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SG_DD(8) SG3_UTILS SG_DD(8)
NAME
sg_dd - copies data to and from files and devices. Specialised for devices that understand
the SCSI command set.
SYNOPSIS
sg_dd [bs=BS] [count=COUNT] [ibs=BS] [if=IFILE] [iflag=FLAGS] [obs=BS] [of=OFILE]
[oflag=FLAGS] [seek=SEEK] [skip=SKIP] [--help] [--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] [retries=RETR] [sync=0|1] [time=0|1] [verbose=VERB]
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) but does not perform any con-
versions.
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.
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 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 out-
put 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 com-
mands).
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 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 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.
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 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 read-
ing 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.
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 pro-
cessing 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.
--version
outputs version number information and exits.
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 read-
ing 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 to
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.
fua causes the FUA (force unit access) bit to be set in SCSI READ and/or WRITE com-
mands. 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.
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.
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 caus-
ing SCSI commands to be sent to the device and that also bypasses most of the actions per-
formed by the block layer (this is only applicable to SCSI devices, not ATA disks). The
same applies for block devices given for OFILE.
COUNT, SKIP, SEEK, BPT and BS may include one of these multiplicative suffixes: c C *1; w
W *2; b B *512; k K KiB *1,024; KB *1,000; m M MiB *1,048,576; MB *1,000,000 . This pat-
tern continues for "G", "T" and "P". The latter two suffixes can only be used for COUNT,
SKIP and SEEK. Also a suffix of the form "x<n>" multiplies the leading number by <n>.
These multiplicative suffixes are compatible with GNU's dd command (since 2002) which
claims compliance with SI and with IEC 60027-2.
Alternatively numerical values can be given in hexadecimal preceded by either "0x" or "0X"
(or with a trailing "h" or "H"). When hex numbers are given, multipliers cannot be used.
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 recov-
ered 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 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.
SIGNALS
The signal handling has been borrowed from dd: SIGINT, SIGQUIT and SIGPIPE output the num-
ber 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 con-
tinues. 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.
AUTHORS
Written by Doug Gilbert and Peter Allworth.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright (C) 2000-2007 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://www.torque.net/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)
See also raw(8), dd(1), ddrescue(GNU)
sg3_utils-1.24 April 2007 SG_DD(8)
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