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Section: DDPT (8) Updated: February 2011
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ddpt - copies data between files and storage devices. Support for devices that understand the SCSI command set.  


ddpt [bpt=BPT[,OBPC]] [bs=BS] [cdbsz=6|10|12|16] [coe=0|1] [coe_limit=CL] [conv=CONVS] [count=COUNT] [ibs=IBS] if=IFILE [iflag=FLAGS] [obs=OBS] [of=OFILE] [of2=OFILE2] [oflag=FLAGS] [retries=RETR] [seek=SEEK] [skip=SKIP] [status=STAT] [verbose=VERB] [--help] [--verbose] [--version] [--wscan]

For comparison here is the synopsis for GNU's dd command:

dd [bs=BS] [cbs=CBS] [conv=CONVS] [count=COUNT] [ibs=IBS] [if=IFILE] [iflag=FLAGS] [obs=OBS] [of=OFILE] [oflag=FLAGS] [seek=SEEK] [skip=SKIP] [status=STAT] [--help] [--version]  


Copy data between files or read data from a file. Specialized for "files" that are storage devices, especially those that can use the SCSI command sets (e.g. SATA and SAS disks, plus DVD drives). Can issue SCSI commands in pass-through ("pt") mode. Similar syntax and semantics to the Unix dd(1) command.

For comparison, the SYNOPSIS section above shows both the ddpt command line options followed by GNU's dd(1) command line options. Broadly speaking ddpt can be considered a super-set of dd. See the section on DD DIFFERENCES for significant differences between ddpt and dd.

ddpt does a segmented copy, first reading in BPT*IBS bytes from IFILE (or less if near the end of the copy) into a copy buffer. In the absence of the various options and conditions that bypass the write operation, the copy buffer is then written out to OFILE. The copy process continues working its way along IFILE and OFILE until either COUNT is exhausted, an end of file is detected, or an error occurs. If IBS and OBS are different, ddpt restricts the value of OBS such that the copy buffer is an integral number output blocks (i.e. (((IBS*BPT) % OBS) == 0) ). In the following descriptions, "segment" refers to all or part of a copy buffer.

The term "pt device" is used for a pass-through device to which SCSI commands like READ(10) and WRITE(10) may be sent. A pt device may only be able to process SCSI commands in which case the "pt" flag is assumed. The ability to recognize such a pt only device may vary depending on the operating system (e.g. in Linux '/dev/sg2' is recognized). However if a device can process either normal UNIX read()/write() calls or pass-through SCSI commands then the default is to use UNIX read()/write() calls. That default can be overridden by using the "pt" flag (e.g. "if=/dev/sdc iflag=pt"). When pt access is specified any partition information is ignored. So "if=/dev/sdc2 iflag=pt skip=3" will start at logical block address 3 of '/dev/sdc'. As a protection measure in version 0.92 ddpt will only accept that if the force flag is given in addition to pt.  


