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<h1> The&nbsp; Linux sg_dd utility<br>
</h1>
</center>
<a href="#Introduction">Introduction</a> <br>
<a href="#dd_like_features">dd like features</a> <br>
<a href="#sg_dd_extras">sg_dd extras</a><br>
<a href="#coe">Continue on error (coe)</a> <br>
<a href="#Recovered_errors">Recovered errors</a> <br>
<a href="#Verbose">Verbose</a><br>
<a href="#Conclusion">Conclusion</a><br>
&nbsp;
<h2><a name="Introduction"></a>Introduction</h2>
The <span style="font-weight: bold;">sg_dd</span> utility is a variant
of the standard Unix command <span style="font-weight: bold;">dd</span>
which
copies files. The sg_dd utility is specialized for devices that use the
SCSI command set in the Linux operating system. The sg_dd utility is
found in the <a href="u_index.html">sg3_utils</a> package which
targets the linux kernel 2.4 and
2.6 series.<br>
<br>
Not all SCSI device types are supported by the sg_dd utility. Obviously
those device types that deal with enclosures and medium changers don't
have addressable blocks and are not supported. The supported device
types are direct access devices (e.g. disks) and cd/dvd devices. The
SCSI tape device type is not be supported (but could be to a limited
degree). When instructed, the sg_dd utility issues SCSI
commands that are defined in SPC-3 (primary commands), SBC-2 (commands
for direct access devices (e.g. disks)) and MMC-4 (commands for CD/DVD
devices). These SCSI command sets can be found at <a
href="http://www.t10.org/">www.t10.org</a>
. <br>
<br>
It is becoming common for non-SCSI devices (e.g. ATA disks) to appear
as SCSI devices to an operating system via a protocol conversion in an
external enclosure and via some transport such as USB or IEEE 1394. The
sg_dd utility should work with most of these devices as it tends to use
exactly the same SCSI commands that the normal block layer would uses.
However, advanced options (e.g. using the 'cdbsz' and 'fua') most
likely will be ignored. Apart from CD players over 10 years old, almost
all CD/DVD players use the Multi Media Command set (MMC or&nbsp;
MMC-2,3,4,5) as their native command set. The fact that the most common
transport is ATAPI (i.e. the ATA packet interface) is irrelevant, to
the
sg_dd utility they are SCSI devices.<br>
<br>
This page outlines the features of the sg_dd utility version 5.38 found
in the <a href="u_index.html">sg3_utils</a> version 1.13 package. This
was released on the 13th
March 2005.<br>
<h2><a name="dd_like_features"></a> dd like features</h2>
The basic syntax of the sg_dd utility is the same as the dd command in
Unix. That said, the syntax of the dd command in Unix is different from
almost all other standard Unix commands. Those familiar with the dd
command
should not be too surprised by the syntax and semantic of the sg_dd
utility. Those not familiar with the dd syntax should be very careful,
especially with the 'of' and 'seek' options, both with dd and sg_dd.
The recent GNU implementation of the dd command is used as a reference
point.<br>
<br>
The main options of dd are:<br>
<ul>
<li>if=&lt;input_file&gt;</li>
<li>of=&lt;output_file&gt;</li>
<li>bs=&lt;block_size&gt;</li>
<li>count=&lt;count_of_blocks&gt;</li>
</ul>
Both dd and sg_dd given these options with suitable arguments will copy
(bs * count) bytes from the beginning of &lt;input_file&gt; to the
beginning of &lt;output_file&gt;. One restriction that sg_dd imposes
when either the &lt;input_file&gt; or &lt;output_file&gt; are accessed
via SCSI commands is that &lt;block_size&gt; must match that of the
device. Further, if both the &lt;input_file&gt; and &lt;output_file&gt;
are accessed via SCSI commands then &lt;block_size&gt; must match that
of both devices. In both dd and sg_dd if the if=&lt;input_file&gt;
option is not given then stdin is assumed. In both dd and sg_dd if
of=&lt;output_file&gt; is not given then stdout is assumed.<br>
<br>
The following extensions are found in sg_dd. An &lt;input_file&gt; of
"-" is interpreted as stdin; an &lt;output_file&gt; of "-" is
interpreted as stdout while an &lt;output_file&gt; of "." is
interpreted as <span style="font-family: monospace;">/dev/null</span>.
