I have to backup my hard disk. I want to use dd and put the image on an external hdd.Can I do this using dd from the OS that resides on the hdd itself or do I have to boot from another device, for example a LiveCD?Is it safe, in general, to take the image of a device, if the device is mounted and working?What if the device is mounted, but I'm sure there's no other I/O operation, while dd is running?I'm sure that rsync is the best tool to use for backups, specially the incremental ones.But I'm interested in dd, because I want to backup up also other storage devices, and it copies also data stored on unpartitioned space. For example, my e-book reader use an unpartitioned space to store uboot, kernel and other data.
Is it safe to take the drive image of the current working drive?
backup;dd;disk image
In general it is not safe. The FS assumes that operations are written in certain order so it can write new data of file and then make a pointer to it from other data, the exact details depend on filesystem. Imagine if following happens:dd reads from location X which contains garbage or some dataFilesystem writes to location XFilesystem writes to location X+1 pointer to location Xdd reads from location X+1 link to location XFrom the point of view of backup you get a garbage data. However there are several ways to workaround it:Freeze filesystem by filesystem specific command (I believe xfs_freeze is one and I don't know any other - but such option exists at least in theory)Create a lvm snapshot and copy from it. The copy will be as-if you rebooted the computer (minus the HDD reordering) so it will be a dirty filesystem but the copy will be atomic. Note that some filesystems like XFS needs to be frozen first.Use rsync as suggested by others. Now the copy is safe and you don't need LVM but the copy is not atomic. So while it avoids the above problem on filesystem level it might still run into problems with files (rather unlikely but one can imagine missing files while mv is executed in background for example)Use filesystem with snapshoting such as btrfs, tux3, zfs, nilfs... Then you avoid both problems - you can just create a snapshot and copy from it by rsync having full atomicity. Note however that such filesystem often tend to be experimental.As a last note - dd might not be a best way of backup. It copies a full disk which is often wasteful as you copy the 'garbage' as well. If you need to have a disk images something like partimage might be better. If you don't a better option is using either using a rsync, tar in differential/incremental mode etc. or a full backup system such as bacula, tarsnap or one of many others. Data deduplication may do wonders for the sizes of backups.