where BPT is Blocks Per Transfer. The copy is made up of multiple transfers, each first reading BPT input blocks (i.e. BPT*IBS bytes) from IFILE into the copy buffer and then from that copy buffer writing BPT*IBS/OBS output blocks to OFILE. This continues until the copy is finished, with the last transfer being potentially shorter. The default BPT value varies depending on IBS. When IBS < 8, BPT is 8192; when IBS < 64, BPT is 1024; when IBS < 1024, BPT is 128; when IBS < 8192, BPT is 16; when IBS < 32768, BPT is 4; else BPT defaults to 1. If BPT is given as 0 it is treated as the default value. For "bs=512", BPT defaults to 128 so that 64 KiB (or less) is read from IFILE into the copy buffer.
The optional OBPC (Output Blocks Per Check) argument controls controls the granularity of sparse writes, write sparing and trim checks. The default granularity is the size of the copy buffer (i.e. BPT*IBS bytes). That can be reduced by specifying OBPC. The finest granularity is when OBPC is 1 which implies the unit of each check is OBS bytes. When OBPC is 0, or not given, the default granularity is used. Large OBPC values are rounded down so that OBPC*OBS does not exceed the size of the copy buffer.
where BS is the IFILE and OFILE block size in bytes. Conflicts with either "ibs=" or "obs=" options. The value of BS is placed in IBS and OBS. If IFILE or OFILE is a "pt" device then BS must be the logical block size of the device. See the DD DIFFERENCES section below. Default is 512 which to date has been correct for hard disks. Other logical block sizes are 2048 bytes for DVDs and 4096 bytes for the coming generation of hard disks.
cdbsz=6 | 10 | 12 | 16
size of SCSI READ and/or WRITE commands issued on pt devices. Default is 10 byte SCSI command blocks (unless calculations indicate that a 4 byte block number may be exceeded or BPT is greater than 16 bits (65535), in which case it defaults to 16 byte SCSI commands).
coe=0 | 1
set to 1 for continue on error. Applies to errors on input and output pt devices plus input from block devices or regular files. Errors on other files will stop ddpt. Default is 0 which implies stop on any error. See the 'coe' flag for more information.
where CL is the maximum number of consecutive bad blocks stepped over (due to "coe=1") on reads before the copy terminates. The default is 0 which is interpreted as no limit. This option is meant to stop the copy soon after unrecorded media is detected while still offering "continue on error" capability.
see the CONVERSIONS section below.
copy COUNT input blocks from IFILE to OFILE. If this option is not given (or COUNT is '-1') then the COUNT may be deduced from either IFILE or OFILE. See the COUNT section below.
where IBS is the IFILE block size in bytes. The default value is BS or its default (512). Conflicts the "bs=" option (e.g. giving both "bs=512 ibs=512" is considered a syntax error).
read from IFILE. This option must be given (i.e. it is mandatory). If IFILE is '-' then stdin is read. Starts reading at the beginning of IFILE unless SKIP is given.
where FLAGS is a comma separated list of one or more flags outlined in the FLAGS section below. These flags are associated with IFILE and are ignored when IFILE is stdin.
where OBS is the OFILE block size in bytes. The default value is BS or its default (512). Conflicts the "bs=" option (e.g. giving both "bs=512 obs=512" is considered a syntax error). If OBS is given then it has the following restriction: the integer expression (((IBS * BPT) % OBS) == 0) must be true. Stated another way: the copy buffer size must be an integral multiple of OBS. If of2=OFILE2 is given then OBS is its block size as well.
write to OFILE. The default value is /dev/null . 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 . If OFILE exists then it is _not_ truncated unless "oflag=trunc" is given. See section on DD DIFFERENCES.
write output to OFILE2. The default action is not to do this additional write (i.e. when this option is not given). OFILE2 is assumed to be a regular file or a fifo (i.e. a named pipe). OFILE2 is opened for writing and is created if necessary. If OFILE2 is a fifo (named pipe) then some other command should be consuming that data (e.g. 'md5sum OFILE2'), otherwise this utility will block. The write to OFILE2 occurs before the write to OFILE and prior to sparse writing and write sparing logic. So everything read is written to OFILE2.
where FLAGS is a comma separated list of one or more flags outlined in the FLAGS section. These flags are associated with OFILE and are ignored when OFILE is /dev/null, '.' (period), or stdout.
sometimes retries at the host are useful, for example when there is a transport error. When RETR is greater than zero then SCSI READs and WRITEs are retried on error, RETR times. Default value is zero. Only applies to errors on pt devices.
start writing SEEK blocks (each of OBS bytes) from the start of OFILE. Default is block 0 (i.e. start of file). The SEEK value may exceed the number of OBS-sized blocks in OFILE.
start reading SKIP blocks (each of IBS bytes) from the start of IFILE. Default is block 0 (i.e. start of file). The SKIP value must be less than or equal to the number of IBS-sized blocks in IFILE.
the STAT value of 'noxfer' suppresses the throughput speed and the copy time output at the end of the copy. The "status=noxfer" option was recently introduced to GNU's dd command. The default action of ddpt is to show the throughput (in megabytes per second) and the time taken to do the copy after the "records in" and "records out" lines at the end of the copy. As a convenience the value 'null' is accepted for STAT and does nothing.
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, plus Unix read() and write() calls, so there can be a lot of output. If VERB is "-1" then output otherwise sent to stderr is redirected to /dev/null .
-h, --help
outputs usage message and exits.
-v, --verbose
equivalent of verbose=1. If --verbose appears twice then that is equivalent to verbose=2. Also -vv is equivalent to verbose=2.
-V, --version
outputs version number information and exits.
-w, --wscan
this option is available in Windows only. It lists storage device names and the corresponding volumes, if any. When used twice it adds the "bus type" of the closest transport (e.g. a SATA disk in a USB connected enclosure has bus type Usb). When used three times a SCSI adapter scan is added. When used four times only a SCSI adapter scan is shown. See EXAMPLES section below and the README.win32 file.


