SGP_DD

NAME

sgp_dd - copies data to and from sg and raw devices

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 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) 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.
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
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
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.

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). 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@interlog.com>.

COPYRIGHT