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man pages section 1M: System Administration Commands

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Updated: July 2017

sg_write_same (1m)


sg_write_same - send the SCSI WRITE SAME command


sg_write_same  [--10]  [--16]  [--32] [--anchor] [--grpnum=GN] [--help]
[--in=IF] [--lba=LBA] [--lbdata] [--num=NUM] [--pbdata]  [--timeout=TO]
[--unmap]  [--verbose]  [--version]  [--wrprotect=WPR]  [--xferlen=LEN]


SG_WRITE_SAME(8)                   SG3_UTILS                  SG_WRITE_SAME(8)

       sg_write_same - send the SCSI WRITE SAME command

       sg_write_same  [--10]  [--16]  [--32] [--anchor] [--grpnum=GN] [--help]
       [--in=IF] [--lba=LBA] [--lbdata] [--num=NUM] [--pbdata]  [--timeout=TO]
       [--unmap]  [--verbose]  [--version]  [--wrprotect=WPR]  [--xferlen=LEN]

       Send the SCSI WRITE SAME (10, 16 or 32 byte) command  to  DEVICE.  This
       command  writes  the given block NUM times to consecutive blocks on the
       DEVICE starting at logical block address LBA.

       The length of the block to be written multiple times is  obtained  from
       either  the  LEN argument, or the length of the given input file IF, or
       by calling READ CAPACITY(16) on DEVICE. The contents of the block to be
       written are obtained from the input file IF or zeroes are used. If READ
       CAPACITY(16) is called (which implies IF was not given) and the PROT_EN
       bit  is  set  then  an extra 8 bytes (i.e.  more than the logical block
       size) of 0xff are sent. If READ CAPACITY(16)  fails  then  READ  CAPAC-
       ITY(10) is used to determine the block size.

       If  neither  --10,  --16  nor --32 is given then WRITE SAME(10) is sent
       unless one of the following conditions  is  met.   If  LBA  (plus  NUM)
       exceeds 32 bits, NUM exceeds 65535, or the --unmap option is given then
       WRITE SAME(16) is sent.  The --10, --16 and --32 options  are  mutually

       In  SBC-3 revision 26 the UNMAP and ANCHOR bits were added to the WRITE
       SAME (10) command. Since the UNMAP bit has been in WRITE SAME (16)  and
       WRITE  SAME  (32)  since  SBC-3 revision 18, the lower of the two (i.e.
       WRITE SAME (16)) is the default when the --unmap option is  given.   To
       send WRITE SAME (10) use the --10 option.

       Take  care:  The  WRITE SAME(10, 16 and 32) commands interpret a NUM of
       zero as write to the end of DEVICE. This utility defaults NUM  to  1  .
       The  WRITE SAME commands have no IMMED bit so if NUM is large (or zero)
       then an invocation of this utility could take a long time,  potentially
       as  long as a FORMAT UNIT command. In such situations the command time-
       out value TO may need to be increased from its default value of 60 sec-
       onds.  In SBC-3 revision 26 the WSNZ (write same no zero) bit was added
       to the Block Limits VPD page [0xB0]. If set  the  WRITE  SAME  commands
       will not accept a NUM of zero. The same SBC-3 revision added the "Maxi-
       mum Write Same Length" field to the Block Limits VPD page.

       The Logical Block Provisioning VPD page [0xB2] contains  the  LBWS  and
       LBW10 bits. If LBWS is set then WRITE SAME (16) supports the UNMAP bit.
       If LBWS10 is set then WRITE SAME (10) supports the UNMAP bit. If either
       LBWS  or  LBWS10 is set and the WRITE SAME (32) is supported then WRITE
       SAME (32) supports the UNMAP bit. This is as of SBC-3 revision 26.

       As a precaution against an  accidental  'sg_write_same  /dev/sda'  (for
       example) overwriting LBA 0 on /dev/sda with zeroes, at least one of the
       --in=IF, --lba=LBA or --num=NUM options must be given.  Obviously  this
       utility can destroy a lot of user data so check the options carefully.

       Arguments to long options are mandatory for short options as well.

       -R, --10
              send  a  SCSI  WRITE SAME (10) command to DEVICE. The ability to
              set the --unmap (and --anchor) options to this command was added
              in SBC-3 revision 26.

       -S, --16
              send a SCSI WRITE SAME (16) command to DEVICE.

       -T, --32
              send a SCSI WRITE SAME (32) command to DEVICE.

       -a, --anchor
              sets the ANCHOR bit in the cdb. Introduced in SBC-3 revision 22.
              That draft requires the --unmap option to also be specified.

       -g, --grpnum=GN
              sets the 'Group number' field to GN.  Defaults  to  a  value  of
              zero.  GN should be a value between 0 and 31.

       -h, --help
              output the usage message then exit.

