Provided by: sg3-utils_1.46-1ubuntu0.22.04.1_amd64 
NAME
sg_dd - copy data to and from files and devices, especially SCSI devices
SYNOPSIS
sg_dd [bs=BS] [conv=CONV] [count=COUNT] [ibs=BS] [if=IFILE] [iflag=FLAGS] [obs=BS]
[of=OFILE] [oflag=FLAGS] [seek=SEEK] [skip=SKIP] [--help] [--verbose] [--version]
[blk_sgio={0|1}] [bpt=BPT] [cdbsz={6|10|12|16}] [cdl=CDL] [coe={0|1|2|3}] [coe_limit=CL]
[dio={0|1}] [odir={0|1}] [of2=OFILE2] [retries=RETR] [sync={0|1}] [time={0|1}[,TO]]
[verbose=VERB] [--dry-run] [--progress] [--verify]
DESCRIPTION
Copy data to and from any files. Specialized for "files" that are Linux SCSI generic (sg)
devices, raw devices or other devices that support the SG_IO ioctl (which are only found
in the lk 2.6 series). Similar syntax and semantics to dd(1) command.
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.
When the --verify option is given, then the read side is the same but the on the write
side, the WRITE SCSI command is replaced by the VERIFY SCSI command. If any VERIFY
commands yields a sense key of MISCOMPARE then the verify operation will stop. The
--verify option can only be used when OFILE is either a sg device or a block device with
oflag=sgio also given. When the --verify option is used, this utility works in a similar
fashion to the Unix cmp(1) command.
This utility is only supported on Linux whereas most other utilities in the sg3_utils
package have been ported to other operating systems. A utility called "ddpt" has similar
syntax and functionality to sg_dd. ddpt drops some Linux specific features while adding
some other generic features. This allows ddpt to be ported to other operating systems.
OPTIONS
blk_sgio={0|1}
when set to 0, block devices (e.g. /dev/sda) are treated like normal files (i.e.
read(2) and write(2) are used for IO). When set to 1, block devices are assumed to
accept the SG_IO ioctl and SCSI commands are issued for IO. This is only supported
for 2.6 series kernels. Note that ATAPI devices (e.g. cd/dvd players) use the SCSI
command set but ATA disks do not (unless there is a protocol conversion as often
occurs in the USB mass storage class). If the input or output device is a block
device partition (e.g. /dev/sda3) then setting this option causes the partition
information to be ignored (since access is directly to the underlying device).
Default is 0. See the 'sgio' flag.
bpt=BPT
each IO transaction will be made using BPT blocks (or less if near the end of the
copy). Default is 128 for logical 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 logical block size is typically 2048 bytes and bpt
defaults to 32 which again implies 64 KiB transfers. The block layer when the
blk_sgio=1 option is used has relatively low upper limits for transfer sizes
(compared to sg device nodes, see /sys/block/<dev_name>/queue/max_sectors_kb ).
bs=BS where BS must be the logical block size of the physical device (if either the input
or output files are accessed via SCSI commands). 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 (or block devices
when 'iflag=sgio' and/or 'oflag=sgio' is given). Default is 10 byte SCSI command
blocks (unless calculations indicate that a 4 byte block number may be exceeded or
BPT is greater than 16 bits (65535), in which case it defaults to 16 byte SCSI
commands).
cdl=CDL
allows setting of command duration limits. CDL is either a single value or two
values separated by a comma. If one value is given, it applies to both IFILE and
OFILE (if they are pass-through devices). If two values are given, the first
applies to IFILE while the second applies to OFILE. The value may be from 0 to 7
where 0 is the default and means there are no command duration limits. Command
duration limits are only supported by 16 byte READ and WRITE commands (plus
READ(32), WRITE(32) and the WRITE SCATTERED command, bit thay are used by this
utility). If the cdbsz operand is not given and would have a value less than 16,
then if CDL is greater than 0, the cdbsz is increased to 16.
Command duration limits can be accesses and changed in the Command duration limit A
and B mode pages, plus the Command duration limit T2A and T2B mode pages. The
sdparm utility may be used to access and change these mode pages.
coe={0|1|2|3}
set to 1 or more for continue on error ('coe'). Only applies to errors on sg
devices or block devices with the 'sgio' flag set. Thus errors on other files will
stop sg_dd. Default is 0 which implies stop on any error. See the 'coe' flag for
more information.
coe_limit=CL
where CL is the maximum number of consecutive bad blocks stepped over (due to
"coe>0") on reads before the copy terminates. This only applies when IFILE is
accessed via the SG_IO ioctl. The default is 0 which is interpreted as no limit.
This option is meant to stop the copy soon after unrecorded media is detected while
still offering "continue on error" capability.
conv=sparse
see the CONVERSIONS section below.
