Using DD for disk cloning

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Using DD for disk cloning
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There’s been a number of questions regarding disk cloning tools and dd has been suggested at least once. I’ve already considered using dd myself, mainly because ease of use, and that it’s readily available on pretty much all bootable Linux distributions.

What is the best way to use dd for cloning a disk? I did a quick Google search, and the first result was an apparent failed attempt. Is there anything I need to do after using dd, i.e. is there anything that CAN’T be read using dd?

Skyhawk

dd is most certainly the best cloning tool, it will create a 100% replica simply by using the following command. I’ve never once had any problems with it.

dd if=/dev/sda of=/dev/sdb bs=32M

Be aware that while cloning every byte, you should not use this on a drive or partition that is being used. Especially applications like databases can’t cope with this very well and you might end up with corrupted data.

MKzero

To save space, you can compress data produced by dd with gzip, e.g.:

dd if=/dev/hdb | gzip -c  > /image.img

You can restore your disk with:

gunzip -c /image.img.gz | dd of=/dev/hdb

To save even more space, defragment the drive/partition you wish to clone beforehand (if appropriate), then zero-out all the remaining unused space, making it easier for gzip to compress:

mkdir /mnt/hdb
mount /dev/hdb /mnt/hdb
dd if=/dev/zero of=/mnt/hdb/zero

Wait a bit, dd will eventually fail with a “disk full” message, then:

rm /mnt/hdb/zero
umount /mnt/hdb
dd if=/dev/hdb | gzip -c  > /image.img

Also, you can get a dd process running in the background to report status by sending it a signal with the kill command, e.g.:

dd if=/dev/hdb of=/image.img &
kill -SIGUSR1 1234

Check your system – the above command is for Linux, OSX and BSD dd commands differ in the signals they accept (OSX uses SIGINFO – you can press Ctrl+T to report the status).

voretaq7

CAUTION: dd’ing a live filesystem can corrupt files. The reason is simple, it has no understanding of the filesystem activity that may be going on, and makes no attempt to mitigate it. If a write is partially underway, you will get a partial write. This is usually not good for things, and generally fatal for databases. Moreover, if you screw up the typo-prone if and of parameters, woe unto you. In most cases, rsync is an equally effective tool written after the advent of multitasking, and will provide consistent views of individual files.

However, DD should accurately capture the bit state of an unmounted drive. Bootloaders, llvm volumes, partition UUIDs and labels, etc. Just make sure that you have a drive capable of mirroring the target drive bit for bit.

To clone a disk, all you really need to do is specify the input and output to dd:

dd if=/dev/hdb of=/image.img

Of course, make sure that you have proper permissions to read directly from /dev/hdb (I’d recommend running as root), and that /dev/hdb isn’t mounted (you don’t want to copy while the disk is being changed – mounting as read-only is also acceptable). Once complete, image.img will be a byte-for-byte clone of the entire disk.

There are a few drawbacks to using dd to clone disks. First, dd will copy your entire disk, even empty space, and if done on a large disk can result in an extremely large image file. Second, dd provides absolutely no progress indications, which can be frustrating because the copy takes a long time. Third, if you copy this image to other drives (again, using dd), they must be as large or larger than the original disk, yet you won’t be able to use any additional space you may have on the target disk until you resize your partitions.

You can also do a direct disk-to-disk copy:

dd if=/dev/hdb of=/dev/hdc

but you’re still subject to the above limitations regarding free space.

As far as issues or gotchas go, dd, for the most part, does an excellent job. However, a while ago I had a hard drive that was about to die, so I used dd to try and copy what information I could off it before it died completely. It was then learned that dd doesn’t handle read errors very well – there were several sectors on the disk that dd couldn’t read, causing dd to give up and stop the copy. At the time I couldn’t find a way to tell dd to continue despite encountering a read error (though it appears as though it does have that setting), so I spent quite a bit of time manually specifying skip and seek to hop over the unreadable sections.

I spent some time researching solutions to this problem (after I had completed the task) and I found a program called ddrescue, which, according to the site, operates like dd but continues reading even if it encounters an error. I’ve never actually used the program, but it’s worth considering, especially if the disk you’re copying from is old, which can have bad sectors even if the system appears fine.

When using dd to clone a disk which may contain bad sectors, use “conv=noerror,sync” to ensure that it doesn’t stop when it encounters an error, and fills in the missing sector(s) with null bytes. This is usually the first step I take if trying to recover from a failed or failing disk — get a copy before doing any recovery attempts, and then do recovery on the good (cloned) disk. I leave it to the recovery tool to cope with any blank sectors that couldn’t be copied.

