NAME¶
sgm_dd - copy data to and from files and devices, especially SCSI 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 Douglas Gilbert and Peter Allworth.
REPORTING BUGS¶
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT¶
Copyright © 2000-2012 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)