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SGM_DD(8) SG3_UTILS SGM_DD(8)
NAME
sgm_dd - copy data to and from files and devices, especially SCSI devices
SYNOPSIS
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] [verbose=VERB] [--dry-run]
[--verbose]
DESCRIPTION
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) de-
vices.
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 direct IO is not available,
then this utility falls back to indirect IO and reports 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.
OPTIONS
bpt=BPT
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 de-
fault (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 transfers.
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).
count=COUNT
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 seek=SEEK are given and the count is derived (i.e. not ex-
plicitly 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.
if=IFILE
read from IFILE instead of stdin. If IFILE is '-' then stdin is read. Starts read-
ing at the beginning of IFILE unless SKIP is given.
iflag=FLAGS
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.
of=OFILE
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 ex-
ists then it is _not_ truncated; it is overwritten from the start of OFILE unless
'oflag=append' or SEEK is given.
oflag=FLAGS
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, '.' (pe-
riod), or stdout.
seek=SEEK
start writing SEEK bs-sized blocks from the start of OFILE. Default is block 0
(i.e. start of file).
skip=SKIP
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.
verbose=VERB
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 ex-
tra 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 pro-
cessing 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.
-d, --dry-run
does all the command line parsing and preparation but bypasses the actual copy or
read. That preparation may include opening IFILE or OFILE to determine their
lengths. This option may be useful for testing the syntax of complex command line
invocations in advance of executing them.
-h, --help
outputs usage message and exits.
-v, --verbose
when used once, this is equivalent to verbose=1. When used twice (e.g. "-vv") this
is equivalent to verbose=2, etc.
-V, --version
outputs version number information and exits.
FLAGS
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 to-
gether 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 OFILE.
fua causes the FUA (force unit access) bit to be set in SCSI READ and/or WRITE com-
mands. 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.
RETIRED OPTIONS
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.
NOTES
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 de-
vices should be checked with the lsscsi utility 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 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 trans-
fer 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.
For sg devices this utility issues SCSI READ and WRITE (SBC) commands which are appropri-
ate for disks and reading from CD/DVD/BD drives. Those commands are not formatted cor-
rectly 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' flag).
EXAMPLES
See the examples given in the man page for sg_dd(8).
SIGNALS
The signal handling has been borrowed from dd: SIGINT, SIGQUIT and SIGPIPE output the num-
ber 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 con-
tinues. All output caused by signals is sent to stderr.
EXIT STATUS
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 exit.
AUTHORS
Written by Douglas Gilbert and Peter Allworth.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright (C) 2000-2019 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.
SEE ALSO
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. dd(1), ddpt(ddpt), raw(8)
sg3_utils-1.45 February 2019 SGM_DD(8)
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