Also, you may find dd’s speed can be affected by the bs (block size) setting. I usually try bs=32768, but you might like to test it on your own systems to see what works the fastest for you. (This assumes that you don’t need to use a specific block size for another reason, e.g. if you’re writing to a tape.)

TimB

The source disk must not have any mounted filesystems.
As a user able to read the block device (root works), run ‘dd if=/dev/sda ….’

Now, one of the neat things here is that you’re creating a stream of bytes… and you can do a lot with that: compress it, send it over the network, chunk it into smaller blobs, etc.

For instance:

dd if=/dev/sda | ssh user@backupserver "cat > backup.img"

But more powerfully:

dd if=/dev/sda | pv -c | gzip | ssh user@backupserver "split -b 2048m -d - backup-`hostname -s`.img.gz"

The above copies a compressed image of the source harddrive to a remote system, where it stores it in numbered 2G chunks using the source host’s name while keeping you updated on progress.

Note that depending on the size of disk, speed of cpu on source, speed of cpu on destination, speed of network, etc. You may want to skip compression, or do the compression on the remote side, or enable ssh’s compression.

If the source drive is damaged at all, you’ll have more luck using dd_rhelp with dd_rescue (my personal preference) or GNU ddrescue.

The reason behind this is that, on read errors, dd keeps trying and trying and trying – potentially waiting for a long time for timeouts to occur. dd_rescue does smart things like reading up to an error, then picking a spot further on on the disk and reading backwards to the last error, and dd_rhelp is basically a dd_rescue session manager – cleverly starting and resuming dd_rescue runs to make it quicker again.

The end result of dd_rhelp is maximum data recovered in minimum time. If you leave dd_rhelp running, in the end it does the exact same job as dd in the same time. However, if dd encountered read errors at byte 100 of your 100Gb disk, you’d have to wait a long time to recover the other 9,999,900 bytes*, whereas dd_rhelp+dd_rescue would recover the bulk of the data much faster.

Tshepang

Another nice thing you can do with dd and rescue disks is copy data over the network:

remote_machine$ nc -l -p 12345

local_machine$ dd if=/dev/sda | nc remote_machine 12345

You can stick gzip in both these pipelines if the network is not local. For progress, use pv. To make local_machine’s netcat quit after it’s done copying, you might add -w 5 or something.

user5692

Keep in mind that dd makes an exact copy, including all the blank space.

That means:

  1. 2nd drive must be at least as big as first
  2. If 2nd drive is larger, extra space will be wasted (filesystem can be expanded mind you)
  3. If the source drive is not full, dd will waste alot of time copying blank space.
  4. You can copy either the entire drive, or a single partition this way.
  5. If this is a bootable drive, I’m pretty sure you need to install the bootloader after using dd

Hope that is helpful

Brent

Another grand feature is copying MBRs, partition tables and boot records.

Just

dd if=/dev/sda of=parttable bs=512 count=1

and the other direction around when you’re writing it. Polish with fdisk after.

You feel much more safe when you have your partition table backed up.

Also, it makes migrating to another hard drive (while changing partion structure) a joy.

alamar

To clone a disk, all you really need to do is specify the input and output to dd:

dd if=/dev/hdb of=hdb.img

Of course, make sure that you have proper permissions to read directly from /dev/hdb (I’d recommend running as root), and that /dev/hdb isn’t mounted (you don’t want to copy while the disk is being changed). Once complete, hdb.img will be a byte-for-byte clone of the entire disk.

There are a few drawbacks to using dd to clone disks. First, dd will copy your entire disk, even empty space, and if done on a large disk can result in an extremely large image file. Second, dd provides absolutely no progress indications, which can be frustrating because the copy takes a long time. Third, if you copy this image to other drives (again, using dd), they must be as large or larger than the original disk, yet you won’t be able to use any additional space you may have on the target disk until you resize your partitions.

You can also do a direct disk-to-disk copy:

dd if=/dev/hdb of=/dev/hdc

but you’re still subject to the above limitations regarding free space.

The first drawback can be resolved by gzipping the data as you make the copy. For example:

dd if=/dev/hdb | gzip -9 > hdb.img.gz

The second drawback can be resolved by using the pipeview (pv) tool. For example:

dd if=/dev/hdb | (pv -s `fdisk -l /dev/hdb | grep -o '[0-9]*{1} MB' | awk '{print $1}'`m) | cat > hdb.img

I know of no way to overcome the third drawback.

