block: default to QD=1 writes for blk-mq rotational zoned devices

For blk-mq rotational zoned block devices (e.g. SMR HDDs), default to
having zone write plugging limit write operations to a maximum queue
depth of 1 for all zones. This significantly reduce write seek overhead
and improves SMR HDD write throughput.

For remotely connected disks with a very high network latency this
features might not be useful. However, remotely connected zoned devices
are rare at the moment, and we cannot know the round trip latency to
pick a good default for network attached devices. System administrators
can however disable this feature in that case.

For BIO based (non blk-mq) rotational zoned block devices, the device
driver (e.g. a DM target driver) can directly set an appropriate
default.

Signed-off-by: Damien Le Moal <[email protected]>
Reviewed-by: Hannes Reinecke <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Reviewed-by: Bart Van Assche <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Signed-off-by: Jens Axboe <[email protected]>

Author: damien-lemoal

block/blk-sysfs.c

@@ -967,6 +967,14 @@ int blk_register_queue(struct gendisk *disk)
 		blk_mq_debugfs_register(q);
 	blk_debugfs_unlock(q, memflags);
 
+	/*
+	 * For blk-mq rotational zoned devices, default to using QD=1
+	 * writes. For non-mq rotational zoned devices, the device driver can
+	 * set an appropriate default.
+	 */
+	if (queue_is_mq(q) && blk_queue_rot(q) && blk_queue_is_zoned(q))
+		blk_queue_flag_set(QUEUE_FLAG_ZONED_QD1_WRITES, q);
+
 	ret = disk_register_independent_access_ranges(disk);
 	if (ret)
 		goto out_debugfs_remove;