path: root/doc/sg_write_same.8

.TH SG_WRITE_SAME "8" "January 2011" "sg3_utils\-1.31" SG3_UTILS
.SH NAME
sg_write_same \- send the SCSI WRITE SAME command
.SH SYNOPSIS
.B sg_write_same
[\fI\-\-10\fR] [\fI\-\-16\fR] [\fI\-\-32\fR] [\fI\-\-anchor\fR]
[\fI\-\-grpnum=GN\fR] [\fI\-\-help\fR] [\fI\-\-in=IF\fR]
[\fI\-\-lba=LBA\fR] [\fI\-\-lbdata\fR] [\fI\-\-num=NUM\fR]
[\fI\-\-pbdata\fR] [\fI\-\-timeout=TO\fR] [\fI\-\-unmap\fR]
[\fI\-\-verbose\fR] [\fI\-\-version\fR] [\fI\-\-wrprotect=WPR\fR]
[\fI\-\-xferlen=LEN\fR] \fIDEVICE\fR
.SH DESCRIPTION
.\" Add any additional description here
Send the SCSI WRITE SAME (10, 16 or 32 byte) command to \fIDEVICE\fR. This
command writes the given block \fINUM\fR times to consecutive blocks on
the \fIDEVICE\fR starting at logical block address \fILBA\fR.
.PP
The length of the block to be written multiple times is obtained from
the \fILEN\fR argument, the length of the given input file \fIIF\fR,
or by calling READ CAPACITY(16) on \fIDEVICE\fR. The contents of the
block to be written are obtained from the input file \fIIF\fR or
zeroes are used. If READ CAPACITY(16) is called (which implies \fIIF\fR
was not given) and the PROT_EN bit is set then the last 8 bytes are
set to 0xff. If READ CAPACITY(16) fails then READ CAPACITY(10) is
used to determine the block size.
.PP
If neither \fI\-\-10\fR, \fI\-\-16\fR nor \fI\-\-32\fR is given then
WRITE SAME(10) is sent unless one of the following conditions is met.
If \fILBA\fR (plus \fINUM\fR) exceeds 32 bits, \fINUM\fR exceeds 65535,
or the \fI\-\-unmap\fR option is given then WRITE SAME(16) is sent.
The \fI\-\-10\fR, \fI\-\-16\fR and \fI\-\-32\fR options are mutually
exclusive.
.PP
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 \fI\-\-unmap\fR option is given.
To send WRITE SAME (10) use the \fI\-\-10\fR option.
.PP
.B Take care:
The WRITE SAME(10, 16 and 32) commands interpret a \fINUM\fR of zero
as write to the end of \fIDEVICE\fR. This utility defaults \fINUM\fR to
1 . The WRITE SAME commands have no IMMED bit so if \fINUM\fR 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 \fITO\fR 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 accepts a \fINUM\fR of zero. The same
SBC\-3 revision added the "Maximum Write Same Length" field to the Block
Limits VPD page.
.PP
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.
.PP
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
\fI\-\-in=IF\fR, \fI\-\-lba=LBA\fR or \fI\-\-num=NUM\fR options must be
given. Obviously this utility can destroy a lot of user data so check the
options carefully.
.SH OPTIONS
Arguments to long options are mandatory for short options as well.
.TP
\fB\-R\fR, \fB\-\-10\fR
send a SCSI WRITE SAME (10) command to \fIDEVICE\fR. The ability to
set the \fI\-\-unmap\fR (and \fI\-\-anchor\fR) options to this command
was added in SBC\-3 revision 26.
.TP
\fB\-S\fR, \fB\-\-16\fR
send a SCSI WRITE SAME (16) command to \fIDEVICE\fR.
.TP
\fB\-T\fR, \fB\-\-32\fR
send a SCSI WRITE SAME (32) command to \fIDEVICE\fR.
.TP
\fB\-a\fR, \fB\-\-anchor\fR
sets the ANCHOR bit in the cdb. Introduced in SBC\-3 revision 22.
That draft requires the \fI\-\-unmap\fR option to also be specified.
.TP
\fB\-g\fR, \fB\-\-grpnum\fR=\fIGN\fR
sets the 'Group number' field to \fIGN\fR. Defaults to a value of zero.
