRITE). Error processing is not considered repetitive. Values of
3 and 4 yield output for all SCSI commands (and Unix read() and
write() calls) so there can be a lot of output.
outputs version number information and exits
A raw device must be bound to a block device prior to using sgm_dd.
See raw(8) for more information about binding raw devices. To be safe,
the sg device mapping to SCSI block devices should be checked with "cat
/proc/scsi/scsi" before use.
Raw device partition information can often be found with fdisk(8) [the
"-ul" argument is useful in this respect].
BYTES and BLOCKS may be followed by one of these multiplicative
suffixes: c C *1; w W *2; b B *512; k K KiB *1,024; KB *1,000; m M MiB
*1,048,576; MB *1,000,000 . This pattern continues for "G", "T" and
"P". The latter two suffixes can only be used for count, skip and seek
values). Also a suffix of the form "x<n>" multiplies the leading number
by <n>. These multiplicative suffixes are compatible with GNU’s dd
command (since 2002) which claims compliance with SI and with IEC
Alternatively numerical values can be given in hexadecimal preceded by
either "0x" or "0X". When hex numbers are given multipliers cannot be
The count, skip and seek parameters can take 64 bit values (i.e. very
big numbers). Other values are limited to what can fit in a signed 32
Data usually gets to the user space in a 2 stage process: first the
SCSI adapter DMAs into kernel buffers and then the sg driver copies
this data into user memory (write operations reverse this sequence).
With memory mapped transfers a kernel buffer reserved by sg is memory
mapped (see the mmap(2) system call) into the user space. When this is
done the second (redundant) copy from kernel buffers to user space is
not needed. Hence the transfer is faster and requires less "grunt" from
All informative, warning and error output is sent to stderr so that
dd’s output file can be stdout and remain unpolluted. If no options are
given, then the usage message is output and nothing else happens.
See the examples given in the man page for sg_dd(8).
For sg devices this utility issues READ and WRITE (SBC) SCSI commands
which are appropriate for disks and reading from CD/DVD drives. Those
commands are not formatted correctly for tape devices so sgm_dd should
not be used on tape devices.
The signal handling has been borrowed from dd: SIGINT, SIGQUIT and
SIGPIPE output the number of remaining blocks to be transferred and the
records in + out counts; then they have their default action. SIGUSR1
causes the same information to be output yet the copy continues. All
output caused by signals is sent to stderr.
Written by Doug Gilbert and Peter Allworth.
Report bugs to <dgilbert at interlog dot com>.
Copyright © 2000-2005 Douglas Gilbert
This software is distributed under the GPL version 2. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR
The simplest variant of this utility is called sg_dd. A POSIX threads
version of this utility called sgp_dd is in the sg3_utils package. The
lmbench package contains lmdd which is also interesting. raw(8), dd(1)