Difference between revisions of "Disk cloning"

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Revision as of 04:31, 21 August 2015

zh-CN:Disk cloning Disk cloning is the process of making an image of a partition or of an entire hard drive. This can be useful for copying the drive to other computers and for backup and recovery purposes.

Using dd

The dd command is a simple, yet versatile and powerful tool. It can be used to copy from source to destination, block-by-block, regardless of their filesystem types or operating systems. A convenient method is to use dd from a live environment, as in a Live CD.

Warning: As with any command of this type, you should be very cautious when using it; it can destroy data. Remember the order of input file (if=) and output file (of=) and do not reverse them! Always ensure that the destination drive or partition (of=) is of equal or greater size than the source (if=).

Cloning a partition

From physical disk /dev/sda, partition 1, to physical disk /dev/sdb, partition 1.

# dd if=/dev/sda1 of=/dev/sdb1 bs=512 conv=noerror,sync
Warning: If output file of= (sdb1 in the example) does not exist, dd will create a file with this name and will start filling up your root file system!

Cloning an entire hard disk

From physical disk /dev/sdX to physical disk /dev/sdY

# dd if=/dev/sdX of=/dev/sdY bs=512 conv=noerror,sync

This will clone the entire drive, including the MBR (and therefore bootloader), all partitions, UUIDs, and data.

  • noerror instructs dd to continue operation, ignoring all read errors. Default behavior for dd is to halt at any error.
  • sync fills input blocks with zeroes if there were any read errors, so data offsets stay in sync.
  • bs=512 sets the block size to 512 bytes, the "classic" block size for hard drives. If and only if your hard drives have a 4 Kib block size, you may use "4096" instead of "512". Also, please read the warning below, because there is more to this than just "block sizes" -it also influences how read errors propagate.
Warning: The block size you specify influences how read errors are handled. Read below.

The dd utility technically has an "input block size" (IBS) and an "output block size" (OBS). When you set bs, you effectively set both IBS and OBS. Normally, if your block size is, say, 1 Mib, dd will read 1024*1024 bytes and write as many bytes. But if a read error occurs, things will go wrong. Many people seem to think that dd will "fill up read errors with zeroes" if you use the noerror,sync options, but this is not what happens. dd will, according to documentation, fill up the OBS to IBS size after completing its read, which means adding zeroes at the end of the block. This means, for a disk, that effectively the whole 1 Mib would become messed up because of a single 512 byte read error in the beginning of the read: 12ERROR89 would become 128900000 instead of 120000089.

If you are positive that your disk does not contain any errors, you could proceed using a larger block size, which will increase the speed of your copying several fold. For example, changing bs from 512 to 64 Ki changed copying speed from 35 MB/s to 120 MB/s on a simple Celeron 2.7 GHz system. But keep in mind that read errors on the source disk will end up as block errors on the destination disk, i.e. a single 512-byte read error will mess up the whole 64 Kib output block.

Tip: If you would like to view dd progressing, you can send it a USR1 signal. pidof dd will tell you the PID of dd, then you can use kill -USR1 on that PID: kill -USR1 $(pidof dd).
Note:
  • To regain unique UUIDs of an ext2/3/4 filesystem, use tune2fs /dev/sdXY -U random on every partition.
  • Partition table changes from dd are not registered by the kernel. To notify of changes without rebooting, use a utility like partprobe (part of GNU parted).

Backing up the MBR

The MBR is stored in the the first 512 bytes of the disk. It consist of 3 parts:

  1. The first 446 bytes contain the boot loader.
  2. The next 64 bytes contain the partition table (4 entries of 16 bytes each, one entry for each primary partition).
  3. The last 2 bytes contain an identifier

To save the MBR as mbr.img:

# dd if=/dev/sdX of=/path/to/mbr_file.img bs=512 count=1

To restore (be careful: this could destroy your existing partition table and with it access to all data on the disk):

# dd if=/path/to/mbr_file.img of=/dev/sdX

If you only want to restore the boot loader, but not the primary partition table entries, just restore the first 446 bytes of the MBR:

# dd if=/path/to/mbr_file.img of=/dev/sdX bs=446 count=1

To restore only the partition table, one must use:

# dd if=/path/to/mbr_file.img of=/dev/sdX bs=1 skip=446 count=64

You can also get the MBR from a full dd disk image:

# dd if=/path/to/disk.img of=/path/to/mbr_file.img bs=512 count=1

Create disk image

1. Boot from a live media.

2. Make sure no partitions are mounted from the source hard drive.

3. Mount the external HD

4. Backup the drive.

# dd if=/dev/sdX conv=sync,noerror bs=64K | gzip -c  > /path/to/backup.img.gz

If necessary (e.g. when the format of the external HD is FAT32) split the disk image in volumes (see also the split man pages).

# dd if=/dev/sdX conv=sync,noerror bs=64K | gzip -c | split -a3 -b2G - /path/to/backup.img.gz

5. Save extra information about the drive geometry necessary in order to interpret the partition table stored within the image. The most important of which is the cylinder size.

# fdisk -l /dev/sdX > /path/to/list_fdisk.info
Note: You may wish to use a block size (bs=) that is equal to the amount of cache on the HD you are backing up. For example, bs=8192K works for an 8 MiB cache. The 64 Kib mentioned in this article is better than the default bs=512 bytes, but it will run faster with a larger bs=.