\fIGN\fR should be a value between 0 and 31.
.TP
\fB\-h\fR, \fB\-\-help\fR
output the usage message then exit.
.TP
\fB\-i\fR, \fB\-\-in\fR=\fIIF\fR
read data (binary) from file named \fIIF\fR and use it as the data out
buffer for the SCSI WRITE SAME command. The length of the data out buffer
is \fI\-\-xferlen=LEN\fR or, if that is not given, the length of the \fIIF\fR
file. If \fIIF\fR 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 \fI\-\-xferlen=LEN\fR 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.
.TP
\fB\-l\fR, \fB\-\-lba\fR=\fILBA\fR
where \fILBA\fR 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'.
.TP
\fB\-L\fR, \fB\-\-lbdata\fR
sets the LBDATA bit in the WRITE SAME cdb.
.TP
\fB\-n\fR, \fB\-\-num\fR=\fINUM\fR
where \fINUM\fR is the number of blocks, starting at \fILBA\fR, to write the
data out buffer to. The default value for \fINUM\fR is 1. The value corresponds
to the 'Number of logical blocks' field in the WRITE SAME cdb. Note that a
value of 0 in \fINUM\fR is interpreted as write the data out buffer on every
block starting at \fILBA\fR to the end of the \fIDEVICE\fR.
.TP
\fB\-P\fR, \fB\-\-pbdata\fR
sets the PBDATA bit in the WRITE SAME cdb.
.TP
\fB\-t\fR, \fB\-\-timeout\fR=\fITO\fR
where \fITO\fR is the command timeout value in seconds. The default value is
60 seconds. If \fINUM\fR is large (or zero) a WRITE SAME command may require
considerably more time than 60 seconds to complete.
.TP
\fB\-U\fR, \fB\-\-unmap\fR
sets the UNMAP bit in the WRITE SAME(10, 16 and 32) cdb. See UNMAP section
below.
.TP
\fB\-v\fR, \fB\-\-verbose\fR
increase the degree of verbosity (debug messages).
.TP
\fB\-V\fR, \fB\-\-version\fR
output version string then exit.
.TP
\fB\-w\fR, \fB\-\-wrprotect\fR=\fIWPR\fR
sets the "Write protect" field in the WRITE SAME cdb to \fIWPR\fR. The
default value is zero. \fIWPR\fR should be a value between 0 and 7.
.TP
\fB\-x\fR, \fB\-\-xferlen\fR=\fILEN\fR
where \fILEN\fR is the data out buffer length. Defaults to the length of
the \fIIF\fR file or, if that is not given, then the READ CAPACITY(16 or 10)
command is called on the \fIDEVICE\fR and the 'Logical block length in
bytes' and the PROT_EN bit in the response are used to determine the
data out buffer length. If both this option and the \fIIF\fR option are
given and \fILEN\fR exceeds the length of the \fIIF\fR file then \fILEN\fR
is the data out buffer length with zeroes used as pad bytes.
.SH 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.
.PP
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).
.PP
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
\fIDEVICE\fR as if the UNMAP bit was zero. In the absence of the
\fI\-\-in=IF\fR 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 \fIDEVICE\fR. 
.PP
Logical blocks may also be unmapped by the SCSI UNMAP and FORMAT UNIT
commands (see the sg_unmap and sg_format utilities).
.SH NOTES
Various numeric arguments (e.g. \fILBA\fR) may include multiplicative
suffixes or be given in hexadecimal. See the "NUMERIC ARGUMENTS" section
in the sg3_utils(8) man page.
.SH EXIT STATUS
The exit status of sg_write_same is 0 when it is successful. Otherwise see
the sg3_utils(8) man page.
.SH AUTHORS
Written by Douglas Gilbert.
.SH "REPORTING BUGS"
Report bugs to <dgilbert at interlog dot com>.
.SH COPYRIGHT
Copyright \(co 2009\-2010 Douglas Gilbert
.br
This software is distributed under a FreeBSD license. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
.SH "SEE ALSO"
.B sg_format,sg_get_lba_status,sg_readcap,sg_unmap(sg3_utils)