[dd interprets input and output file names of
"-" literally. dd interprets an output file of "." as the current
directory and will not accept it.]<br>
<br>
If the 'count' option is not given then an attempt is made to determine
the remaining blocks in the file, device or partition. If the input
file is stdin and no count is given then a copy will continue until an
EOF is detected on the input stream (or something else goes wrong). If
the 'count' option is not given then the remaining blocks on both the
input and output files are determined (if possible) and if both are
found then the minimum of
the two counts is used.&nbsp; The 'skip' option for an input file and
the 'seek' option for an output file are taken into account when
calculating the remaining number of blocks in a file, device or
partition.<br>
<br>
If the 'count' option is given then no further checks regarding the
remaining length of the input and output files are done and the sg_dd
will attempt to copy that number of blocks. A 'count=0' option is valid
and all the normal preparations are made including files being opened
but no copy takes place. Hence the 'count=0' option can be used to
check the syntax is in order and the files are present (see the
"Verbose" section below).<br>
<br>
Other dd options supported by sg_dd:<br>
<ul>
<li>seek=&lt;n_blocks_of&gt;</li>
<li>skip=&lt;n_blocks_if&gt;</li>
<li>ibs=&lt;block_size&gt;</li>
<li>obs=&lt;block_size&gt;</li>
</ul>
If ibs and obs are given to sg_dd then they must be the same as bs
(i.e. they are essentially dummies). Both seek and skip are origin zero
and default to zero (i.e the start of the output and input files
respectively). The 'skip' option cannot be used on an input stream
(e.g. stdin) while the 'seek' option cannot be used on an output stream
(e.g. stdout).<br>
<br>
All numeric arguments can take a multiplier suffix. From sg3_utils
version 1.13 sg_dd's multiplier prefixes have been brought into line
with those of GNU's dd (in coreutils post a change on&nbsp; 2001-12-18):<br>
<br>
<table style="width: 50%; text-align: left;" border="1" cellpadding="2"
cellspacing="2">
<tbody>
<tr>
<td style="vertical-align: top;"><span style="font-weight: bold;">Multiplier</span><br>
</td>
<td style="vertical-align: top;"><span style="font-weight: bold;">Meaning</span><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">x&lt;n&gt;<br>
</td>
<td style="vertical-align: top;">*&lt;n&gt;<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">c<br>
</td>
<td style="vertical-align: top;">*1<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">w<br>
</td>
<td style="vertical-align: top;">*2<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">b<br>
</td>
<td style="vertical-align: top;">*512<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">k&nbsp; K&nbsp; KiB<br>
</td>
<td style="vertical-align: top;">*1024<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">KB<br>
</td>
<td style="vertical-align: top;">*1000<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">m&nbsp; M&nbsp; MiB<br>
</td>
<td style="vertical-align: top;">*1048576 </td>
</tr>
<tr>
<td style="vertical-align: top;">MB<br>
</td>
<td style="vertical-align: top;">*1000000<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">g&nbsp; G&nbsp; GiB<br>
</td>
<td style="vertical-align: top;">2**30<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">GB<br>
</td>
<td style="vertical-align: top;">10**9<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">t&nbsp; T&nbsp; TiB<br>
</td>
<td style="vertical-align: top;">2**40 </td>
</tr>
<tr>
<td style="vertical-align: top;">TB<br>
</td>
<td style="vertical-align: top;">10**12<br>
</td>
</tr>
</tbody>
</table>
<br>
The pattern that starts with "k" and proceeds to "m", "g" and "t" then
to "p", "e", "z" and "y"&nbsp; (not shown in above table). sg_dd only
implements as far as "p" (10**15 or 2**50). sg_dd only allows
multipliers based on "t" and "p" for 'count', 'skip' and 'seek'.<br>
<br>
sg_dd allows numeric arguments to be given in hexadecimal in which case
they must be prefixed by either "0x" or "0X". A numeric argument cannot
both be in hex and have a suffix multiplier. Hence "0x9" is interpreted
as hexadecimal 9 [not (0 * 9)==0]. This is valid "2x4x0xa" and yields
80 (but it isn't very clear).<br>
