Re: magic sysrq keys functionality

On Mon, Jul 26, 2004 at 12:15:02PM -0600, Scott Long wrote:
> On Mon, 26 Jul 2004, Brian Fundakowski Feldman wrote:
> > On Mon, Jul 26, 2004 at 11:49:55AM -0600, Scott Long wrote:
> > > On Mon, 26 Jul 2004, Brian Fundakowski Feldman wrote:
> > > B> On Sun, Jul 25, 2004 at 09:23:36PM +0000, bugghy wrote:
> > > > > Yeah but it sometimes "freezes" (no reboot) ... and I'd rather umount my
> > > > > filesystems before rebooting.
> > > >
> > > > SoftUpdates guarantess that your file systems will not get corrupt.
> > > >
> > >
> > > This isn't entirely correct.  Softupdates guarantees that you won't get
> > > corruption due to metadata pointing to invalid or stale data blocks.
> > > That's not the same as guaranteeing that there won't be any corruption.
> > > Write caching on the drive combined with an in-opportune power loss or
> > > other failure can easily leave you with corrupt or incomplete metadata
> > > and/or data blocks.  A panic while metadata is being committed to disk can
> > > also leave the metadata highly inconsistent and prone to corruption.
> > > This isn't to say the SU is bad or that other strategies are necessarily
> > > better, just that there are definite risks.
> >
> > If you just want to generalize it, you can say that "SoftUpdates
> > guarantees that your file systems will not get corrupt due to just
> > software errors."  I don't particularly think not having UPS is a
> > good idea, but those can fail, and even so the ordering is such
> > that a truncated inode won't result in a truly corrupt filesystem,
> > and the inode doesn't get written until its contents are written
> > out.
> >
> > Also, hw.ata.wc really shouldn't default to 1.
> >
> 
> GAH!  No, please don't start this war again!  The last time that we tried
> turning this off in a release (4.1 IIRC), were were plagqued by months of
> earthquakes, plagues, and deaths of first-born youngsters.  I 100% agree
> that write caching in ATA is not compatible with data integrety, but the
> ATA marketting machine has convinced us that cached+untagged speed is
> better than uncached+tagged safety.  C'est la vie, or so they say here.

I think it would be prudent to add a nice fat "WARNING:" printf to the
boot process.  It's really not obvious that FreeBSD defaults to having
your hard drives run "unsafely," even though it is usually faster.

-- 
Brian Fundakowski Feldman                           \'[ FreeBSD ]''''''''''\
  <> green_at_FreeBSD.org                               \  The Power to Serve! \
 Opinions expressed are my own.                       \,,,,,,,,,,,,,,,,,,,,,,\