Comment by matja

Actual file data ends up in the same transaction group (txg) as metadata if both are changed within the same txg commit (either flushed explicitly, due to recordsize/buffer limit being reached, or txg commit timeout - 5 seconds by default), so if there is a write barrier violation caused by hardware lies followed by an untimely loss of power, the checksums for the txg updates won't match and they get rolled-back until the last valid one when importing the pool - which doesn't end up zero'ing out extents of a file (like in xfs) or ending up with a zero file size (like on ext3/ext4).