<br>
The following dd options are <span style="font-weight: bold;">not</span>
supported by sg_dd:<br>
<ul>
<li>cbs=&lt;bytes&gt;</li>
<li>conv=&lt;keyword&gt;</li>
</ul>
Basically sg_dd does not supported the conversion features of dd. If a
conversion is really needed then sg_dd could be piped through dd (or
vice versa).<br>
<h2><a name="sg_dd_extras"></a>sg_dd extras</h2>
The extra options of sg_dd (not found in GNU's dd) are:<br>
<br>
<ul>
</ul>
<table style="width: 100%; text-align: left;" border="1" cellpadding="2"
cellspacing="2">
<tbody>
<tr>
<td style="vertical-align: top;"><span style="font-weight: bold;">extra
options in sg_dd</span><br>
</td>
<td style="vertical-align: top;"><span style="font-weight: bold;">default</span><br>
</td>
<td style="vertical-align: top;"><span style="font-weight: bold;">Brief
description</span><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">append=0|1<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">append to output file (rather
than overwrite)<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">blk_sgio=0|1<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">when set access devices via SCSI
commands (SG_IO)<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">bpt<br>
</td>
<td style="vertical-align: top;">128<br>
</td>
<td style="vertical-align: top;">blocks_per_transfer (granularity
of each IO)<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">cdbsz=6|10|12|16<br>
</td>
<td style="vertical-align: top;">10 or 16<br>
</td>
<td style="vertical-align: top;">cdb size of SCSI READ and/or
WRITE commands<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">coe=0|1<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">continue_on_error when set<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">dio=0|1<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">direct IO (only via sg device
nodes)<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">fua=0|1|2|3<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">force_unit_access, 1-&gt;if,
2-&gt;of, 3-&gt;if+of<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">odir=0|1<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">O_DIRECT flag on open() when set<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">sync=0|1<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">SYNCHRONIZE CACHE SCSI command
after transfer when set<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">time=0|1<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">when set print elapsed time and
throughput after copy<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">verbose=&lt;n&gt;<br>
</td>
<td style="vertical-align: top;">0<br>
</td>
<td style="vertical-align: top;">larger &lt;n&gt; is the greater
the debug output<br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">--version<br>
</td>
<td style="vertical-align: top;"><br>
</td>
<td style="vertical-align: top;">print the version number and
release date of
sg_dd then exit<br>
</td>
</tr>
</tbody>
</table>
<br>
<br>
The sg_dd utility examines the files it is given and treats them
differently depending on the category and, in some case,s the command
line options given:<br>
<br>
<table style="width: 100%; text-align: left;" border="1" cellpadding="2"
cellspacing="2">
<tbody>
<tr>
<td style="vertical-align: top;"><span style="font-weight: bold;">File
type</span><br>
</td>
<td style="vertical-align: top;"><span style="font-weight: bold;">open
[when input]</span><br>
</td>
<td style="vertical-align: top;"><span style="font-weight: bold;">open
[when output]</span><br>
</td>
<td style="vertical-align: top;"><span style="font-weight: bold;">IO
method</span><br>
</td>
<td style="vertical-align: top;"><span style="font-weight: bold;">Notes</span><br>
</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">normal<br>
</td>
<td style="vertical-align: top;">O_RDONLY<br>
<br>
</td>
<td style="vertical-align: top;">O_WRONLY | O_CREAT<br>
[add O_APPEND if 'append=1']<br>
</td>
<td style="vertical-align: top;">Unix read() write()<br>
</td>
<td style="vertical-align: top;">Add O_DIRECT if<br>
'odir=1'<br>
</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">stdin or stdout<br>
</td>
<td style="vertical-align: top;">[do nothing]<br>
</td>
<td style="vertical-align: top;">[do nothing]<br>
