On Wed, Oct 08, 2008 at 11:49:19AM -0500, Anthony Liguori wrote: > Daniel P. Berrange wrote: > >On Wed, Oct 08, 2008 at 11:06:27AM -0500, Anthony Liguori wrote: > > > >Sorry, it was mistakenly private - fixed now. > > > >Xen does use O_DIRECT for paravirt driver case - blktap is using the combo > >of AIO+O_DIRECT. > > You have to use O_DIRECT with linux-aio. And blktap is well known to > have terrible performance. Most serious users use blkback/blkfront and > blkback does not avoid the host page cache. It maintains data integrity > by passing through barriers from the guest to the host. You can > approximate this in userspace by using fdatasync. > > The issue the bug addresses, iozone performs better than native, can be > addressed in the following way: > > 1) For IDE, you have to disable write-caching in the guest. This should > force an fdatasync in the host. > 2) For virtio-blk, we need to implement barrier support. This is what > blkfront/blkback do. > 3) For SCSI, we should support ordered queuing which would result in an > fdatasync when barriers are injected. Ok, ignore my libvirt patch then. We'll punt this problem back to QEMU & virtio develoers to solve properly Daniel -- |: Red Hat, Engineering, London -o- http://people.redhat.com/berrange/ :| |: http://libvirt.org -o- http://virt-manager.org -o- http://ovirt.org :| |: http://autobuild.org -o- http://search.cpan.org/~danberr/ :| |: GnuPG: 7D3B9505 -o- F3C9 553F A1DA 4AC2 5648 23C1 B3DF F742 7D3B 9505 :| -- Libvir-list mailing list Libvir-list@xxxxxxxxxx https://www.redhat.com/mailman/listinfo/libvir-list