Re: [PATCH] Btrfs: make filesystem read-only when submitting barrier fails

[Chris Mason <chris.mason@fusionio.com>]

On Fri, Oct 05, 2012 at 06:51:59AM -0600, Josef Bacik wrote:
> On Fri, Aug 10, 2012 at 07:38:35AM -0600, Stefan Behrens wrote:
> > So far the return code of barrier_all_devices() is ignored, which
> > means that errors are ignored. The result can be a corrupt
> > filesystem which is not consistent.
> > This commit adds code to evaluate the return code of
> > barrier_all_devices(). The normal btrfs_error() mechanism is used to
> > switch the filesystem into read-only mode when errors are detected.
> > 
> > In order to decide whether barrier_all_devices() should return
> > error or success, the number of disks that are allowed to fail the
> > barrier submission is calculated. This calculation accounts for the
> > worst RAID level of metadata, system and data. If single, dup or
> > RAID0 is in use, a single disk error is already considered to be
> > fatal. Otherwise a single disk error is tolerated.
> > 
> > The calculation of the number of disks that are tolerated to fail
> > the barrier operation is performed when the filesystem gets mounted,
> > when a balance operation is started and finished, and when devices
> > are added or removed.
> > 
> > Signed-off-by: Stefan Behrens <sbehrens@giantdisaster.de>
> 
> So we're going from EOPNOTSUPP resulting in barriers just being turned off to
> the file system being mounted read only?  This is not inline with what every
> other linux file system does, which isn't necessarily a problem but I'm not sure
> it's the kind of change we want to make.  Think about somebody formatting a
> cheap usb stick as btrfs and not understanding why they can't write to it.  I'm
> fine either way, I just want to make sure that we think about the consequences
> of this before we pull it in.  Thanks,

In the past I haven't really trusted the drives to return good errors
when there are problems with cache flushes.  It might be that drives
(and the block layer) are really smart about this now, I know that
Christoph thought any EIOs coming up from a barrier really were eios.

But I still have my doubts, mostly because I don't think anyone tests
these conditions on a regular basis.

-chris