Ubuntu Manpage: sg_write_same - send SCSI WRITE SAME command

Provided by: sg3-utils_1.36-1ubuntu1_amd64

NAME

       sg_write_same - send SCSI WRITE SAME command

SYNOPSIS

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

DESCRIPTION

       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.

       SBC-3  revision  35d  introduced a "no data-out buffer" (NDOB) bit which, if set, bypasses
       the requirement to send a single block of data to the DEVICE together  with  the  command.
       Only WRITE SAME (16 and 32 byte) support the NDOB bit.

       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
       zeros 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 CAPACITY(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 exclusive.

       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 timeout value
       TO may need to be increased from its default value of 60 seconds. 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
       "Maximum 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 zeros, 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.

OPTIONS

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 overwrite 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, --ndob
sets the NDOB bit in the WRITE SAME (16 and 32 byte) commands. Default is to clear
this bit. When this option is given then --in=IF is not allowed and --xferlen=LEN
can only be given if LEN is 0 .

-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 command may require considerably more time
than 60 seconds to complete.

-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 command's data out buffer).

-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 CAPACITY(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 zeros used as pad bytes.

UNMAP

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 zeros 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 Logical 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).

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.

       In  Linux  at  this  time  the sg driver does not support cdb sizes greater than 16 bytes.
       Hence a device node like /dev/sg1 which is associated with the sg driver  will  fail  with
       this  utility  if  the  --32 option is given (or implied by other options). The bsg driver
       with device nodes like /dev/bsg/6:0:0:1 does support cdb sizes greater than 16 bytes.

EXIT STATUS

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

EXAMPLES

       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 zeros is assumed. So 63 blocks of
       zeros (each block containing 512 bytes) will be written from (and including) LBA 0x1234  .
       Note that only one block of zeros 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 zeros) 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 "application tag" and the last four bytes are the  "reference  tag".  With  protection
       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 zeros 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 otherwise 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 zeros 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 LBA:

         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.

AUTHORS

       Written by Douglas Gilbert.

REPORTING BUGS

       Report bugs to <dgilbert at interlog dot com>.

COPYRIGHT

       Copyright © 2009-2013 Douglas Gilbert
       This  software  is distributed under a FreeBSD license. There is NO warranty; not even for
       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.