sg_write_same man page on Scientific
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SG_WRITE_SAME(8) SG3_UTILS SG_WRITE_SAME(8)
NAME
sg_write_same - send the SCSI WRITE SAME command
SYNOPSIS
sg_write_same [--16] [--32] [--grpnum=GN] [--help] [--in=IF]
[--lba=LBA] [--lbdata] [--num=NUM] [--pbdata] [--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.
The length of the block to be written multiple times is obtained from
the LEN argument, 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 CAPAC‐
ITY(16) is called (which implies IF was not given) and the PROT_EN bit
is set then the last 8 bytes are set to 0xff.
By default WRITE SAME(10) is sent unless LBA (plus NUM) exceeds 32
bits, NUM exceeds 65535 or the --unmap option is given. In these cases
WRITE SAME(16) is sent.
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.
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.
This utility is based SBC-3 draft revision 19 found at
http://www.t10.org .
OPTIONS
Arguments to long options are mandatory for short options as well.
-S, --16
send a SCSI WRITE SAME (16) command to DEVICE. The default
action (in the absence of this option and the --32 option) is to
send a SCSI WRITE SAME (10) command.
-T, --32
send a SCSI WRITE SAME (32) command to DEVICE. The default
action (in the absence of this option and the --16 option) is to
send a SCSI WRITE SAME (10) command.
-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). If the response to READ CAPACITY(16) has the
PROT_EN bit set then data out buffer size is modified accord‐
ingly 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‐
plete.
-U, --unmap
sets the UNMAP bit in the WRITE SAME(16 and 32) cdb. When this
option is given the default command becomes WRITE SAME(16) since
the WRITE SAME(10) command does not offer this bit (in SBC-3
draft revision 19). 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.
-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) command is called on the DEVICE 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 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.
UNMAP
Thin provisioning is the sophisticated term used in SBC-3 revision 19
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 thin provisioning is indicated by the TPE bit being set in
the READ CAPACITY(16) command response. That implies at least one of
the UNMAP or WRITE SAME(16) commands is implemented. If the UNMAP com‐
mand 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 TPRZ bit which if set means that if unmapped
blocks are read then zeroes will be returned for the data (and if pro‐
tection information is active, 0xff bytes are returned for that).
In the cases of WRITE SAME(16) and WRITE SAME(32) 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 TPRZ 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). Support for thin
provisioning is indicated by the TPE bit in the response to the SCSI
READ CAPACITY (16) command (see the sg_readcap utility).
NOTES
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.
EXIT STATUS
The exit status of sg_write_same is 0 when it is successful. Otherwise
see the sg3_utils(8) man page.
AUTHORS
Written by Douglas Gilbert.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright © 2009 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‐
POSE.
SEE ALSO
sg_format,sg_readcap,sg_unmap(sg3_utils)
sg3_utils-1.28 June 2009 SG_WRITE_SAME(8)
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