Re: [RFC PATCH] discarding swap

Hugh Dickins
2008-09-12T17:47:02Z

On Fri, 12 Sep 2008, David Woodhouse wrote:
> On Fri, 2008-09-12 at 16:52 +0100, Hugh Dickins wrote:
> > On Fri, 12 Sep 2008, David Woodhouse wrote:
> > > On Fri, 2008-09-12 at 13:10 +0100, Hugh Dickins wrote:
> > > > So long as the I/O schedulers guarantee that a WRITE bio submitted
> > > > to an area already covered by a DISCARD_NOBARRIER bio cannot pass that
> > > > DISCARD_NOBARRIER - ...
> > >
> > > No, that's the point. the I/O schedulers _don't_ give you that guarantee
> > > at all. They can treat DISCARD_NOBARRIER just like a write. That's all
> > > it is, really -- a special kind of WRITE request without any data.
> >
> > Hmmm. In that case I'll need to continue with DISCARD_BARRIER,
> > unless/until I rejig swap allocation to wait for discard completion,
> > which I've no great desire to do.

I'll leave it to Jens to comment on your reply, but I'd like to go
back and add in a further, orthogonal concern or misunderstanding here.

Am I right to be a little perturbed by blk_partition_remap()
and the particular stage at which it's called?

Does it imply that a _BARRIER on swap would have the effect of
inserting a barrier into, say, root and home I/Os too, if swap
and root and home were in separate partitions on the same storage?

Whereas a filesystem would logically only want a barrier to span
its own partition? (I'm ignoring md/dm.)

Hugh