Temporary: Increase BAS_BULKREAD size

Without this we only can execute very little AIO for sequential scans, as
there's just not enough buffers in the ring.  This isn't the right fix, as
just increasing the ring size can have negative performance implications in
workloads where the kernel has all the data cached.

Author:
Reviewed-By:
Discussion: https://postgr.es/m/
Backpatch:

Author: anarazel

src/backend/storage/buffer/freelist.c

@@ -555,7 +555,12 @@ GetAccessStrategy(BufferAccessStrategyType btype)
 			return NULL;
 
 		case BAS_BULKREAD:
-			ring_size_kb = 256;
+
+			/*
+			 * FIXME: Temporary increase to allow large enough streaming reads
+			 * to actually benefit from AIO. This needs a better solution.
+			 */
+			ring_size_kb = 2 * 1024;
 			break;
 		case BAS_BULKWRITE:
 			ring_size_kb = 16 * 1024;