drbd: prepare receiving side for REQ_DISCARD
If the receiver needs to serve a discard request on a queue that does not announce to be discard cabable, it falls back to do synchronous blkdev_issue_zeroout(). We expect only "reasonably" large (up to one activity log extent?) discard requests. We do this to not to not block the receiver for too long in this fallback code path, and to not set/clear too many bits inside one spinlock_irq_save() in drbd_set_in_sync/drbd_set_out_of_sync, 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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