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sg_dd [append=0|1] [blk_sgio=0|1] [bpt=<n>] [bs=<n>] [cdbsz=6|10|12|16]
[coe=0|1] [count=<n>] [dio=0|1] [fua=0|1|2|3] [ibs=<n>] [if=<ifile>]
[obs=<n>] [odir=0|1] [of=<ofile>] [seek=<n>] [skip=<n>] [sync=0|1]
Copy data to and from Linux SCSI generic (sg) devices, raw devices and
those block devices that support the SG_IO ioctl (which are only found
in the lk 2.6 series). Data may be copied to and from normal files as
well. Similar syntax and semantics to dd(1) but does not perform any
append=0 | 1
when set to 1 the output will be appended to the normal file
given to the "of=<name>" argument. Appending only takes place to
normal files: not pipes nor raw files nor sg devices. Error mes-
sage produced if append=1 and seek=<n> where <n> > 0. Default is
0 which starts output at offset of a normal file (subject to
the "seek=" argument).
blk_sgio=0 | 1
when set to 0 block devices (e.g. /dev/sda) are treated like
normal files. When set to 1 block devices are assumed to accept
the SG_IO ioctl; this is only appropriate for kernels from lk
2.6.0 . Default is 0.
each IO transaction will be made using this number of blocks (or
less if near the end of count). Default is 128. So for bs=512
the reads and writes will each convey 64KB of data by default
(less if near the end of the transfer or memory restrictions).
this must be the block size of the physical device. Note that
this differs from dd(1) which permits "bs" to be an integral
multiple. Default is 512 which is usually correct for disks but
incorrect for cdroms (which normally have 2048 byte blocks).
cdbsz=6 | 10 | 12 | 16
size of SCSI READ and/or WRITE commands issued on sg device
names. Default is 10 byte SCSI command blocks (unless calcula-
tions indicate that a 4 byte block number may be exceeded, in
which case it defaults to 16 byte SCSI commands)
coe=0 | 1
set to 1 for continue on error: if reading assume zeros read, if
writing then ignore and continue. Only applies to errors on sg
devices (e.g. errors on normal files will stop sg_dd). Error
messages are still sent to stderr. Similar to "conv=noerror" in
dd(1) command. Default is 0 which implies stop on error.
copy this number of blocks. Default is the minimum number that
sg devices return from READ CAPACITY. Other device types (e.g.
normal files) are _not_ probed for their size. Thus if neither
fua=0 | 1 | 2 | 3
force unit access bit. When 3, fua is set on both "if" and "of",
when 2, fua is set on "if", when 1, fua is set on "of", when 0
(default), fua is cleared on both. 6 byte SCSI READ and WRITE
commands (cdbsz=6) do not support the fua bit. Only active for
sg device file names
if given must be the same as bs
read from FILE instead of stdin. A file name of - is taken to be
if given must be the same as bs
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).
The blk_sgio flag takes precedence if it is also set. Has no
effect on sg, normal or raw files.
write to FILE instead of stdout. A file name of - is taken to be
stdout. If FILE is /dev/null then no actual writes are per-
formed. If FILE is . (period) then it is treated the same way
as /dev/null (this is a shorthand notation).
skip BLOCKS bs-sized blocks at start of output
skip BLOCKS bs-sized blocks at start of input
sync=0 | 1
when 1, does SYNCHRONIZE CACHE command on "of" at the end of the
transfer. Only active when "of" is a sg device file name
time=0 | 1
when 1, times transfer and does throughput calculation, out-
putting the results (to stderr) at completion. When 0 (default)
doesn’t perform timing
outputs version number information and exits
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.
The count is only deduced for sg devices (minimum > 0 if both input and
output are sg devices) otherwise it defaults to 0. This is for safety!
either "0x" or "0X". When hex numbers are given multipliers cannot be
The count, skip and seek parameters can take 64 bit values (i.e. very
big numbers). Other values are limited to what can fit in a signed 32
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 configura-
tion change to activate it. This is typically done with "echo 1 >
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.
Looks quite similar in usage to dd:
sg_dd if=/dev/sg0 of=t bs=512 count=1M
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 of=t bs=512 count=0xf4240
although dd’s speed may improve if bs was larger and count was suitably
reduced. Using a raw device to do something similar on a IDE disk:
raw /dev/raw/raw1 /dev/hda
sg_dd if=/dev/raw/raw1 of=t bs=512 count=1M
To copy a SCSI disk partition to an IDE 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 IDE 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 on a disk this command could
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
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.
Written by Doug Gilbert and Peter Allworth.
Report bugs to <email@example.com>.
Copyright © 2000-2003 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-
A POSIX threads version of this command 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.bit-
net.com/~rmiller/dt.html. See also raw(8), dd(1)
sg3_utils-1.05 November 2003 SG_DD(8)
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