</td>
<td style="vertical-align: top;">Unix read() write()<br>
</td>
<td style="vertical-align: top;"><br>
</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">/dev/null<br>
</td>
<td style="vertical-align: top;">O_RDONLY</td>
<td style="vertical-align: top;">[do nothing]<br>
</td>
<td style="vertical-align: top;">Unix read() if input<br>
</td>
<td style="vertical-align: top;">if output then no IO<br>
</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">block device<br>
</td>
<td style="vertical-align: top;">O_RDONLY</td>
<td style="vertical-align: top;">O_WRONLY | O_CREAT<br>
[add O_APPEND if 'append=1']</td>
<td style="vertical-align: top;">Unix read() write()</td>
<td style="vertical-align: top;">Add O_DIRECT if<br>
'odir=1'</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">block device [blk_sgio=1]<br>
</td>
<td style="vertical-align: top;">O_RDWR or O_RDONLY</td>
<td style="vertical-align: top;">O_RDWR</td>
<td style="vertical-align: top;">SCSI commands</td>
<td style="vertical-align: top;">Opens input O_RDONLY <br>
if O_RDWR fails. Adds <br>
O_DIRECT if 'odir=1' </td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">sg device<br>
</td>
<td style="vertical-align: top;">O_RDWR or O_RDONLY<br>
</td>
<td style="vertical-align: top;">O_RDWR<br>
</td>
<td style="vertical-align: top;">SCSI commands<br>
</td>
<td style="vertical-align: top;">Opens input O_RDONLY<br>
if O_RDWR fails</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">raw device<br>
</td>
<td style="vertical-align: top;">O_RDONLY</td>
<td style="vertical-align: top;">O_WRONLY<br>
</td>
<td style="vertical-align: top;">Unix read() write()<br>
</td>
<td style="vertical-align: top;"><br>
</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
<tr>
<td style="vertical-align: top;">scsi tape device<br>
</td>
<td style="vertical-align: top;">x<br>
</td>
<td style="vertical-align: top;">x<br>
</td>
<td style="vertical-align: top;">no IO<br>
</td>
<td style="vertical-align: top;">error reported<br>
</td>
<td style="vertical-align: top;"><br>
</td>
</tr>
</tbody>
</table>
<br>
Some of the above combinations are not sensible (e.g. 'append=1' on a
block device). Raw devices were introduced in the lk 2.4 series and are
still available in the lk 2.6 series but opening a block device or a
normal file with O_DIRECT (in sg_dd using the option 'odir=1') is
preferred.<br>
<br>
The 'blk_sgio=1' option is only valid in the lk 2.6 series. This option
instructs the sg_dd utility to use the SG_IO ioctl to issue SCSI
commands even though the file type is not a scsi generic (sg) device.
Currently this will work for all cd/dvd devices irrespective of the
transport (except for some very old devices ( &gt; 10 years old)) and
block devices representing SCSI disks (e.g. /dev/sda). Even though SCSI
tape (st) devices now accept the SG_IO ioctl in the lk 2.6 series, the
sg_dd utility expects direct access devices (whereas tapes are
sequential access) so sg_dd generates an error if a st device is
detected. When the input or output file is a sg device then the
'blk_sgio' option is ignored as IO with that file is always via SCSI
commands.<br>
<br>
If a partition of block device is accessed (e.g. /dev/sda2) when
'blk_sgio=0' (or is not given) then logical block address 0 for the
purposes of sg_dd (and its skip and seek options) is the beginning of
that partition while the calculated count (e.g. when a 'count' option
is not given) is the extent of that partition. However if a partition
of block device is accessed (e.g. /dev/sda2) when 'blk_sgio=1' then the
partition is ignored and the underlying device is accessed. This means
logical block address 0 for the purposes of sg_dd (and its skip and
seek options) is the beginning of the device while the calculated count
(e.g. when a 'count' option is not given) is the extent of that device.<br>
<br>
Note about CDs and count (perhaps DVDs don't have this problem). When a
CD is accessed via SCSI commands and the 'count' option is not given
then the READ CAPACITY SCSI command is used by sg_dd to determine the
number of blocks to read. Unfortunately some MMC standards allow a few
of the final blocks to be invalid ("run out" sectors) and this can
result in IO errors when the sg_dd utility is used to copy such a CD.
