diff --git a/src/backend/utils/misc/guc.c b/src/backend/utils/misc/guc.c
index d374f53..cdb6a61 100644
--- a/src/backend/utils/misc/guc.c
+++ b/src/backend/utils/misc/guc.c
@@ -5995,7 +5995,19 @@ convert_to_base_unit(double value, const char *unit,
 		if (base_unit == table[i].base_unit &&
 			strcmp(unitstr, table[i].unit) == 0)
 		{
-			*base_value = value * table[i].multiplier;
+			double		cvalue = value * table[i].multiplier;
+
+			/*
+			 * If the user gave a fractional value such as "30.1GB", round it
+			 * off to the nearest multiple of the next smaller unit, if there
+			 * is one.
+			 */
+			if (*table[i + 1].unit &&
+				base_unit == table[i + 1].base_unit)
+				cvalue = rint(cvalue / table[i + 1].multiplier) *
+					table[i + 1].multiplier;
+
+			*base_value = cvalue;
 			return true;
 		}
 	}