NAME¶
sgm_dd - copies data to and from files and devices. Specialized for devices that
understand the SCSI command set and does memory mapped transfers from sg
devices.
SYNOPSIS¶
sgm_dd [
bs=BS] [
count=COUNT] [
ibs=BS]
[
if=IFILE] [
iflag=FLAGS] [
obs=BS] [
of=OFILE]
[
oflag=FLAGS] [
seek=SEEK] [
skip=SKIP] [
--help]
[
--version]
[
bpt=BPT] [
cdbsz=6|10|12|16] [
dio=0|1] [
sync=0|1] [
time=0|1] [
verbose=VERB]
DESCRIPTION¶
Copy data to and from any files. Specialized for "files" that are
Linux SCSI generic (sg) devices and raw devices. Uses memory mapped transfers
on sg devices. Similar syntax and semantics to
dd(1) but does not
perform any conversions.
Will only perform memory mapped transfers when
IFILE or
OFILE are
SCSI generic (sg) devices.
If both
IFILE and
OFILE are sg devices then memory mapped
transfers are performed on
IFILE. If no other flags are specified then
indirect IO is performed on
OFILE. If 'oflag=dio' is given then direct
IO is attempted on
OFILE. If 'oflag=smmap' is given then shared mmap-ed
IO (sharing the mmap-ed reserve buffer associated with
IFILE) is
attempted. In both latter cases if the faster IO option is not available, they
fall back to indirect IO and report this at the end of the copy.
The first group in the synopsis above are "standard" Unix
dd(1)
operands. The second group are extra options added by this utility. Both
groups are defined below.
OPTIONS¶
- bpt=BPT
- each IO transaction will be made using BPT blocks
(or less if near the end of the copy). Default is 128 for block sizes less
that 2048 bytes, otherwise the default is 32. So for bs=512 the reads and
writes will each convey 64 KiB of data by default (less if near the end of
the transfer or memory restrictions). When cd/dvd drives are accessed, the
block size is typically 2048 bytes and bpt defaults to 32 which again
implies 64 KiB transfers.
- bs=BS
- where BS 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). For this utility the maximum size of each individual IO operation
is BS * BPT bytes.
- 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).
- count=COUNT
- copy COUNT blocks from IFILE to OFILE.
Default is the minimum (of IFILE and OFILE) number of blocks
that sg devices report from SCSI READ CAPACITY commands or that block
devices (or their partitions) report. Normal files are not probed for
their size. If skip=SKIP or skip=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.
- dio=0 | 1
- permits direct IO to be selected on the write-side (i.e. on
OFILE). Only allowed when the read-side (i.e. IFILE) is a sg
device. When 1 there may be a "zero copy" copy (i.e. mmap-ed
transfer 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. The same action as 'dio=1' is also available with 'oflag=dio'.
- ibs=BS
- if given must be the same as BS given to 'bs='
option.
- if=IFILE
- read from IFILE instead of stdin. If IFILE is
'-' then stdin is read. Starts reading at the beginning of IFILE
unless SKIP is given.
- iflag=FLAGS
- where FLAGS is a comma separated list of one or more
flags outlined below. These flags are associated with IFILE and are
ignored when IFILE is stdin.
- obs=BS
- if given must be the same as BS given to 'bs='
option.
- of=OFILE
- write to OFILE instead of stdout. If OFILE is
'-' then writes to stdout. If OFILE is /dev/null then no actual
writes are performed. If OFILE is '.' (period) then it is treated
the same way as /dev/null (this is a shorthand notation). If OFILE
exists then it is _not_ truncated; it is overwritten from the start of
OFILE unless 'oflag=append' or SEEK is given.
- oflag=FLAGS
- where FLAGS is a comma separated list of one or more
flags outlined below. These flags are associated with OFILE and are
ignored when OFILE is /dev/null, '.' (period), or stdout.
- seek=SEEK
- start writing SEEK bs-sized blocks from the start of
OFILE. Default is block 0 (i.e. start of file).
- skip=SKIP
- start reading SKIP bs-sized blocks from the start of
IFILE. Default is block 0 (i.e. start of file).
- sync=0 | 1
- when 1, does SYNCHRONIZE CACHE command on OFILE at
the end of the transfer. Only active when OFILE 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.