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
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 (or count=-1) and cannot be derived then an error
message is issued and no copy takes place.
dio={0|1}
default is 0 which selects indirect (buffered) IO on sg devices. Value of 1
attempts direct IO which, if not available, falls back to indirect IO and notes
this at completion. If direct IO is selected and /proc/scsi/sg/allow_dio has the
value of 0 then a warning is issued (and indirect IO is performed). For finer
grain control use 'iflag=dio' or '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
reading 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.
odir={0|1}
when set to one opens block devices (e.g. /dev/sda) with the O_DIRECT flag. User
memory buffers are aligned to the page size when set. The default is 0 (i.e. the
O_DIRECT flag is not used). Has no effect on sg, normal or raw files. If blk_sgio
is also set then both are honoured: block devices are opened with the O_DIRECT flag
and SCSI commands are issued via the SG_IO ioctl.
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
exists then it is _not_ truncated; it is overwritten from the start of OFILE unless
'oflag=append' or SEEK is given.
of2=OFILE2
write output to OFILE2. The default action is not to do this additional write (i.e.
when this option is not given). OFILE2 is assumed to be a normal file or a fifo
(i.e. a named pipe). OFILE2 is opened for writing, created if necessary, and closed
at the end of the transfer. If OFILE2 is a fifo (named pipe) then some other
command should be consuming that data (e.g. 'md5sum OFILE2'), otherwise this
utility will block.
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, '.'
(period), or stdout.
retries=RETR
sometimes retries at the host are useful, for example when there is a transport
error. When RETR is greater than zero then SCSI READs and WRITEs are retried on
error, RETR times. Default value is zero.
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 or a block device and 'blk_sgio=1' is
given.
time={0|1}[,TO]
when 1, times transfer and does throughput calculation, outputting the results (to
stderr) at completion. When 0 (default) doesn't perform timing.
If that value is followed by a comma, then TO is the command timeout in seconds for
SCSI READ, WRITE or VERIFY commands issued by this utility. The default is 60
seconds.
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
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.
This only occurs for scsi generic (sg) devices and block devices when the
'blk_sgio=1' option is set.
-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.
-p, --progress
this option causes a progress report to be output every two minutes until the copy
is complete. After the copy is complete a line with "completed" is printed to
distinguish the final report from the prior progress reports. When used twice the
progress report is every minute, when used three times the progress report is every
30 seconds.
-v, --verbose
when used once, this is equivalent to verbose=1. When used twice (e.g. "-vv") this
is equivalent to verbose=2, etc.
-x, --verify
do a verify operation (like Unix command cmp(1)) rather than a copy. Cannot be used
with "oflag=sparse". of=OFILE must be given and OFILE must be an sg device or a
block device with "oflag=sgio" also given. Uses the SCSI VERIFY command with the
BYTCHK field set to 1. The VERIFY command is used instead of WRITE when this option
is given. There is no VERIFY(6) command. Stops on the first miscompare unless
oflag=coe is given.
-V, --version
outputs version number information and exits.
CONVERSIONS
One or more conversions can be given to the "conv=" option. If more than one is given,
they should be comma separated. sg_dd does not perform the traditional dd conversions
(e.g. ASCII to EBCDIC). Recently added conversions overlap somewhat with the flags so some
conversions are now supported by sg_dd.
nocreat
this conversion has the same effect as "oflag=nocreat", namely: OFILE must exist,
it will not be created.
noerror
this conversion is very close to "iflag=coe" and is treated as such. See the "coe"
flag. Note that an error on OFILE will stop the copy.
notrunc
this conversion is accepted for compatibility with dd and ignored since the default
action of this utility is not to truncate OFILE.
null has no affect, just a placeholder.
sparse FreeBSD supports "conv=sparse" so the same syntax is supported in sg_dd. See
"sparse" in the FLAGS sections for more information.
sync is ignored by sg_dd. With dd it means supply zero fill (rather than skip) and is
typically used like this "conv=noerror,sync" to have the same functionality as
sg_dd's "iflag=coe".
FLAGS
Here is a list of flags and their meanings:
00 this flag is only active with iflag= and when given replaces if=IFILE. If both are
given an error is generated. The input will be a stream of zeros, similar to using
"if=/dev/zero" alone (but a little quicker).
append causes the O_APPEND flag to be added to the open of OFILE. For regular 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.
coe continue on error. Only active for sg devices and block devices that have the
'sgio' flag set. 'iflag=coe oflag=coe' and 'coe=1' are equivalent. Use this flag
twice (e.g. 'iflag=coe,coe') to have the same action as the 'coe=2'. A medium,
hardware or blank check error while reading will re-read blocks prior to the bad
block, then try to recover the bad block, supplying zeros if that fails, and
finally re-read the blocks after the bad block. A medium, hardware or blank check
error while writing is noted and ignored. A miscompare sense key during a VERIFY
command (i.e. --verify given) is noted and ignored when 'oflag=coe'. The recovery
of the bad block when reading uses the SCSI READ LONG command if 'coe' given twice
or more (also with the command line option 'coe=2'). Further, the READ LONG will
set its CORRCT bit if 'coe' given thrice. SCSI disks may automatically try and
remap faulty sectors (see the AWRE and ARRE in the read write error recovery mode
page (the sdparm utility can access and possibly change these attributes)). Errors
occurring on other files types will stop sg_dd. Error messages are sent to stderr.
