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

[Josef Bacik <jbacik@fusionio.com>]

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,

Josef