sgm_dd - copies data to and from sg and raw devices
Copy data to and from Linux SCSI generic (sg) and raw devices
using memory mapped IO. Similar syntax and semantics to
but does not perform any conversions.
each IO transaction will be made using this number of blocks (or less if
near the end of count). Default is 128.
be the block size of the physical device. Note that this differs from
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).
copy this number of blocks. Default is the minimum number that sg devices
return from READ CAPACITY. Other device types (e.g. normal files)
are _not_ probed for their size. Thus if neither device (i.e. 'if'
nor 'of') is an sg device and count is not given then the command will fail
with an error message requesting a count value.
- 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 IO 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 is 0
- 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. A file name of - is taken to be stdin
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
skip BLOCKS bs-sized blocks at start of output
skip BLOCKS bs-sized blocks at start of input
- 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
outputs version number information and exits
A raw device must be bound to a block device prior to using sgm_dd.
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"
The count is only deduced for sg devices (minimum > 0 if both input and
output are sg devices) otherwise it defaults to 0. This is for safety!
Raw device partition information can often be found with
[the "-ul" argument is useful in this respect].
BYTES and BLOCKS may be followed by the following multiplicative suffixes:
c C *1; b B *512; k *1,024; K *1,000; m *1,048,576; M *1,000,000;
g *1,073,741,824; G *1,000,000,000; t *1,099,511,627,776 and
T *1,000,000,000,000 (the latter two can only be used for count, skip
and seek values).
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 bit number.
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 IO a kernel buffer reserved by sg is memory mapped
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 the CPU.
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
For sg devices this command issues READ_10 and WRITE_10 SCSI commands which
are appropriate for disks and CDROM players. Those commands are not
formatted correctly for tape devices so sgm_dd should not be used on
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 <firstname.lastname@example.org
Copyright © 2000-2002 Douglas Gilbert
This software is distributed under the GPL version 2. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
The simplest variant of this command is called
A POSIX threads version of this command called
is in the sg3_utils package. The lmbench package contains
which is also interesting.
- REPORTING BUGS
- SEE ALSO