This flag is similar to 'conv=noerror,sync' in the dd(1) utility. See note about
READ LONG below.
dio request the sg device node associated with this flag does direct IO. If direct IO
is not available, falls back to indirect IO and notes this at completion. If
direct IO is selected and /proc/scsi/sg/allow_dio has the value of 0 then a warning
is issued (and indirect IO is performed).
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. If 'iflag=sgio' and/or
'oflag=sgio' is also set then both are honoured: block devices are opened with the
O_DIRECT flag and SCSI commands are issued via the SG_IO ioctl.
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.
ff this flag is only active with iflag= and when given replaces if=IFILE. If both are
given an error is generated. The input will be a stream of 0xff bytes (or all bits
set).
flock after opening the associated file (i.e. IFILE and/or OFILE) an attempt is made to
get an advisory exclusive lock with the flock() system call. The flock arguments
are "FLOCK_EX | FLOCK_NB" which will cause the lock to be taken if available else a
"temporarily unavailable" error is generated. An exit status of 90 is produced in
the latter case and no copy is done.
fua causes the FUA (force unit access) bit to be set in SCSI READ and/or WRITE
commands. This only has an effect with sg devices or block devices that have the
'sgio' flag set. The 6 byte variants of the SCSI READ and WRITE commands do not
support the FUA bit.
nocache
use posix_fadvise() to advise corresponding file there is no need to fill the file
buffer with recently read or written blocks.
nocreat
this flag is only active in oflag=FLAGS. If present then OFILE will be opened if it
exists. If OFILE doesn't exist then an error is generated. Without this flag a
regular (empty) file named OFILE will be created (and then filled). For production
quality scripts where OFILE is a device node (e.g. '/dev/sdc') this flag is
recommended. It guards against the remote possibility of 'dev/sdc' disappearing
temporarily (e.g. a USB memory key removed) resulting in a large regular file
called '/dev/sdc' being created.
null has no affect, just a placeholder.
random this flag is only active with iflag= and when given replaces if=IFILE. If both are
given an error is generated. The input will be a stream of pseudo random bytes. The
Linux getrandom(2) system call is used to create a seed and thereadter mrand48_r(3)
is used to generate a pseudo random sequence, 4 bytes at a time. The quality of the
randomness can be viewed with the ent(1) utility. This is not a high quality random
number generator, it is built for speed, not quality. One application is checking
the correctness of the copy and verify operations of this utility.
sgio causes block devices to be accessed via the SG_IO ioctl rather than standard UNIX
read() and write() commands. When the SG_IO ioctl is used the SCSI READ and WRITE
commands are used directly to move data. sg devices always use the SG_IO ioctl.
This flag offers finer grain control compared to the otherwise identical
'blk_sgio=1' option.
sparse after each BS * BPT byte segment is read from the input, it is checked for being
all zeros. If so, nothing is written to the output file unless this is the last
segment of the transfer. This flag is only active with the oflag option. It cannot
be used when the output is not seekable (e.g. stdout). It is ignored if the output
file is /dev/null . Note that this utility does not remove the OFILE prior to
starting to write to it. Hence it may be advantageous to manually remove the OFILE
if it is large prior to using oflag=sparse. The last segment is always written so
regular files will show the same length and so programs like md5sum and sha1sum
will generate the same value regardless of whether oflag=sparse is given or not.
This option may be used when the OFILE is a raw device but is probably only useful
if the device is known to contain zeros (e.g. a SCSI disk after a FORMAT command).
RETIRED OPTIONS
Here are some retired options that are still present:
append=0 | 1
when set, equivalent to 'oflag=append'. When clear the action is to overwrite the
existing file (if it exists); this is the default. See the 'append' flag.
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
Block devices (e.g. /dev/sda and /dev/hda) can be given for IFILE. If neither
'-iflag=direct', 'iflag=sgio' nor 'blk_sgio=1' is given then normal block IO involving
buffering and caching is performed. If only '-iflag=direct' is given then the buffering
and caching is bypassed (this is applicable to both SCSI devices and ATA disks). If
'iflag=sgio' or 'blk_sgio=1' is given then the SG_IO ioctl is used on the given file
causing SCSI commands to be sent to the device and that also bypasses most of the actions
performed by the block layer (this is only applicable to SCSI devices, not ATA disks). The
same applies for block devices given for OFILE.
