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Performance zfs
http://open-zfs.org/wiki/Performance_tuning#Alignment_Shift_.28ashift.29
choose correct value ashift https://illumos.topicbox.com/groups/zfs/T2cf12f740806aa96-Ma3844bffd677334735f94f3d
understanding aio mode https://specs.openstack.org/openstack/nova-specs/specs/mitaka/implemented/libvirt-aio-mode.html
option of qemu https://heiko-sieger.info/qemu-system-x86_64-drive-options/