bionic (8) sg_write_verify.8.gz
NAME
sg_write_and_verify - send the SCSI WRITE AND VERIFY command
SYNOPSIS
sg_write_verify [--16] [--bytchk=BC] [--dpo] [--group=GN] [--help] [--ilen=ILEN] [--in=IF] --lba=LBA
[--num=NUM] [--repeat] [--timeout=TO] [--verbose] [--version] [--wrprotect=WP] DEVICE
DESCRIPTION
Send a SCSI WRITE AND VERIFY (10) or (16) command to DEVICE. The data to be written is read from the IF
file or, in its absence, a buffer full of 0xff bytes is used. The length of the data-out buffer sent with
the command is ILEN bytes or, if that is not given, then it is the length of the IF file.
The write operation is to the DEVICE's medium (optionally to its cache) starting at logical block address
LBA for NUM logical blocks. After the write to medium is performed a verify operation takes place which
may viewed as a medium read (with appropriate checks) but without the data being returned. Additionally,
if BS is set to one, the data read back from the medium in the verify operation is compared to the
original data-out buffer.
The relationship between the number of logical blocks to be written (i.e. NUM) and the length (in bytes)
of the data-out buffer (i.e. ILEN) may be simply found by multiplying the former by the logical block
size. However if the DEVICE has protection information (PI) then it becomes a bit more complicated. Hence
the calculation is left to the user with the default ILEN, in the absence of the IF file, being set to
NUM * 512.
For sending large amounts of data to contiguous logical blocks, a single WRITE AND VERIFY command may not
be appropriate (e.g. due to operating system limitations). In such cases see the REPEAT section below.
OPTIONS
Arguments to long options are mandatory for short options as well. The options are arranged in
alphabetical order based on the long option name.
-S, --16
Send a WRITE AND VERIFY(16) command. The default is to send a WRITE AND VERIFY(10) command unless
LBA or NUM are too large for the 10 byte variant.
-b, --bytchk=BC
where BC is the value to place in the command's BYTCHK field. Values between 0 and 3 (inclusive)
are accepted. The default is value is 0 which implies only a write to the medium then a verify
operation are performed. The only other value T10 defines currently is 1 which does performs an
additional comparison between the data-out buffer that was used by the write operation and the
contents of the logical blocks read back from the medium.
-d, --dpo
Set the DPO (disable page out) bit in the command. The default is to leave it clear.
-g, --group=GN
where GN is the value to place in the command's GROUP NUMBER field. Values between 0 and 31
(inclusive) are accepted. The default is value is 0.
-h, --help
output the usage message then exit.
-I, --ilen=ILEN
where ILEN is the number of bytes that will be placed in the data-out buffer. If the IF file is
given then no more than ILEN bytes are read from that file. If the IF file does not contain ILEN
bytes then an error is reported. If the IF file is not given then a data-out buffer with ILEN
bytes of 0xff is sent.
-i, --in=IF
read data (binary) from file named IF. If IF is "-" then stdin is used. This data will become the
data-out buffer and will be written to the DEVICE's medium. If BC is 1 then that data-out buffer
will be held until after the verify operation and compared to the data read back from the medium.
-l, --lba=LBA
where LBA is the logical block address to start the write to medium. Assumed to be in decimal
unless prefixed with '0x' or has a trailing 'h'. Must be provided.
-n, --num=NUM
where NUM is the number of blocks, starting at LBA, to write to the medium. The default value for
NUM is 1.
-R, --repeat
this option will continue to do WRITE AND VERIFY commands until the IF file is exhausted. This
option requires both the --ilen=ILEN and --in=IF options to be given. Each command starts at the
next logical block address and is for no more than NUM blocks. The last command may be shorter
with the number of blocks scaled as required. If there are residue bytes a warning is sent to
stderr. See the REPEAT section.
-t, --timeout=TO
where TO is the command timeout value in seconds. The default value is 60 seconds. If NUM is large
then 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) in the data-out buffer.
REPEAT
For data sizes around a megabyte and larger, it may be appropriate to send multiple SCSI WRITE AND VERIFY
commands due to operating system limitations (e.g. pass-through SCSI interfaces often limit the amount of
data that can be passed with a SCSI command). With this utility the mechanism for doing that is the
--repeat option.
In this mode the --ilen=ILEN and --in=IF options must be given. The ILEN and NUM values are treated as a
per SCSI command parameters. Up to ILEN bytes will be read from the IF file continually until it is
exhausted. If the IF file is stdin, reading continues until an EOF is detected. The data read from each
iteration becomes the data-out buffer for a new WRITE AND VERIFY command.
The last read from the file (or stdin) may read less than ILEN bytes in which case the number of logical
blocks sent to the last WRITE AND VERIFY is scaled back accordingly. If there is a residual number of
bytes left after that scaling then that is reported to stderr.
If an error occurs then that is reported to stderr and via the exit status and the utility stops at that
point.
NOTES
Other SCSI WRITE commands have a Force Unit Access (FUA) bit but that is set (implicitly) by WRITE AND
VERIFY commands hence there is no option to set it. The data-out buffer may still additionally be placed
in the DEVICE's cache and setting the DPO bit is a hint not to do that.
Normal SCSI WRITEs can be done with the ddpt and the sg_dd utilities. The SCSI WRITE SAME command can be
done with the sg_write_same utility while the SCSI COMPARE AND WRITE command (sg_compare_and_write
utility) offers a "test and set" facility.
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.
EXIT STATUS
The exit status of sg_write_verify is 0 when it is successful. If the verify operation fails that is
typically indicated with a medium error which leads to an exit status of 3.
If BC is set to 1 and the comparison it causes fails this utility will indicate the miscompare with an
exit status of 14. For other exit status values see the EXIT STATUS section in the sg3_utils(8) man page.
EXAMPLES
To start with, a simple example: write 1 block of data held in file t.bin that is 512 bytes long then
write that block to LBA 0x1234 on /dev/sg4 .
# sg_write_verify --lba=0x1234 --in=t.bin /dev/sg4
Since '--num=' is not given then it defaults to 1. Further the ILEN value is obtained from the file size
of t.bin . To additionally do a data-out comparison to the read back data:
# sg_write_verify -l 0x1234 -i t.bin --bytchk=1 /dev/sg4
The ddpt command can do copies between SCSI devices using READ and WRITE commands. However, currently it
has no facility to promote those WRITES to WRITE AND VERIFY commands. Using a pipe, that could be done
like this:
# ddpt if=/dev/sg2 bs=512 bpt=8 count=11 of=- |
sg_write_verify --in=- -l 0x567 -n 8 --ilen=4096 --repeat /dev/sg4
Both ddpt and sg_write_verify are configured for segments of 8 512 byte logical blocks. Since 11 logical
blocks are read then first 8 logical blocks are copied followed by a copy of the remaining 3 blocks.
Since it is assumed that there is no protection information then the data-in and data-out buffers will be
4096 bytes each. For sg_write_verify this needs to be stated explicitly with the --ilen=4096 option.
AUTHORS
Bruno Goncalves and Douglas Gilbert.
REPORTING BUGS
Report bugs to <dgilbert at interlog dot com>.
COPYRIGHT
Copyright © 2014 Douglas Gilbert
This software is distributed under a FreeBSD license. There is NO warranty; not even for MERCHANTABILITY
or FITNESS FOR A PARTICULAR PURPOSE.
SEE ALSO
ddpt(in a package of that name), sg_compare_and_write(8), sg_dd(8), sg_write_same(8)