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 arguments 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). This is called "indirect IO" and there is a 'dio' option to
select "direct IO" which will DMA directly into user memory. Due to some issues "direct
IO" is disabled in the sg driver and needs a configuration change to activate it. This is
typically done with 'echo 1 > /proc/scsi/sg/allow_dio'.
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.
Even if READ LONG succeeds on a "bad" block when 'coe=2' (or 'coe=3') is given, the
recovered data may not be useful. There are no guarantees that the user data will appear
"as is" in the first 512 bytes.
A raw device must be bound to a block device prior to using sg_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', or sg_map before use.
Disk partition information can often be found with fdisk(8) [the "-ul" argument is useful
in this respect].
For sg devices (and block devices when blk_sgio=1 is given) this utility issues SCSI READ
and WRITE (SBC) commands which are appropriate for disks and reading from CD/DVD/HD-DVD/BD
drives. Those commands are not formatted correctly for tape devices so sg_dd should not be
used on tape devices. If the largest block address of the requested transfer exceeds a 32
bit block number (i.e 0xffff) then a warning is issued and the sg device is accessed via
SCSI READ(16) and WRITE(16) commands.
The attributes of a block device (partition) are ignored when 'blk_sgio=1' is used. Hence
the whole device is read (rather than just the second partition) by this invocation:
sg_dd if=/dev/sdb2 blk_sgio=1 of=t bs=512
EXAMPLES
Looks quite similar in usage to dd:
sg_dd if=/dev/sg0 of=t bs=512 count=1MB
This will copy 1 million 512 byte blocks from the device associated with /dev/sg0 (which
should have 512 byte blocks) to a file called t. Assuming /dev/sda and /dev/sg0 are the
same device then the above is equivalent to:
dd if=/dev/sda iflag=direct of=t bs=512 count=1000000
although dd's speed may improve if bs was larger and count was suitably reduced. The use
of the 'iflag=direct' option bypasses the buffering and caching that is usually done on a
block device.
Using a raw device to do something similar on a ATA disk:
raw /dev/raw/raw1 /dev/hda
sg_dd if=/dev/raw/raw1 of=t bs=512 count=1MB
To copy a SCSI disk partition to an ATA disk partition:
raw /dev/raw/raw2 /dev/hda3
sg_dd if=/dev/sg0 skip=10123456 of=/dev/raw/raw2 bs=512
This assumes a valid partition is found on the SCSI disk at the given skip block address
(past the 5 GB point of that disk) and that the partition goes to the end of the SCSI
disk. An explicit count is probably a safer option. The partition is copied to /dev/hda3
which is an offset into the ATA disk /dev/hda . The exact number of blocks read from
/dev/sg0 are written to /dev/hda (i.e. no padding).
To time a streaming read of the first 1 GB (2 ** 30 bytes) on a disk this utility could be
used:
sg_dd if=/dev/sg0 of=/dev/null bs=512 count=2m time=1
On completion this will output a line like: "time to transfer data was 18.779506 secs,
57.18 MB/sec". The "MB/sec" in this case is 1,000,000 bytes per second.
The 'of2=' option can be used to copy data and take a md5sum of it without needing to
re-read the data:
mkfifo fif
md5sum fif &
sg_dd if=/dev/sg3 iflag=coe of=sg3.img oflag=sparse of2=fif bs=512
This will image /dev/sg3 (e.g. an unmounted disk) and place the contents in the (sparse)
file sg3.img . Without re-reading the data it will also perform a md5sum calculation on
the image.
SIGNALS
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.
EXIT STATUS
The exit status of sg_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.
An additional exit status of 90 is generated if the flock flag is given and some other
process holds the advisory exclusive lock.
AUTHORS
Written by Douglas Gilbert and Peter Allworth.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright © 2000-2021 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
cmp(1)
There is a web page discussing sg_dd at http://sg.danny.cz/sg/sg_dd.html
A POSIX threads version of this utility called sgp_dd is in the sg3_utils package. Another
version from that package is called sgm_dd and it uses memory mapped IO to speed transfers
from sg devices.
The lmbench package contains lmdd which is also interesting. For moving data to and from
tapes see dt which is found at http://www.scsifaq.org/RMiller_Tools/index.html
To change mode parameters that effect a SCSI device's caching and error recovery see
sdparm(sdparm)
To verify the data on the media or to verify it against some other copy of the data see
sg_verify(sg3_utils)
See also raw(8), dd(1), ddrescue(GNU), ddpt