On Tue 30-10-12 11:34:41, NeilBrown wrote:

On Tue, 30 Oct 2012 01:10:08 +0100 Jan Kara [off-list ref] wrote:

quoted

On Tue 30-10-12 10:48:37, NeilBrown wrote:

quoted

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

quoted

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()?

  Yes.

quoted

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

  Actually the code is in kernel for quite some time already. The problem
is it is always enabled causing unnecessary performance issues for some
workloads. So these patches try to be more selective in when the code gets
enabled.

Regarding "all filesystems" question: If we update filemap_page_mkwrite()
to call wait_on_page_writeback() then it should be for all filesystems.

Cool.  I didn't realise it had progressed that far.

I guess it is time to look at the possibility of removing the
'copy-into-cache' step for full-page, well-aligned bi_iovecs.

I assume this applies to swap-out as well ??  It has been a minor source of
frustration that when you swap-out to RAID1, you can occasionally get
different data on the two devices because memory changed between the two DMA
events.

  Really? I'm somewhat surprised. I was under the impression that when a
page is added to a swap cache it is unmapped so there should be no
modification to it possible while it is being swapped out. But maybe it
could get mapped back and modified after we unlock the page and submit the
bio. So mm/memory.c:do_swap_page() might need wait_on_page_writeback() as
well. But I'm not an expert on swap code. I guess I'll experiment with this
a bit. Thanks for a pointer.

								Honza