diff --git a/doc/src/sgml/config.sgml b/doc/src/sgml/config.sgml
index 2988865b116..4fd377d1c5f 100644
--- a/doc/src/sgml/config.sgml
+++ b/doc/src/sgml/config.sgml
@@ -2638,6 +2638,8 @@ include_dir 'conf.d'
          This parameter can only be set in
          the <filename>postgresql.conf</filename> file or on the server
          command line.
+         The maximum possible size depends on the operating system and block
+         size, but is typically 1MB on Unix and 128kB on Windows.
          The default is 128kB.
         </para>
        </listitem>
@@ -2655,6 +2657,8 @@ include_dir 'conf.d'
          higher than the <varname>io_max_combine_limit</varname> parameter, the
          lower value will silently be used instead, so both may need to be raised
          to increase the I/O size.
+         The maximum possible size depends on the operating system and block
+         size, but is typically 1MB on Unix and 128kB on Windows.
          The default is 128kB.
         </para>
        </listitem>
diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c
index d65fa07b44c..45bdf819d57 100644
--- a/src/backend/storage/aio/read_stream.c
+++ b/src/backend/storage/aio/read_stream.c
@@ -515,9 +515,10 @@ read_stream_begin_impl(int flags,
 	 * finishes we don't want to have to wait for its buffers to be consumed
 	 * before starting a new one.
 	 *
-	 * Be careful not to allow int16 to overflow (even though that's not
-	 * possible with the current GUC range limits), allowing also for the
-	 * spare entry and the overflow space.
+	 * Be careful not to allow int16 to overflow.  That is possible with the
+	 * current GUC range limits, so this is an artificial limit of ~32k
+	 * buffers and we'd need to adjust the types to exceed that.  We also have
+	 * to allow for the spare entry and the overflow space.
 	 */
 	max_pinned_buffers = (max_ios + 1) * io_combine_limit;
 	max_pinned_buffers = Min(max_pinned_buffers,
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index 66bda60f4ca..6abd1baeac8 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -200,9 +200,9 @@
 #backend_flush_after = 0		# measured in pages, 0 disables
 #effective_io_concurrency = 16		# 1-1000; 0 disables prefetching
 #maintenance_io_concurrency = 16	# 1-1000; 0 disables prefetching
-#io_max_combine_limit = 128kB		# usually 1-32 blocks (depends on OS)
+#io_max_combine_limit = 128kB		# usually 1-128 blocks (depends on OS)
 					# (change requires restart)
-#io_combine_limit = 128kB		# usually 1-32 blocks (depends on OS)
+#io_combine_limit = 128kB		# usually 1-128 blocks (depends on OS)
 
 #io_method = worker			# worker, sync (change requires restart)
 #io_max_concurrency = -1		# Max number of IOs that one process
diff --git a/src/include/port/pg_iovec.h b/src/include/port/pg_iovec.h
index d9891d3805d..df40c7208be 100644
--- a/src/include/port/pg_iovec.h
+++ b/src/include/port/pg_iovec.h
@@ -33,8 +33,12 @@ struct iovec
 
 #endif
 
-/* Define a reasonable maximum that is safe to use on the stack. */
-#define PG_IOV_MAX Min(IOV_MAX, 32)
+/*
+ * Define a reasonable maximum that is safe to use on the stack in arrays of
+ * struct iovec and other small types.  The operating system could limit us to
+ * a number as low as 16, but most systems have 1024.
+ */
+#define PG_IOV_MAX Min(IOV_MAX, 128)
 
 /*
  * Like preadv(), but with a prefix to remind us of a side-effect: on Windows