The dd command is an old command carried over from the Mainframe computing days of the yore. It stands for 'Data Dump'. Also known as 'data destroyer' as a pun, in case not used carefully (since it can destroy disk partitioning very innately and cleanly). It is considered as a good choice of disk to disk backup methods at a very native level, it can even be used to copy whole partitions or filesystems. For details refer below:
-dd backup method:
. sequential file image of a partition irrespective of the filesystem knowledge that reside on the partition. dd cmd does so.
. backup of partition using only the used fs. dump cmd does so.
-dd requires that the restore be done on an equal or bigger partition.
-dump can restore to partition that is sufficient to hold the used fs.
-dd cmd syntax:
# dd if=<input file path> of=<output file name>
-eg: # dd if=/dev/sda3 of=backup-1 [ copies /dev/sda3 partition to a file called 'backup-1' in pwd ]
# dd if=/dev/sda3 of=/dev/fd0 [ copies /dev/sda3 partition to floppy drive ]
# dd if=/dev/sda2 | gzip > backup-2 [ copies /dev/sda2 partition as a gzipped file 'backup-2' in pwd ]
-note:
. dd cmd copies all blocks - whether used or unused (ie empty); also the output location needs to be atleast as big as the input.
. dd cmd can be used an easy way to troubleshoot IO:
# time dd if=/u03/oradata/devenv/one.dbf of=/dev/null [ reads file and times it ]
# dd --help [ or man dd for more help/details ]
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