The isosize utility can be used to determine the actual size of an
ISO9660 file system residing on such a CD. It is often a few blocks
shorter than READ CAPACITY reports (for good reason).<br>
<br>
When an input or output file is being accessed via SCSI commands the
following "extra" options being discussed in this section are active:<br>
<ul>
<li>bpt: each cdb issued will be for this value or less blocks (with
the block size implied)<br>
</li>
<li>cdbsz: defaults to a 10 byte cdb if IO fits in 32 bit lba space
otherwise 16 byte cdb</li>
<li>coe: when set error on read get special treatment (see below)</li>
<li>dio: only active for sg devices (i.e. not block device with
'blk_sgio=1')</li>
<li>fua: force_unit_access flag in cdb; 1-&gt;if, 2-&gt;of;
3-&gt;if+of</li>
<li>odir: active when file is a block device (i.e. inactive for sg
devices)<br>
</li>
<li>sync: when set issues SYNCHRONIZE CACHE command at the end of the
copy</li>
<li>time</li>
<li>verbose: 1 and 2 print setup cdbs, 3 and 4 print all cdbs</li>
</ul>
When an input or output file is being accessed via normal Unix read()
and write() commands the following "extra" options being discussed in
this section are active:<br>
<ul>
<li>append: one active on output file that are "normal" (i.e. neither
devices nor streams)<br>
</li>
<li>bpt: each IO operation transfers (bs * bpt) bytes or less</li>
<li>odir: opens files with the O_DIRECT flag</li>
<li>time</li>
<li>verbose: 1 + 2 print setup system calls, 3 and 4 additional print
read() and write() system calls<br>
</li>
</ul>
If a SIGUSR1 signal is sent to the process identifier (pid) of a
running sg_dd utility then the number of blocks copies to that point is
output. The copy continues.<br>
<br>
When the 'time' option is set, the elapsed time for the copy plus the
throughput measured in megabytes (10**6 bytes)&nbsp; per second is
output when the copy is complete (or an error stops the copy). If a
SIGUSR1 signal is sent to the process identifier (pid) of a running
sg_dd utility which has the 'time' option set the elapsed time and the
throughput of the copy to that point is output and the copy continues.<br>
<h2><a name="coe"></a>Continue on error (coe)</h2>
Recent additions to the sg_dd utility allow it to be used as a copy "of
last resort" from failing media. Read errors on an input file
taking SCSI commands are "tolerated" when the 'coe' option is set.
Write errors from SCSI commands are reported and ignored and the sg_dd
utility continues when the 'coe' option is set. [In the case where
media errors are causing write errors the
reader should check the setting of the AWRE bit in the SCSI "read write
error recovery" mode page (see SBC-2 at http://www.t10.org).] As for
errors in normal files the reader may wish to examine the
"conv=noerror" option of the dd command.<br>
<br>
When a SCSI READ command detects an unrecoverable read error it&nbsp;
responds with a sense key of MEDIUM ERROR or HARDWARE ERROR.
