Provided by: sg3-utils_1.44-1ubuntu2_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}] [coe={0|1|2|3}] [coe_limit=CL] [dio={0|1}] [odir={0|1}]
[of2=OFILE2] [retries=RETR] [sync={0|1}] [time={0|1}] [verbose=VERB] [--dry-run] [-V]
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.
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).
coe={0|1|2|3}
set to 1 or more for continue on error. 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 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 (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}
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 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.
-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.
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.
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:
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 reread the blocks after the bad block. A medium, hardware or blank check error
while writing is noted and ignored. 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
o '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.
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.
null has no affect, just a placeholder.
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 © 2000-2018 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
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
sg3_utils-1.43 August 2018 SG_DD(8)