When the count=COUNT option is not given (or COUNT is '-1') then an attempt is made to deduce COUNT as follows.

When both or either IFILE and OFILE are block devices, then the minimum size, expressed in units of input blocks, is used. When both or either IFILE and OFILE are pass-through devices, then the minimum size, expressed in units of input blocks, is used.

If a regular file is used as input, its size, expressed in units of input blocks (and rounded up if necessary) is used. Note that the rounding up of the deduced COUNT may result in a partial read of the last input block and a corresponding partial write to OFILE if it is a regular file.

The size of pt devices is deduced from the SCSI READ CAPACITY command. Block device sizes (or their partition sizes) are obtained from the operating system, if available.

If skip=SKIP or skip=SEEK are given and the COUNT is deduced (i.e. not explicitly given) then that size is scaled back so that the copy will not overrun the file or device.

If COUNT is not given and IFILE is a fifo (and stdin is treated as a fifo) then IFILE is read until an EOF is detected. If COUNT is not given and IFILE is a /dev/zero (or equivalent) then zeros are read until an error occurs (e.g. file system full).

If COUNT is not given and cannot be deduced then an error message is issued and no copy takes place.  


One or more conversions can be given to the "conv=" option. If more than one is given, they should be comma separated. ddpt does not perform the traditional dd conversions (e.g. ASCII to EBCDIC). Recently added conversions overlap somewhat with the flags so some conversions are now supported by ddpt.
equivalent to "oflag=fdatasync". Flushes data associated with the OFILE to storage at the end of the copy. This conversion is for compatibility with GNU's dd.
equivalent to "oflag=fsync". Flushes data and meta-data associated with the OFILE to storage at the end of the copy. This conversion is for compatibility with GNU's dd.
this conversion is very close to "iflag=coe" and is treated as such. See the "coe" flag. Note that an error on OFILE will stop the copy.
has no affect, just a placeholder.
See "resume" in the FLAGS sections for more information.
See "sparing" in the FLAGS sections for more information.
FreeBSD supports "conv=sparse" so the same syntax is supported in ddpt. See "sparse" in the FLAGS sections for more information.
is ignored by ddpt. With dd it means supply zero fill (rather than skip) and is typically used like this "conv=noerror,sync" to have the same functionality as ddpt's "iflag=coe".
if OFILE is a regular file then truncate it prior to starting the copy. See "trunc" in the FLAGS section.


