zvol_os.c: Increase optimal IO size

Since zvol read and write operations can process up to (DMU_MAX_ACCESS /
2) bytes in a single operation, the current optimal I/O size is set too
low. SCST directly reports this value as the optimal transfer length for
the target SCSI device. Increasing it from the previous volblocksize
significantly improves performance for VDIv2 workloads.

module/os/linux/zfs/zvol_os.c

@@ -1193,7 +1193,7 @@ zvol_queue_limits_init(zvol_queue_limits_t *limits, zvol_state_t *zv,
 		limits->zql_max_segment_size = UINT_MAX;
 	}
 
-	limits->zql_io_opt = zv->zv_volblocksize;
+	limits->zql_io_opt = DMU_MAX_ACCESS/2;
 
 	limits->zql_physical_block_size = zv->zv_volblocksize;
 	limits->zql_max_discard_sectors =