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 [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] [time=0|1] [--version]

DESCRIPTION

Copy data to and from any files. Specialised for "files" that are 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). Similar syntax and semantics to dd(1) but does not perform any conversions.

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 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 (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. If the input or output device is a block device partition then setting this option causes the partition information to be ignored (since access is directly to the underlying device). Default is 0.

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).

bs=BYTES
    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 (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).

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 dd(1) utility. Default is 0 which implies stop on error.

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.

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)

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

ibs=BYTES
    if given must be the same as bs

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.

obs=BYTES
    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.

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.

seek=BLOCKS
    start writing BLOCKS bs-sized blocks from the start of the output file. Default is block 0 (i.e. start of file).

skip=BLOCKS
    start reading BLOCKS bs-sized blocks from the start of input file. Default is block 0 (i.e. start of file).

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.

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

--version
    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.

Raw disk partition information can often be found with fdisk(8) [the "-ul" argument is useful in this respect].

BYTES and BLOCKS may be followed by one of these multiplicative suffixes: c C *1; b B *512; k *1,024; K *1,000; m *1,048,576; M *1,000,000; g *1,073,741,824; G *1,000,000,000; t *1,099,511,627,776 and T *1,000,000,000,000 (the latter two can only be used for count, skip and seek values).

Alternatively numerical values can be given in hexadecimal preceded by either "0x" or "0X". When hex numbers are given multipliers cannot be used.

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.

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.

EXAMPLES

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 equivalent to:

   dd if=/dev/sda of=t bs=512 count=1000000

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 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).

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.

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.

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

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.

AUTHORS
Written by Doug Gilbert and Peter Allworth.

REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.

COPYRIGHT
Copyright © 2000-2004 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
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.bit-net.com/~rmiller/dt.html. See also raw(8), dd(1)