SGM_DD

NAME

sgm_dd - copies data to and from sg and raw devices

SYNOPSIS

sgm_dd [bpt=<n>] [bs=<n>] [cdbsz=6|10|12|16] [count=<n>] [dio=0|1] [fua=0|1|2|3] [ibs=<n>] [if=<ifile>] [obs=<n>] [of=<ofile>] [seek=<n>] [skip=<n>] [sync=0|1] [time=0|1] [--version]

DESCRIPTION

Copy data to and from Linux SCSI generic (sg) and raw devices using memory mapped IO. Similar syntax and semantics to dd(1) but does not perform any conversions.

bpt=BLOCKS: each IO transaction will be made using this number of blocks (or less if near the end of count). Default is 128.
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).
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: permits direct IO to be selected on the write-side (i.e. "of"). Only allowed when the read-side (i.e. "if") is a sg device. When 1 there may be a "zero copy" copy (i.e. mmap-ed IO on the read into the user space and direct IO from there on the write, potentially two DMAs and no data copying from the CPU). Default is 0
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 (the 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
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 sgm_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" 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 device 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). With memory mapped IO a kernel buffer reserved by sg is memory mapped (see the mmap(2) system call) into the user space. When this is done the second (redundant) copy from kernel buffers to user space is not needed. Hence the transfer is faster and requires less "grunt" from the CPU.

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

See the examples given in the man page for sg_dd(8).

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 sgm_dd should not be used on tape devices.

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