A list of flags and their meanings follow. The flag name is followed by one or two indications in square brackets. The first indication is either "[i]", "[o]" or "[io]" indicating this flag is active for the IFILE, OFILE or both the IFILE and the OFILE. The second indication contains some combination of "reg", "blk" or "pt" indicating whether the flag applies to a regular file, a block device (accessed via Unix read() and write() commands) or a pass-through device respectively.
append [o] [reg]
causes the O_APPEND flag to be added to the open of OFILE. For regular files this will lead to data appended to the end of any existing data. Conflicts the seek=SEEK option. The default action of this utility is to overwrite any existing data from the beginning of OFILE 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 [io] [pt], [i] [reg,blk]
continue on error. 'iflag=coe oflag=coe' and 'coe=1' are equivalent. Errors occurring on output regular or block files will stop ddpt. Error messages are sent to stderr. This flag is similar to 'conv=noerror,sync' in the dd(1) utility. Unrecovered errors are counted and output in the summary at the end of the copy.
This paragraph is about coe on pt devices. A medium, hardware or blank check error while reading will re-read blocks prior to the bad block, then try to recover the bad block, supplying zeros if that fails, and finally reread the blocks after the bad block. A medium, hardware or blank check error while writing is noted and ignored. SCSI disks may automatically try and remap faulty sectors (see the AWRE and ARRE in the read write error recovery mode page (the sdparm utility can access these attributes)). If bad LBAs are reported by the pass-through then the LBA of the lowest and highest bad block is also output.
This paragraph is about coe on input regular files and block devices. When a EIO or EREMOTEIO error is detected on a normal segment read then the segment is re-read one block (i.e. IBS bytes) at a time. Any block that yields a EIO or EREMOTEIO error is replaced by zeros. Any other error, a short read or an end of file will terminate the copy, usually after the data that has been read is written to the output file.
direct [io] [reg,blk]
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. May have no effect on pt devices. This flag will bypass caching/buffering normally done by block layer. Beware of data coherency issues if the same locations have been recently accessed via the block layer in its normal mode (i.e. non-direct). See open(2) man page.
dpo [io] [pt]
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.
errblk [i] [pt] [experimental]
attempts to create or append to a file called "errblk.txt" in the current directory the logical block addresses of blocks that cannot be read. The first (appended) line is "# start <timestamp>". That is followed by the LBAs in hex (and prefixed with "0x") of any block that cannot be read, one LBA per line. If the sense data does not correctly identify the LBA of the first error in the range it was asked to read then a LBA range is output in the form of the lowest and the highest LBA in the range separated by a "-". At the end of the copy a line with "# stop <timestamp>" is appended to "errblk.txt". Typically used with "coe".
excl [io] [reg,blk]
causes the O_EXCL flag to be added to the open of IFILE and/or OFILE. See open(2) man page.
fdatasync [o] [reg,blk]
Flushes data associated with the OFILE to storage at the end of the copy.
flock [io] [reg,blk,pt]
after opening the associated file (i.e. IFILE and/or OFILE) an attempt is made to get an advisory exclusive lock with the flock() system call. The flock arguments are "FLOCK_EX | FLOCK_NB" which will cause the lock to be taken if available else a "temporarily unavailable" error is generated. An exit status of 90 is produced in the latter case and no copy is done. See flock(2) man page.
force [io] [pt]
override difference between given block size and the block size found by the SCSI READ CAPACITY command. Use the given block size. Without this flag the copy would not be performed. pt access to what appears to be a block partition is aborted in version 0.92; that can be overridden by the force flag. For related reasons the 'norcap' flag requires this flag when applied to a block device accessed via pt.
fsync [o] [reg,blk]
Flushes data and metadata (describing the file) associated with the OFILE to storage at the end of the copy.
fua [io] [pt]
causes the FUA (force unit access) bit to be set in SCSI READ and/or WRITE commands. The 6 byte variants of the SCSI READ and WRITE commands do not support the FUA bit.
fua_nv [io] [pt]
causes the FUA_NV (force unit access non-volatile cache) bit to be set in SCSI READ and/or WRITE commands. This only has an effect with pt devices. The 6 byte variants of the SCSI READ and WRITE commands do not support the FUA_NV bit.
nocache [io] [reg,blk]
use posix_fadvise() to advise corresponding file there is no need to fill the file buffer with recently read or written blocks. If used with "iflag=" it will increase the read ahead on IFILE.
norcap [io] [pt]
do not perform SCSI READ CAPACITY command on the corresponding pt device. If used on block device accessed via pt then 'force' flag is also required. This is to warn about using pt access on what may be a block device partition.
nowrite [o] [reg,blk,pt]
bypass writes to OFILE. The "records out" count is not incremented. OFILE is still opened but "oflag=trunc" if given is ignored. Also the ftruncate call associated with the sparse flag is ignored (i.e. bypassed). Commands such as trim and SCSI SYNCHRONIZE CACHE are still sent.
null [io]
has no affect, just a placeholder.
pt [io] [blk,pt]
causes a device to be accessed in "pt" mode. In "pt" mode SCSI READ and WRITE commands are sent to access blocks rather than standard UNIX read() and write() commands. The "pt" mode may be implicit if the device is only capable of passing through SCSI commands (e.g. the /dev/sg devices in Linux). This flag is needed for device nodes that can be accessed both via standard UNIX read() and write() commands as well as SCSI commands. Such devices default standard UNIX read() and write() commands in the absence of this flag.
resume [o] [reg]
when a copy is interrupted (e.g. with Control-C from the keyboard) then using the same invocation again with the addition of "oflag=resume" will attempt to restart the copy from the point of the interrupt (or just before that point). It is harmless to use "oflag=resume" when OFILE doesn't exist or is zero length. If the length of OFILE is greater than or equal to the length implied by a ddpt invocation that includes "oflag=resume" then no further data is copied.
self [io] [pt]
used together with trim flag to do a self trim (trim of segments of a pt device that contain all zeros). If OFILE is not given, then it is set to the same as IFILE. If SEEK is not given it set to the same value as SKIP (possibly adjusted if IBS and OBS are different). Implicitly sets "nowrite" flag.
sparing [o] [reg,blk,pt]
during the copy each IBS * BPT byte segment is read from IFILE into a buffer. Then, instead of writing that buffer to OFILE, the corresponding segment is read from OFILE into another buffer. If the two buffers are different, the former buffer is written to the OFILE. If the two buffers compare equal then the write to OFILE is not performed. Write sparing is useful when a write operation is significantly slower than a read. Under some conditions flash memory devices have slow writes plus an upper limit on the number of times the same cell can be rewritten. The granularity of the comparison can be reduced from the default IBS * BPT byte segment with the the OBPC value given to the "bpt=" option. The finest granularity is when OBPC is 1 which implies OBS bytes.
sparse [o] [reg,blk,pt]
after each IBS * BPT byte segment is read from IFILE, it is checked to see if it is all zeros. If so, that segment is not written to OFILE. See the section on SPARSE WRITES below. The granularity of the zero comparison can be reduced from the default IBS * BPT byte segment with the OBPC value given to the "bpt=" option.
ssync [o] [pt]
if OFILE is in "pt" mode then the SCSI SYNCHRONIZE CACHE command is sent to OFILE at the end of the copy.
strunc [o] [reg]
perform a sparse copy with a ftruncate system call to extend the length of the OFILE if required. See the sparse flag and the section on SPARSE WRITES below.
sync [io] [reg,blk]
causes the O_SYNC flag to be added to the open of IFILE and/or OFILE. See open(2) man page.
trim [io] [pt] [experimental]
similar logic to the "sparse" option. However instead of skipping segments that are full of zeros a "trim" command is sent to OFILE. Usually set as an oflag argument but for self trim can be used as an iflag argument (e.g. "iflag=self,trim"). Depending on the usage this may require the device to support "deterministic read zero after trim". See the TRIM, UNMAP AND WRITE SAME section below.
trunc [o] [reg]
if OFILE is a regular file then it is truncated prior to starting the copy. If SEEK is not given or 0 then OFILE is truncated to zero length; when SEEK is larger than zero the truncation takes place at file byte pointer SEEK*OBS. Ignored if "oflag=append". Conflicts with "oflag=sparing".


