sg_dd.8

.TH SG_DD "8" "April 2005" "sg3_utils-1.14" SG3_UTILS
.SH NAME
sg_dd \- copies data to and from files and devices. Specialised for
devices that understand the SCSI command set.
.SH SYNOPSIS
.B sg_dd
[\fIappend=0|1\fR] [\fIblk_sgio=0|1\fR] [\fIbpt=<n>\fR] [\fIbs=<n>\fR]
[\fIcdbsz=6|10|12|16\fR] [\fIcoe=0|1\fR] [\fIcount=<n>\fR] [\fIdio=0|1\fR]
[\fIfua=0|1|2|3\fR] [\fIibs=<n>\fR] [\fIif=<ifile>\fR] [\fIobs=<n>\fR]
[\fIodir=0|1\fR] [\fIof=<ofile>\fR] [\fIseek=<n>\fR] [\fIskip=<n>\fR]
[\fIsync=0|1\fR] [\fItime=0|1\fR] [\fIverbose=<n>\fR] [\fI--version\fR]
.SH DESCRIPTION
.\" Add any additional description here
.PP
Copy data to and from any files. Specialised 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
.B dd(1) 
but does not perform any conversions.
.TP
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 to pipes nor raw files nor sg devices nor block devices. Error
message produced if append=1 and seek=<n> where <n> > 0. Default is 0
which starts output at the beginning of a normal file (or at some other
offset if the "seek=" argument is given).
.TP
blk_sgio=0 | 1
when set to 0, block devices (e.g. /dev/sda) are treated like normal
files (i.e. 
.B read(2)
and 
.B 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.
.TP
bpt=BLOCKS
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 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 the default bpt value may be too
high, especially when blk_sgio is set. The solution is to reduce the bpt
value (64 or 32 may be more appropriate).
.TP
bs=BYTES
this
.B 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
.B 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.
.TP
cdbsz=6 | 10 | 12 | 16
size of SCSI READ and/or WRITE commands issued on sg device 
names (or block devices when "blk_sgio=1" is given).
Default is 10 byte SCSI command blocks (unless calculations indicate
that a 4 byte block number may be exceeded, in which case it defaults
to 16 byte SCSI commands).
.TP
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 the
.B dd(1) 
utility. Default is 0 which implies stop on error.
.TP
count=BLOCKS
copy this number of blocks from 'if' to 'of'. Default is the 
minimum (of 'if' and 'of') number of blocks that sg devices return from
READ CAPACITY SCSI commands or that block devices (or their partitions)
report. Normal files are not probed for their size. If 'skip'
or '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.
.TP
dio=0 | 1
default is 0 which selects indirect IO. 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)
.TP
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
.TP
ibs=BYTES
if given must be the same as bs
.TP
if=FILE
read from FILE instead of stdin. A file name of - is taken to be stdin.
Starts reading at the beginning of FILE unless "skip" is given.
.TP
obs=BYTES
if given must be the same as bs
.TP
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.
.TP
of=FILE
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 performed. If FILE is .
(period) then it is treated the same way as /dev/null (this is a
shorthand notation). If FILE exists then it is _not_ truncated; it is
overwritten from the start of FILE unless "append=1" or "seek" is given.
.TP
seek=BLOCKS
start writing BLOCKS bs-sized blocks from the start of the output file.
Default is block 0 (i.e. start of file).
.TP
skip=BLOCKS
start reading BLOCKS bs-sized blocks from the start of input file.
Default is block 0 (i.e. start of file).
.TP
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 or a block device 
and "blk_sgio=1" is given.
.TP
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
.TP
verbose=<n>
as <n> 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 ouput for all SCSI commands (and
Unix read() and write() calls).
.TP
--version
outputs version number information and exits
.PP
The exit status of the sg_dd utilty is 0 when successful, 1 for
total failure (i.e. failed before copying data) and 2 when some
data has been copied (prior to some problem).
.PP
BYTES and BLOCKS may be followed by 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 pattern continues for "G", "T" and "P". The latter two
suffixes can only be used for count, skip and seek values). 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.
.PP
Alternatively numerical values can be given in hexadecimal preceded by
either "0x" or "0X". When hex numbers are given, multipliers cannot be
used.
.PP
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 bit number.
.PP
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".
.PP
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.
.PP
A raw device must be bound to a block device prior to using sg_dd.
See
.B 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.
.PP
Raw disk partition information can often be found with
.B fdisk(8)
[the "-ul" argument is useful in this respect].
.SH EXAMPLES
.PP
Looks quite similar in usage to dd:
.PP
   sg_dd if=/dev/sg0 of=t bs=512 count=1MB
.PP
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:
.PP
   dd if=/dev/sda of=t bs=512 count=1000000
.PP
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:
.PP
   raw /dev/raw/raw1 /dev/hda
.br
   sg_dd if=/dev/raw/raw1 of=t bs=512 count=1MB
.PP
To copy a SCSI disk partition to an IDE disk partition:
.PP
   raw /dev/raw/raw2 /dev/hda3
.br
   sg_dd if=/dev/sg0 skip=10123456 of=/dev/raw/raw2 bs=512
.PP
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 IDE disk /dev/hda . The exact number of blocks
read from /dev/sg0 are written to /dev/hda (i.e. no padding).
.PP
To time a streaming read of the first 1 GB (2 ** 30 bytes) on a disk
this utility could be used:
.PP
   sg_dd if=/dev/sg0 of=/dev/null bs=512 count=2m time=1
.PP
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.
.SH NOTES
For sg devices (and block devices when blk_sgio=1 is given) this utility
issues READ and WRITE (SBC) SCSI 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 READ_16 and WRITE_16 SCSI commands.
.PP
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:
.PP
   sg_dd if=/dev/sdb2 blk_sgio=1 of=t bs=512
.SH 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.
.SH AUTHORS
Written by Doug Gilbert and Peter Allworth.
.SH "REPORTING BUGS"
Report bugs to <dgilbert at interlog dot com>.
.SH COPYRIGHT
Copyright \(co 2000-2005 Douglas Gilbert
.br
This software is distributed under the GPL version 2. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
.SH "SEE ALSO"
There is a web page discussing sg_dd at http://www.torque.net/sg/sg_dd.html
.PP
A POSIX threads version of this utility called
.B sgp_dd
is in the sg3_utils package. Another version from that package is called
.B sgm_dd
and it uses memory mapped IO to speed transfers from sg devices.
The lmbench package contains
.B lmdd
which is also interesting. For moving data to and from tapes see
.B dt
which is found at http://www.bit-net.com/~rmiller/dt.html. See also
.B raw(8), dd(1)