The mkfs utility will let the user create a filesystem with
profiles that write the logical blocks to 2 physical locations.
Whether there are really 2 physical copies highly depends on the
underlying device type.
For example, a SSD drive can remap the blocks internally to a
single copy—thus deduplicating them. This negates the purpose of
increased redundancy and just wastes filesystem space without
providing the expected level of redundancy.
The duplicated data/metadata may still be useful to statistically
improve the chances on a device that might perform some internal
optimizations. The actual details are not usually disclosed by
vendors. For example we could expect that not all blocks get
deduplicated. This will provide a non-zero probability of
recovery compared to a zero chance if the single profile is used.
The user should make the tradeoff decision. The deduplication in
SSDs is thought to be widely available so the reason behind the
mkfs default is to not give a false sense of redundancy.
As another example, the widely used USB flash or SD cards use a
translation layer between the logical and physical view of the
device. The data lifetime may be affected by frequent plugging.
The memory cells could get damaged, hopefully not destroying both
copies of particular data in case of DUP.
The wear levelling techniques can also lead to reduced
redundancy, even if the device does not do any deduplication. The
controllers may put data written in a short timespan into the
same physical storage unit (cell, block etc). In case this unit
dies, both copies are lost. BTRFS does not add any artificial
delay between metadata writes.
The traditional rotational hard drives usually fail at the sector
level.
In any case, a device that starts to misbehave and repairs from
the DUP copy should be replaced! DUP is not backup.
