Increase default vacuum_buffer_usage_limit to 2MB.

The BAS_VACUUM ring size has been 256kB since commit d526575f introduced
the mechanism 17 years ago.  Commit 1cbbee03 recently made it
configurable but retained the traditional default.  The correct default
size has been debated for years, but 256kB is certainly very small.
VACUUM soon needs to write back data it dirtied only 32 blocks ago,
which usually requires flushing the WAL.  New experiments in prefetching
pages for VACUUM exacerbated the problem by crashing into dirty data
even sooner.  Let's make the default 2MB.  That's 1.6% of the default
toy buffer pool size, and 0.2% of 1GB, which would be a considered a
small shared_buffers setting for a real system these days.  Users are
still free to set the GUC to a different value.

Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/20240403221257.md4gfki3z75cdyf6%40awork3.anarazel.de
Discussion: https://postgr.es/m/CA%2BhUKGLY4Q4ZY4f1rvnFtv6%2BPkjNf8MejdPkcju3Qii9DYqqcQ%40mail.gmail.com

doc/src/sgml/config.sgml

@@ -1961,7 +1961,7 @@ include_dir 'conf.d'
         valid sizes range from <literal>128 kB</literal> to
         <literal>16 GB</literal>.  If the specified size would exceed 1/8 the
         size of <varname>shared_buffers</varname>, the size is silently capped
-        to that value.  The default value is <literal>256 kB</literal>.  If
+        to that value.  The default value is <literal>2MB</literal>.  If
         this value is specified without units, it is taken as kilobytes.  This
         parameter can be set at any time.  It can be overridden for
         <xref linkend="sql-vacuum"/> and <xref linkend="sql-analyze"/>

src/backend/storage/buffer/freelist.c

@@ -561,7 +561,7 @@ GetAccessStrategy(BufferAccessStrategyType btype)
 			ring_size_kb = 16 * 1024;
 			break;
 		case BAS_VACUUM:
-			ring_size_kb = 256;
+			ring_size_kb = 2048;
 			break;
 
 		default:

src/backend/utils/init/globals.c

@@ -143,7 +143,7 @@ int			max_parallel_workers = 8;
 int			MaxBackends = 0;
 
 /* GUC parameters for vacuum */
-int			VacuumBufferUsageLimit = 256;
+int			VacuumBufferUsageLimit = 2048;
 
 int			VacuumCostPageHit = 1;
 int			VacuumCostPageMiss = 2;

src/backend/utils/misc/guc_tables.c

@@ -2275,7 +2275,7 @@ struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_KB
 		},
 		&VacuumBufferUsageLimit,
-		256, 0, MAX_BAS_VAC_RING_SIZE_KB,
+		2048, 0, MAX_BAS_VAC_RING_SIZE_KB,
 		check_vacuum_buffer_usage_limit, NULL, NULL
 	},
 

src/backend/utils/misc/postgresql.conf.sample

@@ -166,7 +166,7 @@
 					#   mmap
 					# (change requires restart)
 #min_dynamic_shared_memory = 0MB	# (change requires restart)
-#vacuum_buffer_usage_limit = 256kB	# size of vacuum and analyze buffer access strategy ring;
+#vacuum_buffer_usage_limit = 2MB	# size of vacuum and analyze buffer access strategy ring;
 					# 0 to disable vacuum buffer access strategy;
 					# range 128kB to 16GB