Hi all, this series adds support for multipathing, that is accessing nvme namespaces through multiple controllers to the nvme core driver. It is a very thin and efficient implementation that relies on close cooperation with other bits of the nvme driver, and few small and simple block helpers. Compared to dm-multipath the important differences are how management of the paths is done, and how the I/O path works. Management of the paths is fully integrated into the nvme driver, for each newly found nvme controller we check if there are other controllers that refer to the same subsystem, and if so we link them up in the nvme driver. Then for each namespace found we check if the namespace id and identifiers match to check if we have multiple controllers that refer to the same namespaces. For now path availability is based entirely on the controller status, which at least for fabrics will be continuously updated based on the mandatory keep alive timer. Once the Asynchronous Namespace Access (ANA) proposal passes in NVMe we will also get per-namespace states in addition to that, but for now any details of that remain confidential to NVMe members. The I/O path is very different from the existing multipath drivers, which is enabled by the fact that NVMe (unlike SCSI) does not support partial completions - a controller will either complete a whole command or not, but never only complete parts of it. Because of that there is no need to clone bios or requests - the I/O path simply redirects the I/O to a suitable path. For successful commands multipath is not in the completion stack at all. For failed commands we decide if the error could be a path failure, and if yes remove the bios from the request structure and requeue them before completing the request. All together this means there is no performance degradation compared to normal nvme operation when using the multipath device node (at least not until I find a dual ported DRAM backed device :)) There are a couple questions left in the individual patches, comments welcome. Note that this series requires the previous series to remove bi_bdev, in doubt use the git tree below for testing. A git tree is available at: git://git.infradead.org/users/hch/block.git nvme-mpath gitweb: http://git.infradead.org/users/hch/block.git/shortlog/refs/heads/nvme-mpath