Additionally it responds with the logical block address of the first
(lowest) block that it failed to read in the current READ command. Any
valid blocks prior to the "bad" block may or may not have been
transferred (depending on several other settings). If 'coe=0' then the
sg_dd utility will simply terminate at this point (with a reasonable
amount of debug information sent to stderr). If 'coe=1' then the first
thing it will try to do is a truncated read up to but not including the
bad block. The remaining discussion in this section assumes 'coe=1'.<br>
<br>
The "bad" block may or may not be readable by the device. If it is
readable then most likely the associated ECC data indicates that it is
corrupt and the ECC data cannot recover it. [It is possible that all
the corruption is in the ECC data and the payload data is ok.] Such
blocks can be retrieved with the READ LONG command which is what the
sg_dd utility does. If the READ LONG fails then a block of zeroes is
substituted for the bad block in the transfer buffer. Only SBC-2
devices (primarily SCSI disks) optionally support READ LONG. CD/DVD
devices (covered in the MMC-4 (draft) standard) do not so a block of
zeroes is substituted for them.<br>
<br>
There still remains blocks after the "bad" block in the initial
transfer to be fetched. Further bad blocks may be detected and if so
the algorithm in the last two paragraph is repeated. The result of this
process is an imperfect copy with blocks that were read properly placed
in the correct relative position in the output. When the 'coe=1' option
is given two addition counters are output on completion:<br>
<ul>
<li>unrecovered errors</li>
<li>read_longs fetched part of unrecovered read errors</li>
</ul>
The first counter is the number of unrecovered (read) errors
encountered. Any number greater than zero flags an imperfect copy. The
second counter (always less than or equal to the first) is the number
of "successful" READ LONG SCSI commands performed. If the source media
is a CD or DVD then this number will be zero.<br>
<br>
<h2><a name="Recovered_errors"></a>Recovered errors</h2>
Often errors are recovered using ECC data or by the device retrying
(usually re-reading) the media. Also at the first sign of trouble,
recoverable errors lead to the block in question being reassigned to
another location on the media (automatically when the AWRE and ARRE
bits are set in the "read write error recovery" mode page).&nbsp; The
user of such a disk may be blissfully unaware that the disk may be
reaching the end of its useful life. Error counters are maintained in
the "Read error counter" and "Write error counter" logs pages which can
be viewed with smartctl (from smartmontools) and sg_logs (from the same
sg3_utils package that sg_dd comes from). Any block that is
automatically or manually re-assigned add a new entry to the "grown"
defect list which cal be viewed with 'sginfo -G' (also found in the
sg3_utils package).<br>
<br>
A disk can be instructed to report RECOVERED ERRORs by setting the PER
bit in the "read write error recover" mode page. Most often this bit is
clear. When sg_dd detects RECOVERED ERRORs it reports them, counts
them and continues the copy. Only the lba of the last recovered error
in a READ or WRITE SCSI
command is reported so there could be more than one recovered error per
SCSI command. The 'bpt=1' option
could be chosen to limit every SCSI command to a single block transfer
(but that would slow things down a fair amount). If the count of
recovered errors is greater than zero at the end of the copy then this
count is output as well.<br>
<br>
There can be other reasons for a sense key of RECOVERED ERROR not
directly related to data being currently read or written. SMART alerts
(called in SCSI documents "Informational Exceptions") can be conveyed
via a RECOVERED ERROR sense key (see the MRIE field in the
Informational Exceptions mode page). Such alerts have additional sense
codes like "Failure prediction threshold exceeded" and those that
contain "impending failure".<br>
<h2><a name="Verbose"></a>Verbose</h2>
In the Unix style, sg_dd doesn't output anything (to stderr) during
large IO transfers. To get a progress report the SIGUSR1 signal can be
sent to the sg_dd process. In the Unix dd command style, sg_dd outputs
two lines on completion that show the number of full and partial
records
in (on the first line) and out (on the second line). Some modern
versions of the dd command also output elapsed time and throughput
figures (to get this with sg_dd use the "time=1" option).<br>
<br>
The sg_dd has a 'verbose' option whose default value is zero. When set
this option has the following actions:<br>
<ol>
<li>show categorization and INQUIRY data (where applicable) for the
input and output files. For files, other than streams, the file/device
size (and device block size) are output. For SCSI devices mode data
relevant to error recovery and caching is output.</li>
<li>same output as 1 plus data for Unix and SCSI commands (cdbs) that
are not repeated (i.e. other than Unix read/write and SCSI READ/WRITE
commands). Increased error reporting for all SCSI commands</li>
<li>same output as 2 plus data for Unix and SCSI commands (cdbs) that
are repeated. For a large copy this will be a lot of output.</li>
<li>maximum amount of debug output. For a large copy this will be a
lot of output.</li>
</ol>
All verbose output is sent to stderr (so that sg_dd usage with "of=-"
(copy output to stdout) is not corrupted).<br>
<br>
Following is an example of using "verbose=1" to find information about
/dev/sda . If no copy is required then setting "count=0" will see to
that. Since "dev/sda" is a block device then it would normally be
accessed via Unix system commands. The "verbose=1" output is relatively
short when "blk_sgio=0" (its default value). The second invocation is
with "blk_sgio=1" and a lot more is output. That includes INQUIRY
standard response data (e.g. "SEAGATE ..." line) followed by the
setting of various mode pages relevant to error processing and caching.
