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# sg_compare_and_write.txt
# This file provides a usage example of sg_compare_and_write.
# sg_compare_and_write accepts a buffer containing 2 logical instances:
# - the verify instance: used to match the current content of the LBA range
# - the write instance: used to write to the LBA if the verify succeeds
#
# In case of failure to verify the data, the command will return with check
# condition with the sense code set to MISCOMPARE DURING VERIFY OPERATION.
#
# The following example shows initialization, successful and unsuccessful
# compare and write using sg3_utils. I am using caw_buf_zero2one and
# caw_buf_one2zero as shown below.
$ hexdump /tmp/caw_buf_zero2one
0000000 0000 0000 0000 0000 0000 0000 0000 0000
*
0000200 1111 1111 1111 1111 1111 1111 1111 1111
*
0000400
$ hexdump /tmp/caw_buf_one2zero
0000000 1111 1111 1111 1111 1111 1111 1111 1111
*
0000200 0000 0000 0000 0000 0000 0000 0000 0000
*
0000400
$ sg_map -i -x
/dev/sg0 0 0 0 0 0 /dev/sda ATA ST3320613AS CC2H
/dev/sg1 3 0 0 0 5 /dev/scd0 HL-DT-ST DVD-RAM GH22NS30 1.01
/dev/sg2 5 0 0 0 0 /dev/sdb KMNRIO K2 0000
/dev/sg3 5 0 0 1 0 /dev/sdc KMNRIO K2 0000
# First I zero out the volume to make sure that the first compare and write
# will succeed
$ sg_write_same --16 -i /dev/zero -n 0x200000 -x 512 /dev/sdc
$ dd if=/dev/sdc bs=512 count=1 skip=100 2>/dev/null | hexdump
0000000 0000 0000 0000 0000 0000 0000 0000 0000
*
0000200
$ ./sg_compare_and_write --in=/tmp/caw_buf_zero2one --lba=100 --xferlen=1024 /dev/sdc
# contents of LBA 100 are a block of ones
$ dd if=/dev/sdc bs=512 count=1 skip=100 2>/dev/null | hexdump
0000000 1111 1111 1111 1111 1111 1111 1111 1111
*
0000200
# We repeat the same compare and write command (zero2one input buffer).
# compare and write fails since the verify failed (compared the zero block to
# the actual 1 block in LBA 100
$ ./sg_compare_and_write --in=/tmp/caw_buf_zero2one --lba=100 --xferlen=1024 /dev/sdc
COMPARE AND WRITE: Fixed format, current; Sense key: Miscompare
Additional sense: Miscompare during verify operation
sg_compare_and_write: SCSI COMPARE AND WRITE failed
# Now we use the second buffer (one2zero)
$ ./sg_compare_and_write --in=/tmp/caw_buf_one2zero --lba=100 --xferlen=1024 /dev/sdc
# operation succeeded, contents of LBA 100 are back to zero
$ dd if=/dev/sdc bs=512 count=1 skip=100 2>/dev/null | hexdump
0000000 0000 0000 0000 0000 0000 0000 0000 0000
*
0000200
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