SGP_DD(8)

NAME
sgp_dd - copies data to and from files and devices. Specialised for devices that understand the SCSI command set.

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

DESCRIPTION

Copy data to and from any files. Specialised for "files" that are Linux SCSI generic (sg) and raw devices. Similar syntax and semantics to dd(1) but does not perform any conversions. Uses POSIX threads to increase the amount of parallelism. This improves speed in some cases.

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). 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

coe=0 | 1
    continue on error is 0 (off) by default. When it is 1 read errors are stepped over (with a block (or blocks) of zeroes being output). When 1, write errors are ignored (and alignment is maintained) Similar to "conv=noerror" in 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.

deb=NUM
    outputs debug information. If NUM is 0 (default) then none and as NUM increases so does the amount of debug (max debug output when NUM is 9)

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 (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. Starts reading at the beginning of FILE unless "skip" is given.

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
    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

thr=NUM
    this is the number or worker threads (default 4) that attempt to copy in parallel. Minimum is 0 and maximum is 16

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 sgp_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.

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). 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.

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.

Why use sgp_dd? Because in some cases it is twice as fast as dd (mainly with sg devices, raw devices give some improvement). Another reason is that big copies fill the block device caches which has a negative impact on other machine activity.

EXAMPLES

Looks quite similar in usage to dd:

   sgp_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
   sgp_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
   sgp_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.

To do a fast copy from one SCSI disk to another one with similar geometry (stepping over errors on the source disk):

   sgp_dd if=/dev/sg0 of=/dev/sg1 bs=512 coe=1

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 simpler, non-threaded version of this utility called sg_dd is in the sg3_utils package. The lmbench package contains lmdd which is also interesting. raw(8), dd(1)