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>.

bpt=BLOCKS

each IO transaction will be made using this number of blocks (or less if near the end of count). Default is 128.

bs=BYTES

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).

count=BLOCKS

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 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.

ibs=BYTES

if given must be the same as bs

if=FILE

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.

obs=BYTES

if given must be the same as bs

of=FILE

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)

seek=BLOCKS

start writing BLOCKS bs-sized blocks from the start of the output file. Default is block 0 (i.e. start of file).

skip=BLOCKS

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

verbose=<n>

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.

--version

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 60027-2.

Alternatively numerical values can be given in hexadecimal preceded by either "0x" or "0X". When hex numbers are given multipliers cannot be used.

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 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 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 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 PURPOSE.

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)