Dd
dd is a utility to create a disk dump by reading every single block on a disk, e.g. your hard drive. However, its architecture is laid out so it can do much more than creating a dump. Here is what dd can do for you:
- manage a disk backup
- create a backup from a disk to a file
- restore a backup from a file to a disk
- clone a harddisk
- create a disk image and transfer it over the network
- create an iso image of a CD
- rescue a file that contains bad blocks
- analyze your disk by displaying selected blocks
- create your own operating system by dumping your bootloader to the boot sector
- benchmark the throughput of your disks
Contents
Disk Backup
Create a backup
Say we have a harddisk /dev/sda that we want to backup entirely (sector-by-sector) to a USB volume /dev/sdb1, mounted on /mnt/sdb1. We call this a dump or an image of /dev/sda. The dump shall be named backup.img. Here is the dd command: <source>
dd if=/dev/sda of=/mnt/sdb1/backup.img
</source> In this command, if stands for input file and of for output file.
Restore a backup
To restore this backup, we boot from a live CD and do the command vice versa. This can overwrite all content on your harddisk, this is the intention. <source>
dd if=/mnt/sdb1/backup.img of=/dev/sda
</source>
Clone a harddisk
To clone a disk A to B, both disks need to have the same capacity. It is very convenient for USB disks. Say our USB disk source is called /dev/sdb and the target is called /dev/sdc. Do it like this: <source>
dd if=/dev/sdb of=/dev/sdc
</source> Now if sdc has a bigger capacity, this capacity will be lost because the file system is not aware of it.
Transfer a disk image
To transfer a disk image over the network to a computer named target, use <source>
dd if=/dev/sdb | ssh root@target "(cat >backup.img)"
</source>
create an iso image of a CD
To create an iso image of a CD, read it block-by-block and save the blocks to a file. In case of an error, dd is not supposed to abort: <source>
dd if=/dev/cdrom of=cdimage.iso conv=noerror
</source>
create a disk image
To create an empty image of a harddisk, e.g. for qemu virtualization: <source>
dd if=/dev/zero of=harddisk.img bs=516096 seek=6241 count=0
</source> creates a 3GB-Image. For a 6GB-Image, use seek=12482.
rescue a file that contains bad blocks
If your favorite movie or song cannot be played any longer because the file is corrupt, you can use dd to ignore the corrupt part: <source>
dd if=movie.avi of=rescued_movie.avi conv=noerror
</source>
analyze your disk
With dd you can read any byte from your disk. For example to read the first 40 bytes from your first disk, issue <source>
dd if=/dev/sda bs=40 count=1
</source> The output will be displayed on the console. It will look about like this: <source>
tweedleburg:~ # dd if=/dev/sda bs=40 count=1 3���؎м|��W���������RR�A��U1�1+0 records in 1+0 records out 40 bytes (40 B) copied, 4.6373e-05 s, 863 kB/s
</source> You see there are many glyphs that are no characters or numbers. To make sense out of this, pipe it to hexdump: <source>
tweedleburg:~ # dd if=/dev/sda bs=40 count=1 | hexdump -C 1+0 records in 1+0 records out 40 bytes (40 B) copied, 5.4665e-05 s, 732 kB/s 00000000 33 c0 fa 8e d8 8e d0 bc 00 7c 89 e6 06 57 8e c0 |3........|...W..| 00000010 fb fc bf 00 06 b9 00 01 f3 a5 ea 1f 06 00 00 52 |...............R| 00000020 52 b4 41 bb aa 55 31 c9 |R.A..U1.| 00000028
</source> Looks much better, right? Now you know the first byte on your first harddisk is 33 (hexadecimal).
To analyze your disk by displaying selected blocks, in this case block 1001 use: <source>
dd if=/dev/sdc1 count=1 skip=1000
</source>
Create your own bootloader
To create your own operating system by dumping your bootloader to the boot sector of a bootable disk image use <source>
dd conv=notrunc if=bootloader of=qemu.img
</source>
benchmark the throughput of your disks
To benchmark the throughput of your disk /dev/sda1, e.g. for different block sizes, proceed like this: <source>
# dd if=/dev/sda1 of=/dev/null bs=512 count=1000000 1000000+0 records in 1000000+0 records out 512000000 bytes (512 MB) copied, 5.16588 s, 99.1 MB/s # dd if=/dev/sda1 of=/dev/null bs=4096 count=1000000 1000000+0 records in 1000000+0 records out 4096000000 bytes (4.1 GB) copied, 36.0667 s, 114 MB/s
</source> However, make sure you have read Background:How caching works first otherwise you will be surprised by a mysterious accelleration like this: <source>
# dd if=/dev/sda1 of=/dev/null bs=512 count=1000000 1000000+0 records in 1000000+0 records out 512000000 bytes (512 MB) copied, 5.32254 s, 96.2 MB/s # dd if=/dev/sda1 of=/dev/null bs=512 count=1000000 1000000+0 records in 1000000+0 records out 512000000 bytes (512 MB) copied, 1.09851 s, 466 MB/s
</source>
get the progress
During a long dd run you may want to know the progress. dd will output it for you if it receives a special signal, USR1. So,
- find out the number of SIGUSR
<source>
# kill -l 1) SIGHUP 2) SIGINT 3) SIGQUIT 4) SIGILL 5) SIGTRAP 6) SIGABRT 7) SIGBUS 8) SIGFPE 9) SIGKILL 10) SIGUSR1 11) SIGSEGV 12) SIGUSR2 13) SIGPIPE 14) SIGALRM 15) SIGTERM 16) SIGSTKFLT 17) SIGCHLD 18) SIGCONT 19) SIGSTOP 20) SIGTSTP 21) SIGTTIN 22) SIGTTOU 23) SIGURG 24) SIGXCPU 25) SIGXFSZ 26) SIGVTALRM 27) SIGPROF 28) SIGWINCH 29) SIGIO 30) SIGPWR 31) SIGSYS 34) SIGRTMIN 35) SIGRTMIN+1 36) SIGRTMIN+2 37) SIGRTMIN+3 38) SIGRTMIN+4 39) SIGRTMIN+5 40) SIGRTMIN+6 41) SIGRTMIN+7 42) SIGRTMIN+8 43) SIGRTMIN+9 44) SIGRTMIN+10 45) SIGRTMIN+11 46) SIGRTMIN+12 47) SIGRTMIN+13 48) SIGRTMIN+14 49) SIGRTMIN+15 50) SIGRTMAX-14 51) SIGRTMAX-13 52) SIGRTMAX-12 53) SIGRTMAX-11 54) SIGRTMAX-10 55) SIGRTMAX-9 56) SIGRTMAX-8 57) SIGRTMAX-7 58) SIGRTMAX-6 59) SIGRTMAX-5 60) SIGRTMAX-4 61) SIGRTMAX-3 62) SIGRTMAX-2
</source>
- send SIGUSR to all dd processes:
<source>
killall -10 dd
</source> The output will read like this: <source>
2989696+0 records in 2989696+0 records out 1530724352 bytes (1.5 GB) copied, 583.336 s, 2.6 MB/s
</source>
