On Mon, 29 Oct 2012 19:30:51 +0100 Jan Kara [off-list ref] wrote:

On Mon 29-10-12 19:13:58, Jan Kara wrote:

quoted

On Fri 26-10-12 18:35:24, Darrick J. Wong wrote:

quoted

This creates BDI_CAP_STABLE_WRITES, which indicates that a device requires
stable page writes.  It also plumbs in a sysfs attribute so that admins can
check the device status.

Signed-off-by: Darrick J. Wong <redacted>

  I guess Jens Axboe [off-list ref] would be the best target for this
patch (so that he can merge it). The patch looks OK to me. You can add:
  Reviewed-by: Jan Kara [off-list ref]

  One more thing popped up in my mind: What about NFS, Ceph or md RAID5?
These could (at least theoretically) care about stable writes as well. I'm
not sure if they really started to use them but it would be good to at
least let them know.

What exactly are the semantics of BDI_CAP_STABLE_WRITES ?

If I set it for md/RAID5, do I get a cast-iron guarantee that no byte in any
page submitted for write will ever change until after I call bio_endio()?

If so, is this true for all filesystems? - I would expect a bigger patch would
be needed for that.

If not, what exactly are the semantics?

Thanks,
NeilBrown