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SGP_DD(8) SG3_UTILS SGP_DD(8)
NAME
sgp_dd - copy data to and from files and devices, especially SCSI devices
SYNOPSIS
sgp_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] [coe=0|1] [cdbsz=6|10|12|16] [deb=VERB] [dio=0|1] [sync=0|1] [thr=THR]
[time=0|1] [verbose=VERB] [--dry-run] [--verbose]
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 conver-
sions. Uses POSIX threads (often called "pthreads") to increase the amount of parallelism.
This improves speed in some cases.
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 de-
fault (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 of the actual device block
size. 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 (unless calculations indicate that a 4 byte block number
may be exceeded, in which case it defaults to 16 byte SCSI commands).
coe=0 | 1
set to 1 for continue on error. Only applies to errors on sg devices. Thus errors
on other files will stop sgp_dd. Default is 0 which implies stop on any error. See
the 'coe' flag for more information.
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 seek=SEEK are given and the count is deduced (i.e. not ex-
plicitly given) then that 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 deduced then an error message is issued and no
copy takes place.
deb=VERB
outputs debug information. If VERB is 0 (default) then there is minimal debug in-
formation and as VERB increases so does the amount of debug (max debug output when
VERB 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) For finer grain control use 'iflag=dio' or
'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 read-
ing 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 ex-
ists 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, '.' (pe-
riod), 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.
thr=THR
where THR is the number or worker threads (default 4) that attempt to copy in par-
allel. Minimum is 1 and maximum is 1024.
time=0 | 1
when 1, the transfer is timed and throughput calculation is performed, outputting
the results (to stderr) at completion. When 0 (default) no timing is performed.
verbose=VERB
increase verbosity. Same as deb=VERB. Added for compatibility with sg_dd and
sgm_dd.
-d, --dry-run
does all the command line parsing and preparation but bypasses the actual copy or
read. That preparation may include opening IFILE or OFILE to determine their
lengths. This option may be useful for testing the syntax of complex command line
invocations in advance of executing them.
-h, --help
outputs usage message and exits.
-v, --verbose
when used once, this is equivalent to verbose=1. When used twice (e.g. "-vv") this
is equivalent to verbose=2, etc.
-V, --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 to-
gether 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.
coe continue on error. When given with 'iflag=', an error that is detected in a single
SCSI command (typically 'bpt' blocks) is noted (by an error message sent to
stderr), then zeros are substituted into the buffer for the corresponding write op-
eration and the copy continues. Note that the sg_dd utility is more sophisticated
in such error situations when 'iflag=coe'. When given with 'oflag=', any error re-
ported by a SCSI WRITE command is reported to stderr and the copy continues (as if
nothing went wrong).
dio request the sg device node associated with this flag does direct IO. If direct IO
is 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 is-
sued (and indirect IO is performed).
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.
mmap can only be used in the iflag=FLAGS or the oflag=FLAGS argument list but not both.
The nominated side of the copy will use memory mapped IO based on the mmap(2) sys-
tem call. The sg driver will remap its DMA destination or source buffer into the
user space when the mmap(2) system call is used on a sg device.
fua causes the FUA (force unit access) bit to be set in SCSI READ and/or WRITE com-
mands. 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.
RETIRED OPTIONS
Here are some retired options that are still present:
coe=0 | 1
continue on error is 0 (off) by default. When it is 1, it is equivalent to
'iflag=coe oflag=coe' described in the FLAGS section above. Similar to 'conv=noer-
ror,sync' in dd(1) utility. Default is 0 which implies stop on error. More advanced
coe=1 processing on reads is performed by the sg_dd utility.
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 sgp_dd. See raw(8) for more
information about binding raw devices. To be safe, the sg device mapping to SCSI block de-
vices 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 arguments 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 se-
lect "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 de-
vice caches which has a negative impact on other machine activity.
SIGNALS
The signal handling has been borrowed from dd: SIGINT, SIGQUIT and SIGPIPE output the num-
ber 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 con-
tinues. All output caused by signals is sent to stderr.
EXAMPLES
Looks quite similar in usage to dd:
sgp_dd if=/dev/sg0 of=t bs=512 count=1MB
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 correspondingly scaled. Us-
ing a raw device to do something similar on a ATA disk:
raw /dev/raw/raw1 /dev/hda
sgp_dd if=/dev/raw/raw1 of=t bs=512 count=1MB
To copy a SCSI disk partition to an ATA 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
EXIT STATUS
The exit status of sgp_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 (C) 2000-2020 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 but with more advanced "continue on error"
logic is called sg_dd and is also found in the sg3_utils package. The lmbench package con-
tains lmdd which is also interesting. raw(8), dd(1)
sg3_utils-1.45 February 2020 SGP_DD(8)
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