drbd: allow parallel flushes for multi-volume resources
To maintain write-order fidelity accros all volumes in a DRBD resource, the receiver of a P_BARRIER needs to issue flushes to all volumes. We used to do this by calling blkdev_issue_flush(), synchronously, one volume at a time. We now submit all flushes to all volumes in parallel, then wait for all completions, to reduce worst-case latencies on multi-volume resources. Signed-off-by: NPhilipp Reisner <philipp.reisner@linbit.com> Signed-off-by: NLars Ellenberg <lars.ellenberg@linbit.com> Signed-off-by: NJens Axboe <axboe@fb.com>
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