ircd::fs: Add nodelay option to write_opts; relax the flush on write by default.

include/ircd/fs/write.h

@@ -83,6 +83,11 @@ struct ircd::fs::write_opts
 	/// an exception will be thrown. We default to true because we have faith
 	/// in the useful propagation of an exception for this event.
 	bool interruptible {true};
+
+	/// Submits the I/O request immediately rather than allowing IRCd to
+	/// queue requests for a few iterations of the ircd::ios event loop.
+	/// (only relevant to aio).
+	bool nodelay {false};
 };
 
 inline

ircd/aio.cc

@@ -544,19 +544,32 @@ ircd::fs::aio::kernel::submit(request &request)
 	queue.at(qcount++) = static_cast<iocb *>(&request);
 	stats.cur_queued++;
 
+	// Determine whether the user wants (or needs) to submit without delay.
+	bool nodelay; switch(request.aio_lio_opcode)
+	{
+		case IOCB_CMD_PREADV:
+			nodelay = request.ropts->nodelay;
+			break;
+
+		case IOCB_CMD_PWRITEV:
+			nodelay = request.wopts->nodelay;
+			break;
+
+		default:
+			nodelay = true;
+			break;
+	}
+
 	const bool flush_now
 	{
+		// The nodelay flag is set
+		nodelay
+
 		// The queue has reached the configured size
-		qcount >= size_t(max_submit)
+		|| qcount >= size_t(max_submit)
 
 		// The queue has reached its maximum size
 		|| qcount >= queue.size()
-
-		// The request causes serialization. This is considered true for all
-		// non-reading events, even for different files and locations. It may
-		// be possible to optimize this condition.
-		|| request.aio_lio_opcode != IOCB_CMD_PREADV
-		|| request.ropts->nodelay
 	};
 
 	if(flush_now)