Re: [virt-tools-list] cache write back barriers
On Fri, Jun 14, 2013 at 12:53:04PM +0200, Stefan Hajnoczi wrote: On Thu, Jun 13, 2013 at 10:47:32AM +0200, folkert wrote: Hi, In virt-manager I saw that there's the option for cache writeback for storage devices. I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? Looking at current git, the cache types supported by virt-manager are: - none - writethrough - writeback - default [virt-manager only, not in virt-install] These translate directly into the libvirt driver ... cache=... field which you can find documented here: http://libvirt.org/formatdomain.html#elementsDisks As far as I can tell (from looking at libvirt sources) as long as you have a modern qemu these will translate to the same names on the qemu command line. No, that would be unsafe. When the guest issues a flush then QEMU will ensure that data reaches the disk with -drive cache=writeback. Aha so the writeback behaves like the consume harddisks with write-cache on them. In answer to the original question by 'folkert': In that case maybe an extra note could be added to the virt-manager (excellent software by the way!) that if the client vm supports barriers, that write-back in that case then is safe. Agree? I suspect the problem with doing this is it depends on the hypervisor. Likely for qemu and Xen (since it uses a qemu device model) this would be true. Possibly not for other hypervisors that virt-manager can control. Generally speaking, it would be nice to document these properly and also how they are implemented in different hypervisors, because I know I for one don't find these settings very obvious. So, patches welcome! Rich. -- Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones virt-p2v converts physical machines to virtual machines. Boot with a live CD or over the network (PXE) and turn machines into KVM guests. http://libguestfs.org/virt-v2v -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: cache write back barriers
On Thu, Jun 13, 2013 at 10:47:32AM +0200, folkert wrote: Hi, In virt-manager I saw that there's the option for cache writeback for storage devices. I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? No, that would be unsafe. When the guest issues a flush then QEMU will ensure that data reaches the disk with -drive cache=writeback. Aha so the writeback behaves like the consume harddisks with write-cache on them. In that case maybe an extra note could be added to the virt-manager (excellent software by the way!) that if the client vm supports barriers, that write-back in that case then is safe. Agree? CCed virt-manager mailing list so they can see your request. Stefan -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: cache write back barriers
On Wed, Jun 12, 2013 at 10:03:10AM +0200, folkert wrote: In virt-manager I saw that there's the option for cache writeback for storage devices. I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? No, that would be unsafe. When the guest issues a flush then QEMU will ensure that data reaches the disk with -drive cache=writeback. Stefan -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: cache write back barriers
Hi, In virt-manager I saw that there's the option for cache writeback for storage devices. I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? No, that would be unsafe. When the guest issues a flush then QEMU will ensure that data reaches the disk with -drive cache=writeback. Aha so the writeback behaves like the consume harddisks with write-cache on them. In that case maybe an extra note could be added to the virt-manager (excellent software by the way!) that if the client vm supports barriers, that write-back in that case then is safe. Agree? Folkert van Heusden -- Ever wonder what is out there? Any alien races? Then please support the seti@home project: setiathome.ssl.berkeley.edu -- Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
Re: cache write back barriers
I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? no, cache=writeback is ok, write barriers are working correctly only with cache=unsafe,it doesn't care about write flush. - Mail original - De: folkert folk...@vanheusden.com À: kvm@vger.kernel.org Envoyé: Mercredi 12 Juin 2013 10:03:10 Objet: cache write back barriers Hi, In virt-manager I saw that there's the option for cache writeback for storage devices. I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? regards, Folkert van Heusden -- Always wondered what the latency of your webserver is? Or how much more latency you get when you go through a proxy server/tor? The numbers tell the tale and with HTTPing you know them! http://www.vanheusden.com/httping/ --- Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
cache write back barriers
Hi, In virt-manager I saw that there's the option for cache writeback for storage devices. I'm wondering: does this also make kvm to ignore write barriers invoked by the virtual machine? regards, Folkert van Heusden -- Always wondered what the latency of your webserver is? Or how much more latency you get when you go through a proxy server/tor? The numbers tell the tale and with HTTPing you know them! http://www.vanheusden.com/httping/ --- Phone: +31-6-41278122, PGP-key: 1F28D8AE, www.vanheusden.com -- To unsubscribe from this list: send the line unsubscribe kvm in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
