# phpman > man > sg_compare_and_write(8) COMPARE AND [WRITE(8)](https://www.chedong.com/phpMan.php/man/WRITE/8/markdown) SG3_UTILS COMPARE AND [WRITE(8)](https://www.chedong.com/phpMan.php/man/WRITE/8/markdown) ## NAME sg_compare_and_write - send the SCSI COMPARE AND WRITE command ## SYNOPSIS **sg**___**compare**___**and**___**write** [_--dpo_] [_--fua_] [_--fua_nv_] [_--grpnum=GN_] [_--help_] _--in=IF_ [_--inw=WF_] _--lba=LBA_ [_--num=NUM_] [_--quiet_] [_--timeout=TO_] [_--verbose_] [_--version_] [_--wrprotect=WP_] [_--xferlen=LEN_] _DEVICE_ ## DESCRIPTION Send the SCSI COMPARE AND WRITE command to _DEVICE_. This utility fetches a compare buffer and a write buffer from either one or two files. If the _--inw=WF_ option is given then the compare buffer is fetched from the file indicated by the _--in=IF_ while the write buffer is fetched from the file indicated by the _--inw=WF_. If the _--inw=WF_ option is not given then the con‐ catenated compare and write buffers are fetched from the file indicated by the _--in=IF_ op‐ tion. Those buffers are expected to each contain _NUM_ blocks of data. The compare starts at logical block address _LBA_ on the _DEVICE_ and if the comparison fails (i.e. the provided compare buffer does not equal the data at _LBA_ on the _DEVICE_) then the COMPARE AND WRITE command finishes with a sense key of MISCOMPARE. In this case this utility will complete and set an exit sta‐ tus of 14 (which happens to be the sense key value of MISCOMPARE). If the comparison succeeds then the provided write buffer is stored starting at _LBA_ for _NUM_ blocks on the _DEVICE_. The actual number of bytes transferred in the data-out buffer of the COMPARE AND WRITE com‐ mand may need to be given by the user with the _--xferlen=LEN_ option. _LEN_ defaults to (2 * _NUM_ * 512) which is 1024 for the default _NUM_ of 1. If the block size is other than 512 then the user will need to use _--xferlen=LEN_ option. If protection information is given (indicated by a value of _WP_ other than 0 (the default)) then for a _NUM_ of 1 _LEN_ should be 1040 . Note that the SCSI READ CAPACITY command is not performed by this utility (e.g. to find the block size). The T10 definition of the SCSI COMPARE AND WRITE command requires that the _DEVICE_ implement the compare and optional write as an uninterrupted series of actions. Depending on some other _DEVICE_ settings a verify operation may occur prior to the compare. When a mismatch occurs between the compare buffer and the blocks starting at _LBA_ read from the _DEVICE_ the sense buffer containing the MISCOMPARE sense key causes several messages to be sent to stderr (including the offset of the first byte mismatch). To suppress these messages use the _--quiet_ option. With or without the _--quiet_ option the exit status will be set to 14. This command is defined in SBC-3 whose most recent revision is 36. SBC-3 and other SCSI docu‐ ments can be found at . ## OPTIONS Arguments to long options are mandatory for short options as well. The options are arranged in alphabetical order based on the long option name. ### -d --dpo Set the DPO bit in the COMPARE AND WRITE CDB ### -f --fua Set the FUA bit in the COMPARE AND WRITE CDB ### -F --fua Set the FUA_NV bit in the COMPARE AND WRITE CDB. This bit was removed in SBC-3 revi‐ sion 35d and its position marked as "reserved". ### -g --grpnum where _GN_ is the value to be placed in the group number field in the COMPARE AND WRITE CDB. ### -h --help output the usage message then exit. ### -i --in read data (binary) from file named _IF_. This will either be the combined compare and write buffers (when the _--inw=WF_ option is not given) or just the compare buffer (when the _--inw=WF_ option is given). If _IF_ is '-' then stdin (e.g. a pipe) is read. ### -C --inc The same as the **--in** option. ### -D --inw read data (binary) from file named _WF_. This will the write buffer that will become the second half of the data-out buffer sent to the _DEVICE_ associated with the COMPARE AND WRITE command. Note that when this option is given then the _--in=IF_ is expected to hold the associated compare buffer. ### -l --lba where _LBA_ is the logical block address to start the COMPARE AND WRITE command. Assumed to be in decimal unless prefixed with '0x' or has a trailing 'h'. ### -n --num where _NUM_ is the number of blocks, starting at _LBA_, to read and compare with the ver‐ ify instance. And given a match, the _NUM_ of blocks to write starting _LBA_. The default value for _NUM_ is 1. ### -q --quiet suppress the sense buffer messages associated with a MISCOMPARE sense key that would otherwise be sent to stderr. Still set the exit status to 14 which is the sense key value indicating a MISCOMPARE. ### -t --timeout where _TO_ is the command timeout value in seconds. The default value is 60 seconds. If _NUM_ is large (or zero) a WRITE SAME command may require considerably more time than 60 seconds