Restore system

To restore your system:

# gunzip -c /path/to/backup.img.gz | dd of=/dev/sdX

When the image has been split, use the following instead:

# cat /path/to/backup.img.gz* | gunzip -c | dd of=/dev/sdX

Examples with compression

When you need to create the hard drive or a single partition compressed backup image file you must use compression tools which can do backup from stdout and the dd command. Those compressed files cannot be mounted by the mount command but it is useful to know how to create and restore them.

xz

Install the xz package from the official repositories.

Backup:

# dd if=/dev/sdXY | xz > image-file.xz

Restore:

# xzcat image-file.xz | dd of=/dev/sdXY

7zip

Install the p7zip package from the official repositories. This backup example will split the dd command output in the files by up to 100 mebibyte each:

# dd if=/dev/sdXY | 7zr a -v100m -t7z -si image-file.7z

Restore with 7zip:

# 7zr x -so image-file.7z | dd of=/dev/sdXY
Note: 7zip can split only the 7z compression type files

Zip

Install the zip package from the official repositories, which contains zipsplit among other utilities for the management of zip archives. It will create a file named - inside the image-file.zip file which will contain data from the dd command output. To make a raw output of the file you can use the -cp option with unzip in stdout for the dd command.

Backup:

# dd if=/dev/sdXY | zip --compression-method bzip2 image-file.zip - 

Restore:

# unzip -cp image-file.zip | dd of=/dev/sdXY

The zip tool cannot split files on the fly but you can use the zipsplit utility on an already-created file.

See also man zip for more information.

Rar

Install the rarAUR and unrar packages from the AUR.

This backs up on the fly:

# dd if=/dev/sdXY | rar a -sisdXY image-file.rar

The image-file.rar will contain, if opened using file-roller, for instance, a single file named sdXY

Unfortunately, -v, which serves to chunk the resulting archive, cannot be used in combination with -si, according to the documentation.

This restores:

# unrar p -ierr image-file.rar | dd of=/dev/sdXY

The -ierr prevents unrar's stdout from being prefixed and suffixed with unrar's comments. You could instead use -inul, which omits the comments entirely.

You could also use rar instead of the unrar utility.

Bzip2

Creation by using dd is safer:

# dd if=/dev/sdXY | bzip2 -f5 > compressedfile.bzip2
937016+0 records in
937016+0 records out
479752192 bytes (480 MB) copied, 94.7002 s, 5.1 MB/s

And a safe way of restoring with combination of dd:

# bunzip2 -dc compressedfile.bzip2 | dd of=/dev/sdXY

or

# bzcat compressedfile.bzip2 | dd of=/dev/sdXY
Warning: Never ever use the bzip2 -kdc imgage.bzip2 > /dev/sdXY and bzip2 -kc /dev/sdXY > imgage.bzip2 methods for serious backup of partitions and disks. The errors might be due to the end of the device or partition and the restore process also gives errors due the truncated end.

Using cp

The cp program can be used to clone a disk, one partition at a time. An advantage to using cp is that the filesystem type of the destination partition(s) may be the same as or different from the source. For safety, perform the process from a live environment.

Note: This method should not be considered in the same category as disk cloning on the level at which dd operates. Also, it has been reported that even with the -a switch, some extended attributes may not be copied. For better results, rsync or tar should be used.

The basic procedure from a live environment will be:

  • Create the new destination partition(s) using fdisk, cfdisk or other tools available in the live environment.
  • Create a filesystem on each of the newly created partitions. Example:
# mkfs -t ext3 /dev/sdXY
  • Mount the source and destination partitions. Example:
# mount -t ext3 /dev/sdXY /mnt/source
# mount -t ext3 /dev/sdZY /mnt/destination
  • Copy the files from the source partition to the destination:
# cp -a /path/to/source/* /path/to/destination

-a: preserve all attributes, never follow symbolic links and copy recursively

  • Change the mount points of the newly cloned partitions in /etc/fstab accordingly
  • Finally, install the GRUB bootloader if necessary. (See GRUB)

Disk cloning software

Disk cloning in Arch

  • Partclone provides utilities to save and restore used blocks on a partition and supports ext2, ext3, ext4, hfs+, reiserfs, reiser4, btrfs, vmfs3, vmfs5, xfs, jfs, ufs, ntfs, fat(12/16/32), exfat. Optionally, an ncurses interface can be used. Partclone is available in the community repository.
  • Partimage, an ncurses program, is available in the community repos. Partimage does not currently support ext4 or btrfs filesystems. NTFS is experimental.

Disk cloning outside of Arch

If you wish to backup or propagate your Arch install root, you are probably better off booting into something else and clone the partition from there. Some suggestions:

  • PartedMagic has a very nice live cd/usb with PartImage and other recovery tools.
  • Mindi is a linux distribution specifically for disk clone backup. It comes with its own cloning program, Mondo Rescue.
  • Acronis True Image is a commercial disk cloner for Windows. It allows you to create a live (from within Windows), so you do not need a working Windows install on the actual machine to use it. After registration of the Acronis software on their website, you will be able to download a Linux-based Live CD and/or plugins for BartPE for creation of the Windows-based live CD. It can also create a WinPE Live CD based on Windows. The created ISO Live CD image by Acronis doesn't have the hybrid boot ability and cannot be written to USB storage as a raw file.
  • FSArchiver allows you to save the contents of a file system to a compressed archive file. Can be found on the System Rescue CD.
  • Clonezilla is an enhanced partition imager which can also restore entire disks as well as partitions. Clonezilla is included on the Arch Linux installation media.
  • Redo Backup and Recovery is a Live CD featuring a graphical front-end to partclone.

External Links