- verbose=VERB
- as VERB increases so does the amount of debug output
sent to stderr. Default value is zero which yields the minimum amount of
debug output. A value of 1 reports extra information that is not
repetitive. A value 2 reports cdbs and responses for SCSI commands that
are not repetitive (i.e. other that READ and WRITE). Error processing is
not considered repetitive. Values of 3 and 4 yield output for all SCSI
commands (and Unix read() and write() calls) so there can be a lot of
output.
- --help
- outputs usage message and exits.
- --version
- outputs version number information and exits.
FLAGS¶
Here is a list of flags and their meanings:
- append
- causes the O_APPEND flag to be added to the open of
OFILE. For normal files this will lead to data appended to the end
of any existing data. Cannot be used together with the seek=SEEK
option as they conflict. The default action of this utility is to
overwrite any existing data from the beginning of the file or, if
SEEK is given, starting at block SEEK. Note that attempting
to 'append' to a device file (e.g. a disk) will usually be ignored or may
cause an error to be reported.
- dio
- is only active with oflag (i.e. 'oflag=dio'). Its action is
described in the 'dio=1' option description above.
- direct
- causes the O_DIRECT flag to be added to the open of
IFILE and/or OFILE. This flag requires some memory alignment
on IO. Hence user memory buffers are aligned to the page size. Has no
effect on sg, normal or raw files.
- dpo
- set the DPO bit (disable page out) in SCSI READ and WRITE
commands. Not supported for 6 byte cdb variants of READ and WRITE.
Indicates that data is unlikely to be required to stay in device (e.g.
disk) cache. May speed media copy and/or cause a media copy to have less
impact on other device users.
- dsync
- causes the O_SYNC flag to be added to the open of
IFILE and/or OFILE. The "d" is prepended to lower
confusion with the 'sync=0|1' option which has another action (i.e. a
synchronisation to media at the end of the transfer).
- excl
- causes the O_EXCL flag to be added to the open of
IFILE and/or OFILE.
- fua
- causes the FUA (force unit access) bit to be set in SCSI
READ and/or WRITE commands. This only has effect with sg devices. The 6
byte variants of the SCSI READ and WRITE commands do not support the FUA
bit. Only active for sg device file names.
- null
- has no affect, just a placeholder.
- smmap
- is only active for oflag. It sets shared mmap IO usage on
OFILE if it is a sg device node. The IFILE also needs to be
a sg device node (or there is no mmap-ed reserve buffer to share).
RETIRED OPTIONS¶
Here are some retired options that are still present:
- fua=0 | 1 | 2 | 3
- force unit access bit. When 3, fua is set on both
IFILE and OFILE; when 2, fua is set on IFILE; when 1,
fua is set on OFILE; when 0 (default), fua is cleared on both. See
the 'fua' flag.
NOTES¶
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.
Raw device partition information can often be found with
fdisk(8) [the
"-ul" argument is useful in this respect].
Various numeric arguments (e.g.
SKIP) may include multiplicative suffixes
or be given in hexadecimal. See the "NUMERIC ARGUMENTS" section in
the
sg3_utils(8) man page.
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 transfers
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.
For sg devices this utility issues SCSI READ and WRITE (SBC) commands which are
appropriate for disks and reading from CD/DVD/BD drives. Those commands are
not formatted correctly for tape devices so sgm_dd should not be used on tape
devices.
This utility stops the copy if any error is encountered. For more advanced
"copy on error" logic see the
sg_dd utility (and its 'coe'
flag).
EXAMPLES¶
See the examples given in the man page for
sg_dd(8).
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.
EXIT STATUS¶
The exit status of sgm_dd is 0 when it is successful. Otherwise see the
sg3_utils(8) man page. Since this utility works at a higher level than
individual commands, and there are 'coe' and 'retries' flags, individual SCSI
command failures do not necessary cause the process to exit.
AUTHORS¶
Written by Doug Gilbert and Peter Allworth.
REPORTING BUGS¶
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT¶
Copyright © 2000-2009 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¶
The simplest variant of this utility is called
sg_dd. A POSIX threads
version of this utility called
sgp_dd is in the sg3_utils package. The
lmbench package contains
lmdd which is also interesting.
raw(8),
dd(1)