SG_DD(8) SG3_UTILS SG_DD(8)
NAME
sg_dd - copy data to and from files and devices, especially SCSI de-
vices
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 play-
ers) 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 parti-
tion (e.g. /dev/sda3) then setting this option causes the parti-
tion 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 restric-
tions). When cd/dvd drives are accessed, the logical block size
is typically 2048 bytes and bpt defaults to 32 which again im-
plies 64 KiB transfers. The block layer when the blk_sgio=1 op-
tion 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 com-
mands). 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 individ-
ual 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 calcula-
tions 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 sin-
gle 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 ap-
plies 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 sup-
ported 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 Com-
mand duration limit A and B mode pages, plus the Command dura-
tion 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 informa-
tion.
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 de-
fault 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 de-
rived 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 out-
lined below. These flags are associated with IFILE and are ig-
nored 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 per-
formed. 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 ad-
ditional 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 out-
lined below. These flags are associated with OFILE and are ig-
nored 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. De-
fault is block 0 (i.e. start of file).
skip=SKIP
start reading SKIP bs-sized blocks from the start of IFILE. De-
fault 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, out-
putting 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 is-
sued 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 oc-
curs 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 invoca-
tions 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 min-
utes 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 re-
port 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 per-
form 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 ig-
nored since the default action of this utility is not to trun-
cate 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 informa-
tion.
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 over-
write 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 pos-
sibly 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 is-
sued (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 com-
mands. 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 synchronisa-
tion 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 vari-
ants 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 er-
ror 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 re-
moved) 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 ig-
nored 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 pro-
grams like md5sum and sha1sum will generate the same value re-
gardless 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 de-
fault. 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 suf-
fixes 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 "di-
rect 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 util-
ity 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 par-
tition 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 con-
tents 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 SIG-
PIPE 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, individ-
ual 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 war-
ranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR-
POSE.
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.scsi-
faq.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-1.46 March 2021 SG_DD(8)
Generated by dwww version 1.16 on Tue Dec 16 14:03:17 CET 2025.