magic sysrq keys functionality

[Brian Fundakowski Feldman]

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.

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