path: root/sgp_dd.8

.TH SGP_DD "8" "February 2002" "sg3_utils-0.98" SG3_UTILS
.SH NAME
sgp_dd \- copies data to and from sg and raw devices
.SH SYNOPSIS
.B sgp_dd
[\fIOPTION\fR]...
.SH DESCRIPTION
.\" Add any additional description here
.PP
Copy data to and from Linux SCSI generic (sg) and raw devices. 
Similar syntax and semantics to 
.B dd(1) 
but does not perform any conversions. Uses POSIX threads to increase
the amount of parallelism. This improves speed in some cases.
.TP
bpt=BLOCKS
each IO transaction will be made using this number of blocks (or less if 
near the end of count). Default is 128.
.TP
bs=BYTES
this 
.B must 
be the block size of the physical device. Note that this differs from
.B 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
.TP
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)
.TP
count=BLOCKS
copy this number of blocks. Default is minimum number that sg devices
return from READ CAPACITY (if that works) or 0
.TP
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)
.TP
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)
.TP
gen=0 | 1
when 0 (default) either input or output must be a sg or raw device.
When it is 1 then this condition is relaxed.
.TP
ibs=BYTES
if given must be the same as bs
.TP
if=FILE
read from FILE instead of stdin. A file name of - is taken to be stdin
.TP
obs=BYTES
if given must be the same as bs
.TP
of=FILE
write to FILE instead of stdout. A file name of - is taken to be stdout
.TP
seek=BLOCKS
skip BLOCKS bs-sized blocks at start of output
.TP
skip=BLOCKS
skip BLOCKS bs-sized blocks at start of input
.TP
thr=NUM
this is the number or worker threads (default 4) that attempt to
copy in parallel. Minimum is 0 and maximum is 16
.TP
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
.TP
--version
outputs version number information and exits
.PP
Either the input file or the output file should be a sg or raw device
(unless gen=1).
A raw device must be bound to a block device prior to using sgp_dd.
See
.B 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.
.PP
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
.B fdisk(8)
[the "-ul" argument is useful in this respect].
.PP
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; and G *1,000,000,000
.PP
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.
.PP
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.
.PP
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.
.SH EXAMPLES
.PP
Looks quite similar in usage to dd:
.PP
   sgp_dd if=/dev/sg0 of=t bs=512 count=1M
.PP
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:
.PP
   dd if=/dev/sda of=t bs=512 count=1000000
.PP
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:
.PP
   raw /dev/raw/raw1 /dev/hda
.br
   sgp_dd if=/dev/raw/raw1 of=t bs=512 count=1M
.PP
To copy a SCSI disk partition to an IDE disk partition:
.PP
   raw /dev/raw/raw2 /dev/hda3
.br
   sgp_dd if=/dev/sg0 skip=10123456 of=/dev/raw/raw2 bs=512
.PP
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.
.PP
To do a fast copy from one SCSI disk to another one with similar
geometry (stepping over errors on the source disk):
.PP
   sgp_dd if=/dev/sg0 of=/dev/sg1 bs=512 coe=1
.SH AUTHORS
Written by Doug Gilbert and Peter Allworth.
.SH "REPORTING BUGS"
Report bugs to <dgilbert@interlog.com>.
.SH COPYRIGHT
Copyright \(co 2000, 2001 Douglas Gilbert
.br
This software is distributed under the GPL version 2. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
.SH "SEE ALSO"
A simpler, non-threaded version of this command called
.B sg_dd
is in the sg3_utils package. The lmbench package contains
.B lmdd
which is also interesting.
.B raw(8), dd(1)