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authormstsirkin <mstsirkin@0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652>2015-10-21 14:20:04 +0000
committermstsirkin <mstsirkin@0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652>2015-10-21 14:20:04 +0000
commitff6b2bb653ae98cd0e71523d9c82f908bc95f035 (patch)
treea95662c5b32c051f83dd1c861b1fcc79e432b219 /specvars.tex
parentcff819e3c64b19873746fdeb4307d3e6f0a66ff2 (diff)
virtio-blk: restore VIRTIO_BLK_F_FLUSH and VIRTIO_BLK_F_CONFIG_WCE
VIRTIO_BLK_F_CONFIG_WCE is important in order to achieve good performance (up to 2x, though more realistically +30-40%) in latency-bound workloads. However, it was removed by mistake together with VIRTIO_BLK_F_FLUSH. In addition, even removing VIRTIO_BLK_F_FLUSH was probably not a great idea, because it simplifies simple drivers (e.g. firmware) that are okay with a writethrough cache but still need data to persist after power loss. What really should have been removed is just the possibility that devices not propose VIRTIO_BLK_F_FLUSH, but even that only deserves a "SHOULD" in the new world of conformance statements. Restore these, with the following changes: * clarify and use conformance statements in order to define writeback and writethrough caching according to what is commonly done by high-end storage. * clarify (with conformance statements) the influence of the VIRTIO_BLK_F_FLUSH feature on caching and how to proceed if only one of VIRTIO_BLK_F_FLUSH and VIRTIO_BLK_F_CONFIG_WCE is negotiated. * strengthen the requirement for persisting writes to MUST after a VIRTIO_BLK_T_FLUSH request (and in other cases too involving the new features). The suggested behavior upon feature negotiation is okay for the Linux implementation of virtio1, even after the implementation is modified to support the two new features. This fixes VIRTIO-144. Signed-off-by: Paolo Bonzini <pbonzini@redhat.com> Accepted by electronic ballot: https://www.oasis-open.org/apps/org/workgroup/virtio/ballot.php?id=2864 Signed-off-by: Michael S. Tsirkin <mst@redhat.com> git-svn-id: https://tools.oasis-open.org/version-control/svn/virtio/branches/v1.0@546 0c8fb4dd-22a2-4bb5-bc14-6c75a5f43652
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