On Thu, 27 Aug 2009 11:12:39 pm Christoph Hellwig wrote: > On Thu, Aug 27, 2009 at 08:21:55PM +0930, Rusty Russell wrote: > > > - virtio-blk needs to advertise ordered queue by default. > > > This makes cache=writethrough safe on virtio. > > > > >From a guest POV, that's "we don't know, let's say we're ordered because that > > may make us safer". Of course, it may not help: how much does it cost to > > drain the queue? > > > > The bug, IMHO is that we *should* know. And in future I'd like to fix that, > > either by adding an VIRTIO_BLK_F_ORDERED feature, or a VIRTIO_BLK_F_UNORDERED > > feature. > > > > > Action plan for QEMU: > > > > > > - IDE needs to set the write cache enabled bit > > > - virtio needs to implement a cache flush command and advertise it > > > (also needs a small change to the host driver) > > > > So, virtio-blk needs to be enhanced for this as well. > > Really, enabling volatile write caches without advertising a cache flush > command is a bug in the storage, where in our case qemu is the storage. > So I don't really see the need for two feature bits. Here's my plan for > virtio-blk: > > - add a new VIRTIO_BLK_F_WCACHE feature. If this feature is set we > do > (a) implement the prepare_flush queue operation to send a > standalone cache flush > (b) set a proper barrier ordering flag on the queue OK, I buy that. I'll update the virtio_pci spec accordingly, too. I've applied your previous patch. > The complex (not to say over engineered) verison would be to split > the caching and data integrity setting into two options: > > (1) hostcache=on|off > use buffered vs O_DIRECT I/O > (2) integrity=osync|fsync|none > use O_SYNC, use f(data)sync or do not care about data integrity If we were starting from scratch, I'd agree. But seems like too much user-visible churn. Thanks, Rusty. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