Bypassing writes of blocks full of zeros can save a lot of IO. However with regular files, bypassed writes at the end of the copy can lead to an OFILE which is shorter than it would have been without sparse writes. This can lead to integrity checking programs like md5sum and sha1sum generating different values.

This utility has two ways of handling this file length problem: writing the last block (even if it is full of zeros) or using the ftruncate system call. A third approach is to ignore the problem (i.e. leaving OFILE shorter). The ftruncate approach is used when "oflag=strunc" while the last block is written when "oflag=sparse". To ignore the file length issue use "oflag=sparse,sparse". Note that if OFILE's length is already correct or longer than required, no action is taken.

The support for sparse writing of regular files may depend on the OS, the file system and the settings of OFILE. POSIX makes few guarantees when the ftruncate system call is used to extend a file's length, as may occur when "oflag=strunc". Further, primitive file systems like VFAT may not accept sparse writes or simulate the effect by writing blocks of zeros. The latter approach will defeat any sparse writing performance gain.  


This is a new storage feature often associated with Solid State Disks (SSDs) or disk arrays with "thin provisioning". In the ATA command set (ACS-2) the relevant command is DATA SET MANAGEMENT with the TRIM bit set. In the SCSI command set (SBC-3) it is either the UNMAP or WRITE SAME command. Note there is no TRIM command however the term is frequently used in the technical press.

Trim is a way of telling a storage device that blocks are no longer needed. Keeping the pool of unwritten blocks large is important for the write performance of SSDs and the thrifty use of real storage in thin provisioned arrays. Currently file systems in recent OSes may issue trims associated with file deletes. The trim option in ddpt may be useful when a partition or a whole SSD is to be "deleted". Note that ddpt is bypassing file systems in that it only offers trim on pass-through (pt) devices.

