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COMPARE AND WRITE(8)                        SG3_UTILS                        COMPARE AND WRITE(8)

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  concatenated  compare  and  write  buffers are fetched from the file indicated by the
       --in=IF option.

       Those buffers are expected to each contain NUM blocks of data. The compare starts at logi-
       cal 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 status 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
       command 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 imple-
       ment 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
       documents can be found at http://www.t10.org .

OPTIONS
       Arguments to long options are mandatory for short options as well.  The  options  are  ar-
       ranged 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_nv
              Set  the FUA_NV bit in the COMPARE AND WRITE CDB. This bit was removed in SBC-3 re-
              vision 35d and its position marked as "reserved".

       -g, --grpnum=GN
              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=IF
              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=IF
              The same as the --in option.

       -D, --inw=WF
              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  COM-
              PARE AND WRITE command. Note that when this option is given then the --in=IF is ex-
              pected to hold the associated compare buffer.

       -l, --lba=LBA
              where LBA is the logical block address to start the COMPARE AND WRITE command.  As-
              sumed to be in decimal unless prefixed with '0x' or has a trailing 'h'.

       -n, --num=NUM
              where  NUM  is  the number of blocks, starting at LBA, to read and compare with the
              verify 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=TO
              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=WP
              set  the  WRPROTECT field in the cdb to WP. The default value is 0 which implies no
              protection information is sent (along with the user data) by this utility.

       -x, --xferlen=LEN
              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
              additional protection information) then this default will  be  incorrect;  the  use
              must supply the correct value for LEN

NOTES
       Various  numeric  arguments  (e.g. LBA) may include multiplicative suffixes or be given in
       hexadecimal. See the "NUMERIC ARGUMENTS" section in the sg3_utils(8) 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 immedi-
       ately (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
       binary 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
       COMPARE 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) 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 shahar.salzman AT kaminario.com or dgilbert AT interlog.com

COPYRIGHT
       Copyright (C) 2012-2020 Kaminario Technologies LTD
       Redistribution and use in source and binary forms, with or without modification, are  per-
       mitted provided that the following conditions are met:
       * Redistributions of source code must retain the above copyright notice, this list of con-
       ditions and the following disclaimer.
       * Redistributions in binary form must reproduce the above copyright notice, this  list  of
       conditions  and  the following disclaimer in the documentation and/or other materials pro-
       vided with the distribution.
       * Neither the name of the <organization> nor the names of its contributors may be used  to
       endorse or promote products derived from this software without specific prior written per-
       mission.

       THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"  AND  ANY  EX-
       PRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF MER-
       CHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL  Kami-
       nario Technologies LTD BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY,
       OR CONSEQUENTIAL 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 NEG-
       LIGENCE  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)

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