Additionally, you can speed up the copy time by telling dd to work with larger chunks of data. For example:

dd if=/dev/hdb of=hdb.img bs=1024

jsumners

For future reference it might be of interest to check out ddrescue. It has saved my day a couple of times.

Anders Hansson

This is kind of a cheap hack, but it’s a quick and dirty way to monitor your DD process.

Run your dd command. Open a new shell and do a ps awx to find your dd process’ PID. Now in the new shell type watch -n 10 kill -USR1 {pid of your DD process}

This will do nothing in the watch output window, but back in the original DD shell, DD will start outputting status reports every 10 seconds. You can change the -n 10 in the watch command to any other time frame of course.

Tachyon

dd does provide progress information – well most versions in linux. I’ve seen some which don’t but don’t recall the unix flavour.

The man page says:
Sending a USR1 signal to a running ‘dd’ process makes it print I/O statistics to standard error and then resume copying.

I use this feature regularly.

Steven

How to copy using dd (in this case to a remote machine, but the same principle applies to a local copy) which shows progress.

It works by storing the pid via file descriptor 3 in /tmp/pid, which is then used for the subsequent kills with signal USR1. A wrinkle was to filter the output of the progress on stderr to only one line via filtering stderr through a subshell.

(dd bs=1M if=$lv-snapshot & echo $! >&3 ) 3>/tmp/pid  2> >(grep 'copied' 1>&2) | gzip --fast | ssh $DEST "gzip -d | dd bs=1M of=$lv" &
# Need this sleep to give the above time to run
sleep 1
PID=$(</tmp/pid)

while kill -0 $PID; do
  kill -USR1 $PID
  sleep 5
done

Edward Groenendaal

You could actually try something like this

dd if=/dev/sda2 of=/dev/sdb2 bs=4096 conv=notrunc,noerror

to skip all errors and have exact clone of a hard drive

Steven Penny

Someone had to say this: give Clonezilla a try (http:// clonezilla.org/)

What do you get? To copy only used parts of the filesystem. Clonezilla uses dd, grub, sfdisk, parted, partimage, ntfsclone, and/or partclone. Depending on the options you choose.

Decent documentation can be found at:
http:// clonezilla.org/clonezilla-live/doc/

For some reason, dd fails when imaging CDs with audio tracks. You need to use cdrdao or something similar to get an image + TOC file.

Matt

A note on speed: in my experience dd is twice as fast if you specify bs=1024 instead of the default bs=512. Using an even larger block size gives no noticeable speedup over bs=1024.

One thing you must be aware of when dd-ing a full disk is that doing so will overwrite the master boot record of the receiving disk. This contains the partition table and other vital information. If the new disk is not the same as the old disk, this can create all sorts of tables. Copying over partitions is generally safer (and swap partitions do not have to be copied over)

Paul de Vrieze

I’ve been out of the admin role for many years now, but I know that ‘dd’ is up to the job. I used this technique regularly in the late 80s on Sun Sparc and 386i computers. I had one client order over 30 386i systems running CAD software that was distributed on multiple QIC tapes.

We installed on the first computer, configured the app, ran SunOS’ sys-unconfig, placed the drive in a shoebox with a different SCSI address and then proceeded to ‘dd’ to the other 30 drives.

pbrooks100

As others have mentioned above, one of the gotchas with cloning a mounted file system is potential data corruption. This obviously won’t apply to full drive clones, but if you’re using LVM you can Snapshot the LogicalVolume and dd from the snapshot to get a consistent image.

Ophidian

You can create a compressed image file of the partition (or disk) on the fly using bzip2 or gzip instead of dd. This is nice for storing away images in removable media:

bzip2 -c /dev/sdaX >imagefile.bz2
or
gzip -c /dev/sdaX >imagefile.gz

If the disk has been heavily used before, you can enhance the compression by filling all unused space with zeros before the imaging:

mkdir /mnt/mymountpoint
mount /dev/sdaX /mnt/mymountpoint
cat /dev/zero >/mnt/mymountpoint/dummyfile.bin
(Wait for it to end with a "disk full" error)
rm /mnt/mymountpoint/dummyfile.bin
umount /mnt/mymountpoint

To restore the image into another disk, all you have to do is:

bzcat imagefile.bz2 >/dev/sdbY
or
zcat imagefile.gz >/dev/sdbY

JCCyC

For NTFS volumes, I prefer using ntfsclone. It’s part of the ntfsprogs package.

Just a warning to beginners that needs to be said: At least with some Versions, bs=X means that memory to the size of X will literally be allocated. bs=2GB on a system with 1GB of RAM and insufficient swap WILL cause bad things to happen.

rackandboneman

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