sg_dd - copies data to and from files and devices. Specialised for
devices that understand the SCSI command set.
sg_dd [bs=BS] [conv=CONV] [count=COUNT] [ibs=BS] [if=IFILE]
[iflag=FLAGS] [obs=BS] [of=OFILE] [oflag=FLAGS] [seek=SEEK] [skip=SKIP]
[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] [-V]
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) but only one type of "conversion" is
supported. That optional conversion is sparse writing of the output
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.
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.
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
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 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
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.
only one conversion is permitted: sparse writing of the OFILE.
This option has the same action as ’oflag=sparse’. This
alternate syntax was added for FreeBSD which uses this syntax in
its dd command. See the FLAGS section below.
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.
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.
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.
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.
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.
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.
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
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 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.
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.
--help outputs usage message and exits.
outputs version number information and exits.
-V 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
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
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
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
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
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
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).
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’
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
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
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
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:
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.
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 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
An additional exit status of 90 is generated if the flock flag is given
and some other process holds the advisory exclusive lock.
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
There is a web page discussing sg_dd at
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
To change mode parameters that effect a SCSI device’s caching and error
recovery see sdparm(sdparm)
See also raw(8), dd(1), ddrescue(GNU), ddpt