This utility issues SCSI commands to pt devices and for "trim" currently issues a SCSI WRITE SAME(16) command with the UNMAP bit set. If the pt device is a SSD with a ATA interface then recent versions of Linux will translate the SCSI WRITE SAME to the ATA DATA SET MANAGEMENT command with the TRIM bit set. The maximum size of each "trim" command sent is the size of the copy buffer (i.e. IBS * BPT bytes). And that maximum can be reduced with the OBPC argument of the "bpt=" option.

The trim can be used various ways. One way is a copy where the copy buffer (or some part of it) is checked for zeros as is done by the sparse oflag. When a zero segment is found, a trim "command" is sent to the OFILE. For example:

   ddpt if=dsk.img bs=512 of=/dev/sdc oflag=pt,trim

The copy buffer is 64 KiB (since BPT and OBPC default to 128 when "bs=512") and it is checked for all zeros. If it is all zeros then a trim command is sent to the corresponding location of /dev/sdc which is accessed via the pt interface. If it is not all zeros then a SCSI WRITE command is sent. Another way is to trim all or part of a disk. To trim a whole disk (i.e. deleting all its data):

    ddpt if=/dev/zero bs=512 of=/dev/sdc oflag=pt,trim

A third way is to "self-trim" which is to only trim those parts of a disk that contain segments full of zeros:

    ddpt if=/dev/sdc skip=0x2300 bs=512 iflag=pt,self,trim count=0x1234f0

The "self" oflag automatically sets up the output side of the copy to send trim commands (if required) back the the same device (i.e. /dev/sdc). If this example was self-trimming a partition then the partition would start at LBA 0x2300 and be 0x1234f0 blocks long.

Some random product examples: the Intel X25-M G2 SSDs have trim with recent firmware and they do deterministic read zero after trim. The Seagate Pulsar SSD has an ATA interface which supports the deterministic reads of zero after the DATA SET MANAGEMENT command with the TRIM option.  


dd defaults "if=" and "of=" to stdin and stdout respectively. This follows Unix filter conventions. However since dd and ddpt are often used to read binary data for timing purposes, having to supply "of=/dev/null" can be easily forgotten. Without it dd will potentially spew binary data on the console. So ddpt has changed its defaults: the "if=IFILE" is now mandatory and to read from stdin "if=-" can be used; "of=OFILE" remains optional but its default changes to "/dev/null" (or "NUL" in Windows). To send output to stdout ddpt accepts "of=-".

dd truncates OFILE unless "conv=notrunc" is given. When dd truncates, it truncates to zero length unless SEEK is greater than zero. ddpt does not truncate OFILE by default. If OFILE exists it will be overwritten. The overwrite starts at block zero unless SEEK or "oflag=append" is given. If OFILE is a regular file then "oflag=trunc" (or "conv=trunc") will truncate OFILE prior to the copy.

Numeric arguments to ddpt can be given in hexadecimal, either with a leading "0x" or "0X" or with a trailing "h". Note that dd accepts "0x123" but interprets it as "0 * 123" (i.e. zero). ddpt will also interpret "x" as multiplies unless the left operand is zero (e.g. "0x123"). So both dd and ddpt will interpret "skip=2x123" as "skip=246".

Terabyte size disks make it impractical to copy all the data into a buffer before writing it out. Therefore both dd and ddpt read a relatively small amount of data into a copy (or transfer) buffer then write it out to the destination, repeating this process until the COUNT is exhausted.

A major difference in ddpt is the addition of BPT to control the size of the copy buffer. With dd, IBS is the size of the copy buffer and the unit of SKIP and COUNT. With ddpt, IBS * BPT is the size of the copy buffer and IBS is the unit of SKIP and COUNT. This allows ddpt to have its IBS set to the logical block size of IFILE without unduly restricting the size of the copy buffer. And setting IBS (and OBS for OFILE) accurately is required when the pass-through interface is used since with the SCSI READ and WRITE commands the logical block size is implicit.

The way dd handles its copy buffer (outlined in SUSv4 description of dd) is relatively complex, especially when IBS and OBS are different sizes. The restriction that ddpt places on IBS and OBS ( i.e. (((IBS*BPT) % OBS) == 0) ) means that a single copy buffer can be used since its size is a multiple of both IBS and OBS. Being able to precisely define the copy buffer size in ddpt makes sparse writing, write sparing and trim operations simpler to define and the user to control.

ddpt does not support dd's "cbs=" option (conversion block size). If the "cbs=" option is given to ddpt then it is ignored.  