See the SBC-2 drafts at <a href="http://www.t10.org/">www.t10.org</a>
for more information. As an example a AWRE bit of 1 means that the disk
will automatically reallocate a logical block address (to another
sector)
if a recoverable error is detected on a WRITE operation. RECOVERED
ERRORs are only report on read and write operations when the PER [Post
Error] bit is set.<br>
<br>
<span style="font-family: monospace;">$ sg_dd if=/dev/sda of=. bs=512
verbose=1 count=0 blk_sgio=0</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&gt;&gt; Input file type:
block device</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
open input, flags=0x0</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&gt;&gt; Output file type:
null device</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
[bgs64] number of blocks=17781521 [0x10f5311], block size=512</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">0+0 records in</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">0+0 records out</span><br
style="font-family: monospace;">
<span style="font-family: monospace;"><br>
<br>
$ sg_dd if=/dev/sda of=. bs=512 verbose=1 count=0 blk_sgio=1</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&gt;&gt; Input file type:
block device</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
open input(sg_io), flags=0x2</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp; /dev/sda:
SEAGATE&nbsp;&nbsp;
ST39173LC&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1234&nbsp;
[pdt=0]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp; Read write error recovery
mode page:</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
AWRE:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1&nbsp; [Changeable: y, def: 1,
saved: 1]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
ARRE:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1&nbsp; [Changeable: y, def: 1,
saved: 1]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
RC:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0&nbsp; [Changeable: y,
def: 0, saved: 0]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
EER:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1&nbsp; [Changeable: y, def:
0, saved: 1]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
PER:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1&nbsp; [Changeable: y, def:
0, saved: 1]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
DTE:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0&nbsp; [Changeable: y, def:
0, saved: 0]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
DCR:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0&nbsp; [Changeable: y, def:
0, saved: 0]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp; Caching mode page:</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
WRE:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 1&nbsp; [Changeable: y, def:
1, saved: 1]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
RCD:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0&nbsp; [Changeable: y, def:
0, saved: 0]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp; Control mode page:</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;
SWP:&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; 0&nbsp; [Changeable: n, def:
0, saved: 0]</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&gt;&gt; Output file type:
null device</span><br style="font-family: monospace;">
<span style="font-family: monospace;">&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;
number of blocks=17781521 [0x10f5311], block size=512</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">0+0 records in</span><br
style="font-family: monospace;">
<span style="font-family: monospace;">0+0 records out</span><br>
<br>
Some of the output above is out of order, the "number of blocks" line
relates to the input file.<br>
<h2><a name="Conclusion"></a>Conclusion</h2>
The sg_dd utility maintains syntactic and semantic compatibility with
the Unix dd command while allowing precise control over SCSI devices
such as SCSI disks and CD/DVD drives. Amongst other uses it can copy
data from failing media, continuing as best it can when medium errors
are encountered.<br>
<br>
<br>
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<p>Last updated: 14th March 2005<br>
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