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 [bs=BS] [count=COUNT] [ibs=BS] [if=IFILE] [iflag=FLAGS] [obs=BS]
[of=OFILE] [oflag=FLAGS] [seek=SEEK] [skip=SKIP] [--help] [--version]
[bpt=BPT] [cdbsz=6|10|12|16] [dio=0|1] [sync=0|1] [time=0|1]
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
performed on IFILE. If no other flags are specified then indirect IO is
performed on OFILE. If ’oflag=dio’ is given then direct IO is attempted
on OFILE. If ’oflag=smmap’ is given then shared mmap-ed IO (sharing the
mmap-ed reserve buffer associated with IFILE) is attempted. In both
latter cases if the faster IO option is not available, they fall back
to indirect IO and report this at the end of the copy.
The first group in the synopsis above are "standard" Unix dd(1)
operands. The second group are extra options added by this utility.
Both groups are defined below.
each IO transaction will be made using BPT blocks (or less if
near the end of the copy). Default is 128 for block sizes less
that 2048 bytes, otherwise the default is 32. So for bs=512 the
reads and writes will each convey 64 KiB of data by default
(less if near the end of the transfer or memory restrictions).
When cd/dvd drives are accessed, the block size is typically
2048 bytes and bpt defaults to 32 which again implies 64 KiB
bs=BS where BS 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). For
this utility the maximum size of each individual IO operation is
BS * BPT bytes.
cdbsz=6 | 10 | 12 | 16
size of SCSI READ and/or WRITE commands issued on sg device
names. 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 COUNT blocks from IFILE to OFILE. Default is the minimum
(of IFILE and OFILE) number of blocks that sg devices report
from SCSI READ CAPACITY commands or that block devices (or their
partitions) report. Normal files are not probed for their size.
If skip=SKIP or skip=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. on
OFILE). Only allowed when the read-side (i.e. IFILE) 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. The same action as ’dio=1’ is also
available with ’oflag=dio’.
ibs=BS if given must be the same as BS given to ’bs=’ option.
read from IFILE instead of stdin. If IFILE is ’-’ then stdin is
read. Starts reading at the beginning of IFILE unless SKIP is
where FLAGS is a comma separated list of one or more flags
outlined below. These flags are associated with IFILE and are
ignored when IFILE is stdin.
obs=BS if given must be the same as BS given to ’bs=’ option.
write to OFILE instead of stdout. If OFILE is ’-’ then writes to
stdout. If OFILE is /dev/null then no actual writes are
performed. If OFILE is ’.’ (period) then it is treated the same
way as /dev/null (this is a shorthand notation). If OFILE exists
then it is _not_ truncated; it is overwritten from the start of
OFILE unless ’oflag=append’ or SEEK is given.
where FLAGS is a comma separated list of one or more flags
outlined below. These flags are associated with OFILE and are
ignored when OFILE is /dev/null, ’.’ (period), or stdout.
start writing SEEK bs-sized blocks from the start of OFILE.
Default is block 0 (i.e. start of file).
start reading SKIP bs-sized blocks from the start of IFILE.
Default is block 0 (i.e. start of file).
sync=0 | 1
when 1, does SYNCHRONIZE CACHE command on OFILE at the end of
the transfer. Only active when OFILE 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 VERB 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.
--help outputs usage message and exits.
outputs version number information and exits.
Here is a list of flags and their meanings:
append causes the O_APPEND flag to be added to the open of OFILE. For
normal files this will lead to data appended to the end of any
existing data. Cannot be used together with the seek=SEEK
option as they conflict. The default action of this utility is
to overwrite any existing data from the beginning of the file
or, if SEEK is given, starting at block SEEK. Note that
attempting to ’append’ to a device file (e.g. a disk) will
usually be ignored or may cause an error to be reported.
dio is only active with oflag (i.e. ’oflag=dio’). Its action is
described in the ’dio=1’ option description above.
direct causes the O_DIRECT flag to be added to the open of IFILE and/or
OFILE. This flag requires some memory alignment on IO. Hence
user memory buffers are aligned to the page size. Has no effect
on sg, normal or raw files.
dpo set the DPO bit (disable page out) in SCSI READ and WRITE
commands. Not supported for 6 byte cdb variants of READ and
WRITE. Indicates that data is unlikely to be required to stay in
device (e.g. disk) cache. May speed media copy and/or cause a
media copy to have less impact on other device users.
dsync causes the O_SYNC flag to be added to the open of IFILE and/or
OFILE. The "d" is prepended to lower confusion with the
’sync=0|1’ option which has another action (i.e. a
synchronisation to media at the end of the transfer).
excl causes the O_EXCL flag to be added to the open of IFILE and/or
fua causes the FUA (force unit access) bit to be set in SCSI READ
and/or WRITE commands. This only has effect with sg devices. The
6 byte variants of the SCSI READ and WRITE commands do not
support the FUA bit. Only active for sg device file names.
null has no affect, just a placeholder.
smmap is only active for oflag. It sets shared mmap IO usage on OFILE
if it is a sg device node. The IFILE also needs to be a sg
device node (or there is no mmap-ed reserve buffer to share).
Here are some retired options that are still present:
fua=0 | 1 | 2 | 3
force unit access bit. When 3, fua is set on both IFILE and
OFILE; when 2, fua is set on IFILE; when 1, fua is set on OFILE;
when 0 (default), fua is cleared on both. See the ’fua’ flag.
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].
Various numeric arguments (e.g. SKIP) may include multiplicative
suffixes or be given in hexadecimal. See the "NUMERIC ARGUMENTS"
section in the sg3_utils(8) man page.
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.
For sg devices this utility issues SCSI READ and WRITE (SBC) commands
which are appropriate for disks and reading from CD/DVD/BD drives.
Those commands are not formatted correctly for tape devices so sgm_dd
should not be used on tape devices.
This utility stops the copy if any error is encountered. For more
advanced "copy on error" logic see the sg_dd utility (and its ’coe’
See the examples given in the man page for sg_dd(8).
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.
The exit status of sgm_dd is 0 when it is successful. Otherwise see the
sg3_utils(8) man page. Since this utility works at a higher level than
individual commands, and there are ’coe’ and ’retries’ flags,
individual SCSI command failures do not necessary cause the process to
Written by Doug Gilbert and Peter Allworth.
Report bugs to <dgilbert at interlog dot com>.
Copyright © 2000-2009 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)