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 - ... > > > > > That seems a reasonable guarantee to me, and perhaps it's trivially > > > obvious to those who know their I/O schedulers; but I don't, so I'd > > > like to hear such assurance given. > > > > 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. > > Is there any particular reason why DISCARD_NOBARRIER shouldn't be > enhanced to give the intuitive guarantee I suggest? It is distinct > from a WRITE, I don't see why it has to be treated in the same way > if that's unhelpful to its users. The semantics we want would be something like "when a WRITE or DISCARD request is submitted, automatically turn it into a soft barrier if there is already an outstanding WRITE or DISCARD request overlapping the same sectors". Detecting overlap isn't hard in the single-queue case, but things like CFQ make it interesting -- you'd have to search _every_ queue. And you couldn't just do it when inserting barriers -- you need a write to gain the barrier flag, if it's inserted after a discard. So we really do care about the performance. I agree it would be nice to have if we can do it cheaply enough, though. > I expect the answer will be: it could be so enhanced, but we really > don't know if it's worth adding special code for that without the > experience of more users. That too. We don't yet really know how much the DISCARD requests buy us in terms of performance or device lifetime. It'll depend a lot on the internals of the devices, and we don't get told a lot about that. -- David Woodhouse Open Source Technology Centre David.Woodhouse@xxxxxxxxx Intel Corporation -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