A partial write is a write to the OFILE of less than OBS bytes. This typically occurs at the end of a copy. dd can do partial writes. ddpt does partial writes to regular files and fifos (including stdout). However ddpt ignores partial writes when OFILE is a block device or a pt device. When ddpt ignores a partial write, it sends a warning to the console (stderr).

At the end of the copy two lines are output to the console:

   <in_full>+<in_partial> records in

   <out_full>+<out_partial> records out

The "records in" line is the number of full input blocks (each of IBS bytes) that have been read plus the number of partial blocks ( usually less than IBS bytes) that have been read. Following the lead of dd when 'iflag=coe' is active a block that cannot be read (and has zeros substituted for its output) is regarded as a partial read. The "records out" line is the number of full output blocks (each of OBS bytes) that have been written plus the number of partial blocks (usually less than OBS bytes) that have been written.

Block devices (e.g. /dev/sda and /dev/hda) can be given for IFILE. If neither 'iflag=direct' nor 'iflag=pt' is given then normal block IO involving buffering and caching is performed. If 'iflag=direct' is given then the buffering and caching is bypassed (this is applicable to both SCSI devices and ATA disks). When 'iflag=pt' is given SCSI commands are sent to the device which bypasses most of the actions performed by the block layer. The same applies for block devices given for OFILE.

BPT, BS, COUNT, IBS, OBPC, OBS, SKIP and SEEK may include one of these multiplicative 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 continues 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>; however the combinations "0x" and "0X" are treated differently, see the next paragraph. These multiplicative suffixes are compatible with GNU's dd command (since 2002) which claims compliance with the SI and with IEC 60027-2 standards.

Alternatively numerical values can be given in hexadecimal preceded by either "0x" or "0X" (or with a trailing "h" or "H"). When hex numbers are given, multipliers cannot be used.

The COUNT, SKIP and SEEK arguments can take 64 bit values (i.e. very big numbers). Other numerical values are limited to what can fit in a signed 32 bit number.

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.

Disk partition information can often be found with fdisk(8) [the "-ul" argument is useful in this respect]. Also parted(8) can be used like this: 'parted /dev/sda unit s print' .

For pt 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 ddpt should not be used on tape devices. If the largest block address of the requested transfer exceeds a 32 bit block number (i.e 0xffffffff) then a warning is issued and the sg device is accessed via SCSI READ(16) and WRITE(16) commands.

The attributes of a block device (e.g. partitions) are ignored when the pt flag is used. Hence the whole device is read (rather than just the second partition) by this invocation:

   ddpt if=/dev/sdb2 iflag=pt of=t bs=512

Assuming /dev/sdb and /dev/sg2 refer to the same device, then after the following two invocations, the contents of the files "t", "tt" and "ttt" should be same:

   ddpt if=/dev/sdb of=tt bs=512

   ddpt if=/dev/sg2 of=ttt bs=512  


The examples in this page use Linux device names. For suitable device names in other supported Operating Systems see this web page: . The sg3_utils(8) man page in the sg3_utils package also covers device naming.

ddpt usage looks quite similar to dd:

   ddpt 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 iflag=direct of=t bs=512 count=1000000

although dd's speed may improve if bs was larger and count was suitably reduced. The use of the 'iflag=direct' option bypasses the buffering and caching that is usually done on a block device.

The dd command's bs argument can be thought of as roughly equivalent to ddpt's bs*bpt . dd almost assumes buffering on a block device and will work as long as bs is a multiple of the actual logical block size. Since ddpt can work at a lower level in some cases the bs argument must be a disk's actual logical block size. Thus the bpt argument was introduced to make the copy more efficient. So these two invocations are roughly equivalent:

   dd if=/dev/sda of=t bs=8k count=64

   ddpt if=/dev/sda of=t bs=512 bpt=16 count=1k

In both cases the total number of bytes moved is bs*count . And that will be done by reading 8k (8192 bytes) into a buffer then writing out that buffer to the file t. The read write sequence continues until the count is complete or an error occurs.

