btrfs win8 preview kvm VERY SLOW IO

[ 7776.947491] ------------[ cut here ]------------
[ 7776.947546] WARNING: at fs/btrfs/tree-log.c:3716 btrfs_log_inode_parent+0x2af/0x2c0 [btrfs]()
[ 7776.947574] Hardware name: GA-990FXA-UD5
[ 7776.947579] Modules linked in: vhost_net macvtap macvlan tun ebtable_nat ebtables raid10 fuse xt_tcpudp xt_pkttype xt_physdev xt_LOG xt_limit bridge stp llc ip6t_REJECT nf_conntrack_ipv6 nf_defrag_ipv6 ip6table_raw ipt_REJECT it87 hwmon_vid iptable_raw iptable_filter ip6table_mangle nf_conntrack_netbios_ns nf_conntrack_broadcast nf_conntrack_ipv4 nf_defrag_ipv4 ip_tables xt_conntrack nf_conntrack ip6table_filter ip6_tables x_tables cpufreq_conservative cpufreq_userspace cpufreq_powersave acpi_cpufreq mperf microcode binfmt_misc dm_mod snd_hda_codec_hdmi crc32c_intel snd_hda_codec_realtek ghash_clmulni_intel snd_hda_intel snd_hda_codec aesni_intel raid1 ablk_helper firewire_ohci edd cryptd snd_hwdep firewire_core lrw joydev sp5100_tco aes_x86_64 sr_mod snd_pcm k10temp i2c_piix4 snd_timer button hid_generic mxm_wmi cdrom snd xts crc_itu_t fam15h_power gf128mul wmi sg soundcore edac_core xhci_hcd r8169 snd_page_alloc edac_mce_amd pcspkr serio_raw autofs4 btrfs zlib_deflate libcrc32c usbhid ohci_hcd radeon ehci_hcd usbcore usb_common ttm drm_kms_helper drm i2c_algo_bit processor thermal_sys [last unloaded: microcode]
[ 7776.947719] Pid: 9562, comm: qemu-kvm Not tainted 3.7.0-rc4-1-default #7
[ 7776.947721] Call Trace:
[ 7776.947739]  [<ffffffff810a010a>] warn_slowpath_common+0x7a/0xb0
[ 7776.947744]  [<ffffffff810a0155>] warn_slowpath_null+0x15/0x20
[ 7776.947779]  [<ffffffffa018635f>] btrfs_log_inode_parent+0x2af/0x2c0 [btrfs]
[ 7776.947814]  [<ffffffffa01863b1>] btrfs_log_dentry_safe+0x41/0x70 [btrfs]
[ 7776.947849]  [<ffffffffa015ee84>] btrfs_sync_file+0x164/0x200 [btrfs]
[ 7776.947856]  [<ffffffff811e8980>] generic_write_sync+0x50/0x70
[ 7776.947913]  [<ffffffffa01603a6>] btrfs_file_aio_write+0x2b6/0x310 [btrfs]
[ 7776.947921]  [<ffffffff811bb493>] do_sync_write+0xa3/0xe0
[ 7776.947943]  [<ffffffff811bbb0e>] vfs_write+0xae/0x180
[ 7776.947949]  [<ffffffff811bbfaa>] sys_pwrite64+0x9a/0xa0
[ 7776.947971]  [<ffffffff81606029>] system_call_fastpath+0x16/0x1b
[ 7776.947974] ---[ end trace 37a41a586d58e58e ]---