Hi all,

Can some kind soul summarize the rules of using dd viz-a-viz when must and/or mustn't source and destination drives/partitions be mounted or unmounted during the copying process?
I've read many contractory views on the matter which is highly confusing.

cheers,

CC.

Pretty simple in my mind.

When you're reading/writing to a file on a partition, it should be mounted (obviously, in order to access the file).
When you're reading/writing to a raw disk, it should be unmounted to prevent corruption or inconsistency.

The reasons given don't make sense to me.
By your reasoning, when you're reading/writing to a file on a partition, it should also be UNmounted - "to prevent corruption or inconsistency"

There's no need to unmount the entire filesystem to prevent writing to a single file, provided it's not some kind of system file that's constantly in use. You just want to make sure it's not being written to by some other process while you're using dd on it, which should be pretty easy to do for the vast majority of files somebody might want to use dd on.

Besides, as I said before, once you unmount the filesystem you lose access to individual files, so it's not an option anyway. If you're truly worried about it you can always remount the partition in read-only mode first, as long as you're just trying to read from the file.

Perhaps this will be a bit more specific:

There are four use-cases for dd and dealing with files. Writing to a file, reading from a file, writing to a raw disk/partition, reading from a raw disk/partition.

1) Writing to a file - the filesystem obviously must be mounted in rw mode in order to write to a file on it, so that's your only option.

2) Reading from a file - You want to make sure the file's contents do not change mid-read. 99.9% of the time you can directly control when/how the file you'll be reading from dd gets written to, so just shut off whatever process writes to it and you're fine. If you want to be on the safe side, remount the filesystem in read-only mode. Since you're dealing with a file on a partition, unmounting the partition entirely is not an option.

3) Writing to a raw disk/partition - It must be unmounted.

4) Reading from a raw disk/partition - Same rule as #2, but it is usually MUCH more difficult to control writes to an entire disk/partition (remember this includes access timestamping on files, etc.), so you're much better off just unmounting entirely, or at a minimum remounting in read only mode.

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That makes a great deal more sense now, many thanks indeed.