sgp_dd man page on YellowDog
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SGP_DD(8) SG3_UTILS SGP_DD(8)
NAME
sgp_dd - copies data to and from files and devices. Specialized for
devices that understand the SCSI command set.
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]
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 conversions. Uses POSIX threads to
increase the amount of parallelism. This improves speed in some cases.
The first group in the synopsis above are "standard" Unix dd(1) oper‐
ands. 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 inte‐
gral 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 calcula‐
tions 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 skip=SEEK are given and the count is deduced
(i.e. not explicitly 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 mes‐
sage is issued and no copy takes place.
deb=VERB
outputs debug information. If VERB is 0 (default) then there is
minimal debug information 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 reading at the beginning of IFILE unless SKIP is
given.
iflag=FLAGS
where FLAGS is a comma separated list of one or more flags out‐
lined 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 per‐
formed. 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 out‐
lined 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.
thr=THR
where THR is the number or worker threads (default 4) that
attempt to copy in parallel. Minimum is 1 and maximum is 16.
time=0 | 1
when 1, the transfer is timed and throughput calculation is per‐
formed, 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.
--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 attempt‐
ing 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 sub‐
stituted into the buffer for the corresponding write operation
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 reported 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
issued (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 com‐
mands. 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 synchronisa‐
tion 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 sup‐
port 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 sec‐
tion above. Similar to 'conv=noerror,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 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].
COUNT, SKIP, SEEK, BPT and BS may include one of these multi‐
plicative suffixes: c C *1; w W *2; b B *512; k K KiB *1,024; KB
*1,000; m M MiB *1,048,576; MB *1,000,000 . This pattern contin‐
ues for "G", "T" and "P". The latter two suffixes can only be
used for COUNT, SKIP and SEEK. Also a suffix of the form "x<n>"
multiplies the leading number by <n>. These multiplicative suf‐
fixes are compatible with GNU's dd command (since 2002) which
claims compliance with SI and with IEC 60027-2.
Alternatively numerical values can be given in hexadecimal pre‐
ceded by either "0x" or "0X". When hex numbers are given, multi‐
pliers cannot be used.
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 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.
SIGNALS
The signal handling has been borrowed from dd: SIGINT, SIGQUIT
and SIGPIPE output the number of remaining blocks to be trans‐
ferred and the records in + out counts; then they have their
default action. SIGUSR1 causes the same information to be out‐
put yet the copy continues. 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 associ‐
ated 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. Using a raw device to do something simi‐
lar 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 neces‐
sary 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 © 2000-2007 Douglas Gilbert
This software is distributed under the GPL version 2. There is
NO warranty; not even for MERCHANTABILITY or FITNESS FOR A PAR‐
TICULAR 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 contains
lmdd which is also interesting. raw(8), dd(1)
sg3_utils-1.25 July 2007 SGP_DD(8)
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