Why is it better not to use dd as a backup type?
Many users of raspiBackup use dd as backup type.
Especially users who work more with Windows than with Linux, because they can restore a
dd backup under Windows with win32imager or similar tools.
dd can of course be used as a backup tool, but there is a certain risk that does not exist with the tar and rsync backup methods. It is therefore recommended that every user of raspiBackup should use tar or rsync instead.
Why?
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dd backs up an entire partition 1 to 1 at bit level. Errors are reported are reported if the bits cannot be read. However, there are also file system errors occur from time to time. These are usually caused by sudden power failure. These errors are not noticed by dd. This means that dd backs up a partition including the file system errors, which are then also restored during the restore. restored. Thus you have an exact copy of the system with file system errors. You keep creating backups in the belief that everything is OK and then delete old backups at some point. Thus, little by little unnoticed, backups without file system errors are replaced by backups with file system errors. replaced. If a restore is then required, you no longer have a backup without file system errors and can only hope that the file system errors have not have damaged any important system data. Otherwise the backup is useless.
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When restoring a dd backup, the backup is restored 1 to 1 at bit level. is restored. If the SD card is not OK, often no error is reported by dd is reported. If you then boot the system, it does not start or there are error messages from system services. Then the users get in touch and create a problem report because raspiBackup is not working correctly. After they are then asked to perform a restore to a new SD card the problem disappears. These problem reports are unnecessary effort that should be avoided.
What now?
Switching to tar is quick and easy. The same file system can be used on the on the backup partition can be used as for dd.
However, the restore can no longer be carried out with Windows programs such as Win32DiskImager or Etcher.
Instead, you can start raspiBackup on a Raspberry. If you only have one Raspberry available, you should get a one-time "emergency SD card" with Raspbian and raspiBackup and put it aside, until you need it...
For rsync you need an ext2/3/4 file system on the backup partition. This can be created under Linux. The restore is also done with raspiBackup on a Raspberry.
Further disadvantages of dd backups
With a dd backup, the entire partition is always backed up - even if only a fraction of the partition (e.g. 33%) is used. This means that with a 64GB partition, 42GB are always backed up for nothing, the backup process takes unnecessarily takes 66% longer and the backup occupies 66% more disk space. more disk space. There is the option DD_BACKUP_SAVE_USED_PARTITIONS_ONLY with which only the existing root partition is backed up and not the entire device.
Which backup type is the best?
The most efficient backup type is rsync. By using hardlinks only files that have changed are copied and therefore all but the first the first backup process is completed relatively quickly. In addition, the files are unpacked and can be accessed directly if only a few files from the backup are needed. a few files from the backup are required. With tar and dd you have to the backups have to be unpacked first.