to complete. ### -v --verbose increase the degree of verbosity (debug messages). ### -V --version output version string then exit. ### -w --wrprotect set the WRPROTECT field in the cdb to _WP_. The default value is 0 which implies no pro‐ tection information is sent (along with the user data) by this utility. ### -x --xferlen where _LEN_ is the data out buffer length in byte. It defaults to (2 * _NUM_ * 512) bytes. If the _DEVICE_ block size is other than 512 bytes or _WP_ is non-zero (implying addi‐ tional protection information) then this default will be incorrect; the use must sup‐ ply the correct value for _LEN_ ## NOTES Various numeric arguments (e.g. _LBA_) may include multiplicative suffixes or be given in hexa‐ decimal. See the "NUMERIC ARGUMENTS" section in the [sg3_utils(8)](https://www.chedong.com/phpMan.php/man/sg3utils/8/markdown) man page. ## EXAMPLES Before overwriting the first two blocks of whatever (SCSI) storage device that is chosen, take a small backup. The logical block size is assumed to be 512 bytes. Take a copy (in backup01.bin) of the first two blocks:: # sg_dd if=/dev/sg1 bs=512 of=backup01.bin count=2 2+0 records in 2+0 records out **WARNING:** if /dev/sg1 corresponds to a disk on your system that contains currently mounted file systems, do _not_ continue. If you can, unmount all file systems on that disk. If that is not possible, use another disk with no mounted file systems on it. In Linux the scsi_debug driver is a good candidate for experimentation. Now fill the first block with 0xff bytes: # sg_dd iflag=ff bs=512 of=/dev/sg1 count=1 1+0 records in 1+0 records out and the second block with 0x0 bytes: # sg_dd iflag=00 bs=512 seek=1 of=/dev/sg1 count=1 1+0 records in 1+0 records out Now copy those two blocks into a file: # sg_dd if=/dev/sg1 bs=512 of=ff00.bin count=2 2+0 records in 2+0 records out Now we can do a compare and write command. It is told to compare the first block (i.e. LBA 0) with the first block in the given file (i.e. ff00.bin). If they are equal (they should be both full of 0xff bytes). Since the compare succeeds, it will write the second block in ff00.bin over LBA 0: # sg_compare_and_write --in=ff00.bin --lba=0 --num=1 /dev/sg1 No news is good news. Now if we do that command again: # sg_compare_and_write --in=ff00.bin --lba=0 --num=1 /dev/sg1 Miscompare at byte offset: 0 [0x0] sg_compare_and_write failed: Miscompare This is expected. The first sg_compare_and_write ended up writing 0x0 bytes over LBA 0x0. The second sg_compare_and_write command compares LBA 0x0 with 0xff bytes and fails immediately (i.e. at byte offset: 0). Now we will overwrite the first 3 bytes of ff00.bin with 0x0: # sg_dd bs=1 iflag=00 of=ff00.bin count=3 3+0 records in 3+0 records out Notice the 'bs=1' operand. The dd utility (and thus sg_dd) is very useful for doing small bi‐ nary edits on a file. Now if we do that sg_compare_and_write again, it still fails but with a small difference: # sg_compare_and_write --in=ff00.bin --lba=0 --num=1 /dev/sg1 Miscompare at byte offset: 3 [0x3] sg_compare_and_write failed: Miscompare So the bytes at offset 0, 1, and 2 compared equal but not the byte at offset 3. The SCSI COM‐ PARE AND WRITE will stop on the first micompared byte. ## EXIT STATUS The exit status of sg_compare_and_write is 0 when it is successful. If the compare step fails then the exit status is 14. For other exit status values see the EXIT STATUS section in the [sg3_utils(8)](https://www.chedong.com/phpMan.php/man/sg3utils/8/markdown) man page. Earlier versions of this utility set an exit status of 98 when there was a MISCOMPARE. ## AUTHORS Written by Shahar Salzman. Maintained by Douglas Gilbert. Additions by Eric Seppanen. ## REPORTING BUGS Report bugs to or ## COPYRIGHT Copyright © 2012-2020 Kaminario Technologies LTD Redistribution and use in source and binary forms, with or without modification, are permit‐ ted provided that the following conditions are met: * Redistributions of source code must retain the above copyright notice, this list of condi‐ tions and the following disclaimer. * Redistributions in binary form must reproduce the above copyright notice, this list of con‐ ditions and the following disclaimer in the documentation and/or other materials provided with the distribution. * Neither the name of the nor the names of its contributors may be used to en‐ dorse or promote products derived from this software without specific prior written permis‐ sion. THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABIL‐ ITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL Kaminario Tech‐ nologies LTD BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSE‐ QUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE. ## SEE ALSO **sg**___**xcopy,** **sg**___**receive**___**copy**___**results(sg3**___**utils)** sg3_utils-1.46 November 2020 COMPARE AND [WRITE(8)](https://www.chedong.com/phpMan.php/man/WRITE/8/markdown)