dangerous situation with shutdown process

[Matthias Buelow]

Lowell Gilbert <freebsd-stable-local at be-well.ilk.org> writes:

>We keep trying to point out that barriers *can't* be enforced on the
>hardware with many (most, and apparently an increasing percentage of)
>ATA drives.  There is no semantic on these drives that allows you to
>guarantee the journal block will be written before the corresponding
>data block.  If you are sure that your drives do this properly, then
>you are safe, but in that case there's no reliability problem with
>softupdates, either.

See my other mail(s) about other systems using cache disabling/enabling
to make up for a drive that ignores (or does not implement) a flush
command.

Then the advice of "disable the wb-cache on disks to ensure data safety"
doesn't make sense:

Either

 * the drive supports disabling the write-back-cache,
   then this method can be used to flush data to the platters,

or else

 * the drive does not support disabling the write-back-cache, or lies
   about it, then the advice to disable the write-back-cache for
   softupdates is meaningless.

I know my drive allows disabling of the write cache, as, apparently, the
majority of IDE/SATA drives do.

mkb.