Filesystems in general and UFS with soft updates in particular rely on
disks providing accurate response to writes. When write caching is
enabled the disk will lie and tell the operating system that the write
has completed successfully, in reality the data is only cached in disk
RAM. When the power disappears the data will be gone forever.
No. This used to be the case with some cheaper disks which ignored the
ATA "flush cache" command to score higher on benchmarks, but I doubt
you'll find any disks on the market that still do that (at least from
reputable manufacturers). ZFS makes extensive use of the "flush cache"
command to ensure file system integrity (and in particular to ensure
that the intent log is written to disk so it can be replayed in case of