       -i, --in=IF
              read data (binary) from file named IF and use it as the data out
              buffer for the SCSI WRITE SAME command. The length of  the  data
              out buffer is --xferlen=LEN or, if that is not given, the length
              of the IF file. If IF is "-" then stdin is read. If this  option
              is not given then 0x00 bytes are used as fill with the length of
              the data out buffer obtained from --xferlen=LEN  or  by  calling
              READ  CAPACITY(16  or 10).  If the response to READ CAPACITY(16)
              has the PROT_EN bit set then data out buffer  size  is  modified
              accordingly with the last 8 bytes set to 0xff.

       -l, --lba=LBA
              where  LBA  is the logical block address to start the WRITE SAME
              command.  Defaults to lba 0 which is a dangerous block to  over-
              write  on a disk that is in use. Assumed to be in decimal unless
              prefixed with '0x' or has a trailing 'h'.

       -L, --lbdata
              sets the LBDATA bit in the WRITE SAME cdb.

       -n, --num=NUM
              where NUM is the number of blocks, starting at LBA, to write the
              data  out  buffer  to. The default value for NUM is 1. The value
              corresponds to the 'Number of logical blocks' field in the WRITE
              SAME  cdb. Note that a value of 0 in NUM is interpreted as write
              the data out buffer on every block starting at LBA to the end of
              the DEVICE.

       -P, --pbdata
              sets the PBDATA bit in the WRITE SAME cdb.

       -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 com-
              mand  may require considerably more time than 60 seconds to com-

       -U, --unmap
              sets the UNMAP bit in the WRITE SAME(10, 16  and  32)  cdb.  See
              UNMAP section below.

       -v, --verbose
              increase the degree of verbosity (debug messages).

       -V, --version
              output version string then exit.

       -w, --wrprotect=WPR
              sets the "Write protect" field in the WRITE SAME cdb to WPR. The
              default value is zero. WPR should be a value between  0  and  7.
              When WPR is 1 or greater, and the disk's protection type is 1 or
              greater, then  8  extra  bytes  of  protection  information  are
              expected  or generated (to place in the commmand's data out buf-

       -x, --xferlen=LEN
              where LEN is the data out buffer length. Defaults to the  length
              of  the  IF  file or, if that is not given, then the READ CAPAC-
              ITY(16 or 10) command is used to find the 'Logical block  length
              in  bytes'.  That  figure  may  be  increased  by 8 bytes if the
              DEVICE's protection type is 1 or greater and the WRPROTECT field
              (see  --wrprotect=WPR)  is 1 or greater. If both this option and
              the IF option are given and LEN exceeds the  length  of  the  IF
              file  then LEN is the data out buffer length with zeroes used as
              pad bytes.

       Logical block provisioning is a new term introduced in  SBC-3  revision
       25  for  the ability to mark blocks as unused. It is closely related to
       the ATA DATA SET MANAGEMENT command with the "Trim" bit set. For  large
       storage arrays, it is a way to provision less physical storage than the
       READ CAPACITY command reports is available, potentially allocating more
       physical storage when WRITE commands require it. For flash memory it is
       a way of potentially saving power (and perhaps access time) when it  is
       known large sections (or almost all) of the flash memory is not in use.

       Support  for  logical  block provisioning is indicated by the LBPME bit
       being set in the READ CAPACITY(16) command response (see the sg_readcap
       utility).   That  implies  at  least one of the UNMAP or WRITE SAME(16)
       commands is implemented. If the UNMAP command is implemented  then  the
       "Maximum  unmap  LBA  count" and "Maximum unmap block descriptor count"
       fields in the Block Limits VPD page should both be greater  than  zero.
       The  READ CAPACITY(16) command response also contains a LBPRZ bit which
       if set means that if unmapped blocks  are  read  then  zeroes  will  be
       returned  for  the  data (and if protection information is active, 0xff
       bytes are returned for that). In SBC-3 revision 27 the same  LBPRZ  bit
       was added to the Logical Block Provisioning VPD page.

       In SBC-3 revision 25 the LBPU and ANC_SUP bits where added to the Logi-
       cal Block Provisioning VPD page. When LBPU is set it indicates that the
       device  supports the UNMAP command (see the sg_unmap utility). When the
       ANC_SUP bit is set it indicates the device supports anchored LBAs.

       When the UNMAP bit is set in the cdb then the data out buffer  is  also
       sent.   Additionally the data section of that data out buffer should be
       full of 0x0 bytes while the data protection block, 8 bytes at  the  end
       if  present,  should  be set to 0xff bytes. If these conditions are not
       met and the LBPRZ bit is set then the UNMAP bit is ignored and the data
       out  buffer  is  written to the DEVICE as if the UNMAP bit was zero. In
       the absence of the --in=IF option, this utility will  attempt  build  a
       data  out  buffer  that meets the requirements for the UNMAP bit in the
       cdb to be acted on by the DEVICE.

       Logical blocks may also be unmapped by the SCSI UNMAP and  FORMAT  UNIT
       commands (see the sg_unmap and sg_format utilities).

