SG_DD

NAME

sg_dd - copies data to and from sg and raw devices

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 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 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 pipes nor raw files nor sg 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. 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.
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 64KB 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. 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 dd(1) command. Default is 0 which implies stop on error.
count=BLOCKS: 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 device (i.e. 'if' nor 'of') is an sg device and count is not given then the command will fail with an error message requesting a count value.
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
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).
seek=BLOCKS: skip BLOCKS bs-sized blocks at start of output
skip=BLOCKS: 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, 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.

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! 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 the following 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=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 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 on a disk this command 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.

NOTE

For sg devices this command issues READ_10 and WRITE_10 SCSI commands which are appropriate for disks and CDROM players. 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.

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@interlog.com>.

COPYRIGHT