The 'of2=' option can save time when the input would otherwise need to be read twice. For example, to copy data and take a md5sum of it without needing to re-read the data:

  mkfifo fif

  md5sum fif &

  ddpt if=/dev/sg3 iflag=coe of=sg3.img oflag=sparse of2=fif bs=512

This will image /dev/sg3 (e.g. an unmounted disk) and place the contents in the (sparse) file sg3.img . Without re-reading the data it will also perform a md5sum calculation on the image.

Now we use sparse writing logic to get some idea of how many blocks on a disk are full of zeros. After a SCSI FORMAT or an ATA SECURITY ERASE command a disk may be all zeros.

   ddpt if=/dev/sdc bs=512 oflag=sparse

Since no "of=" option is given, output goes to /dev/null so nothing is actually written so the "records out" will be zero. However there will be a count of "records in" and "bypassed records out". If /dev/sdc is full of zeros then "records in" and "bypassed records out" will be the same. Since the "bpt=" option is not given it defaults to "bpt=128,128" so the copy buffer will be 64 KiB and the sparse check for zeros will be done with 64 KiB (128 block) granularity.

For examples of the trim and self,trim options see the section above on TRIM, UNMAP AND WRITE SAME.

Following is an example run on a Windows OS using the '--wscan' option which shows the available device names (e.g. PD1) and the associated volume name(s):

   ddpt -w
PD0 [C] FUJITSU MHY2160BH 0000
PD1 [DF] WD 2500BEV External 1.05 WD-WXE90

So, for example, volumes D: and F: reside on PhysicalDisk1 (abbreviated to "PD1") which is manufactured by WD (Western Digital).

Further examples can be found on this web page: . There is a text file called ddpt_examples.txt in the "doc" directory of this package's distribution tarball.  


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 and the copy continues. All output caused by signals is sent to stderr.  


To aid scripts that call ddpt, the exit status is set to indicate success (0) or failure (1 or more). Note that some of the lower values correspond to the SCSI sense key values. The exit status values are:
syntax error. Either illegal command line options, options with bad arguments or a combination of options that is not permitted.
the device reports that it is not ready for the operation requested. The device may be in the process of becoming ready (e.g. spinning up but not at speed) so the utility may work after a wait.
the device reports a medium or hardware error (or a blank check). For example an attempt to read a corrupted block on a disk will yield this value.
the device reports an "illegal request" with an additional sense code other than "invalid operation code". This is often a supported command with a field set requesting an unsupported capability.
the device reports a "unit attention" condition. This usually indicates that something unrelated to the requested command has occurred (e.g. a device reset) potentially before the current SCSI command was sent. The requested command has not been executed by the device. Note that unit attention conditions are usually only reported once by a device.
the device reports an illegal request with an additional sense code of "invalid operation code" which means that it doesn't support the requested command.
the device reports an aborted command. In some cases aborted commands can be retried immediately (e.g. if the transport aborted the command due to congestion).
the utility is unable to open, close or use the given IFILE or OFILE. The given file name could be incorrect or there may be permission problems. Adding the -v option may give more information.
the device reports it has a check condition but "no sense". It is unlikely that this value will occur as an exit status.
the device reports a "recovered error". The requested command was successful. Most likely a utility will report a recovered error to stderr and continue, probably leaving the utility with an exit status of 0 .
the command sent to device has timed out. This occurs in Linux only; in other ports a command timeout will appear as a transport (or OS) error.
the flock flag has been given on a device and some other process holds the advisory exclusive lock.
the response to a SCSI command failed sanity checks.
the device reports it has a check condition but the error doesn't fit into any of the above categories.
any errors that can't be categorized into values 1 to 98 may yield this value. This includes transport and operating system errors after the command has been sent to the device.


Written by Doug Gilbert  


Report bugs to <dgilbert at interlog dot com>.  


Copyright © 2008-2011 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.  


There is a web page discussing ddpt at

The lmbench package contains lmdd which is also interesting. For moving data to and from tapes see dt which is found at

To change mode parameters that effect a SCSI device's caching and error recovery see sdparm(sdparm)

To scan and repair disk partitions see TestDisk (testdisk).

Additional references: dd(1), ddrescue(GNU), open(2), flock(2), sg_dd,sg3_utils(sg3_utils)




This document was created by man2html, using the manual pages.
Time: 08:00:48 GMT, June 15, 2011