       See attributes(5) for descriptions of the following attributes:

       |Availability   | system/storage/sg3_utils |
       |Stability      | Uncommitted              |
       Various  numeric  arguments  (e.g. LBA) may include multiplicative suf-
       fixes or be given in hexadecimal. See the "NUMERIC  ARGUMENTS"  section
       in the sg3_utils(8) man page.

       The  exit status of sg_write_same is 0 when it is successful. Otherwise
       see the sg3_utils(8) man page.

       One simple usage is to write blocks of  zero  from  (and  including)  a
       given LBA:

         sg_write_same --lba=0x1234 --num=63 /dev/sdc

       Since --xferlen=LEN has not been given, then this utility will call the
       READ CAPACITY command on /dev/sdc to determine the number of bytes in a
       logical  block.   Let us assume that is 512 bytes. Since --in=IF is not
       given a block of zeroes is assumed. So 63 blocks of zeroes (each  block
       containing 512 bytes) will be written from (and including) LBA 0x1234 .
       Note that only one block of zeroes is passed to  the  SCSI  WRITE  SAME
       command in the data out buffer (as required by SBC-3).

       A  similar  example  follows but in this case the blocks are "unmapped"
       ("trimmed" in ATA speak) rather than zeroed:

         sg_write_same --unmap -L 0x1234 -n 63 /dev/sdc

       Note that if the LBPRZ bit in the READ  CAPACITY(16)  response  is  set
       (i.e.   LPPRZ is an acronym for logical block provisioning read zeroes)
       then these two examples do the same thing, at least seen from the point
       of view of subsequent reads.

       This  utility  can also be used to write protection information (PI) on
       disks formatted with a protection type greater than zero. PI is 8 bytes
       of  extra  data appended to the user data of a logical block: the first
       two bytes are a CRC (the "guard"), the next two bytes are the "applica-
       tion tag" and the last four bytes are the "reference tag". With protec-
       tion types 1 and 2 if the application tag  is  0xffff  then  the  guard
       should not be checked (against the user data).

       In  this example we assume the logical block size (of the user data) is
       512 bytes and the disk has been formatted with protection type 1. Since
       we are going to modify LBA 2468 then we take a copy of it first:

         dd if=/dev/sdb skip=2468 bs=512 of=2468.bin count=1

       The following command line sets the user data to zeroes and the PI to 8
       0xFF bytes on LBA 2468:

         sg_write_same --lba=2468 /dev/sdb

       Reading back that block should be successful  because  the  application
       tag  is 0xffff which suppresses the guard (CRC) check (which would oth-
       erwise be wrong):

         dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1

       Now an attempt is made to create a binary file with zeroes in the  user
       data,  0x0000 in the application tag and 0xff bytes in the other two PI
       fields. It is awkward to create 0xff bytes in a file (in Unix)  as  the
       "tr" command below shows:

         dd if=/dev/zero bs=1 count=512 of=ud.bin
         tr "\000" "\377" < /dev/zero | dd bs=1 of=ff_s.bin count=8
         cat ud.bin ff_s.bin > lb.bin
         dd if=/dev/zero bs=1 count=2 seek=514 conv=notrunc of=lb.bin

       The  resulting  file  can be viewed with 'hexdump -C lb.bin' and should
       contain 520 bytes. Now that file can be written to LBA 2468 as follows:

         sg_write_same --lba=2468 wrprotect=3 --in=lb.bin /dev/sdb

       Note the --wrprotect=3 rather than being set to 1, since  we  want  the
       WRITE SAME command to succeed even though the PI data now indicates the
       user data is corrupted. When an attempt is made to  read  the  LBA,  an
       error should occur:

         dd if=/dev/sdb skip=2468 bs=512 of=/dev/null count=1

       dd  errors are not very expressive, if dmesg is checked there should be
       a line something like this: "[sdb]  Add.  Sense:  Logical  block  guard
       check  failed".  The  block  can be corrected by doing a "sg_write_same
       --lba=1234 /dev/sdb" again or restoring the original contents  of  that

         dd if=2468.bin bs=512 seek=2468 of=/dev/sdb conv=notrunc count=1

       Hopefully  the  dd  command would never try to truncate the output file
       when it is a block device.

       Written by Douglas Gilbert.

       Report bugs to <dgilbert at interlog dot com>.

       Copyright (C) 2009-2011 Douglas Gilbert
       This software is distributed under a FreeBSD license. There is NO  war-
       ranty;  not  even  for MERCHANTABILITY or FITNESS FOR A PARTICULAR PUR-


       This    software    was    built    from    source     available     at
       https://java.net/projects/solaris-userland.    The  original  community
       source was downloaded from  http://sg.danny.cz/sg/p/sg3_utils-1.33.tgz

       Further information about this software can be found on the open source
       community website at http://sg.danny.cz/sg/sg3_utils.html.

sg3_utils-1.33                   November 2011                SG_WRITE_SAME(8)