aio: Infrastructure for io_method=worker

This commit contains the basic, system-wide, infrastructure for
io_method=worker. It does not yet actually execute IO, this commit just
provides the infrastructure for running IO workers, kept separate for easier
review.

The number of IO workers can be adjusted with a PGC_SIGHUP GUC. Eventually
we'd like to make the number of workers dynamically scale up/down based on the
current "IO load".

To allow the number of IO workers to be increased without a restart, we need
to reserve PGPROC entries for the workers unconditionally. This has been
judged to be worth the cost. If it turns out to be problematic, we can
introduce a PGC_POSTMASTER GUC to control the maximum number.

As io workers might be needed during shutdown, e.g. for AIO during the
shutdown checkpoint, a new PMState phase is added. IO workers are shut down
after the shutdown checkpoint has been performed and walsender/archiver have
shut down, but before the checkpointer itself shuts down. See also
87a6690cc69.

Updates PGSTAT_FILE_FORMAT_ID due to the addition of a new BackendType.

Reviewed-by: Noah Misch <noah@leadboat.com>
Co-authored-by: Thomas Munro <thomas.munro@gmail.com>
Co-authored-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/uvrtrknj4kdytuboidbhwclo4gxhswwcpgadptsjvjqcluzmah%40brqs62irg4dt
Discussion: https://postgr.es/m/20210223100344.llw5an2aklengrmn@alap3.anarazel.de
Discussion: https://postgr.es/m/stj36ea6yyhoxtqkhpieia2z4krnam7qyetc57rfezgk4zgapf@gcnactj4z56m

13 files changed, 282 insertions, 12 deletions

diff --git a/src/backend/postmaster/launch_backend.c b/src/backend/postmaster/launch_backend.c
index 77fb877dbad..bf6b55ee830 100644
--- a/src/backend/postmaster/launch_backend.c
+++ b/src/backend/postmaster/launch_backend.c
@@ -48,6 +48,7 @@
 #include "replication/slotsync.h"
 #include "replication/walreceiver.h"
 #include "storage/dsm.h"
+#include "storage/io_worker.h"
 #include "storage/pg_shmem.h"
 #include "tcop/backend_startup.h"
 #include "utils/memutils.h"
@@ -197,6 +198,7 @@ static child_process_kind child_process_kinds[] = {
 	[B_ARCHIVER] = {"archiver", PgArchiverMain, true},
 	[B_BG_WRITER] = {"bgwriter", BackgroundWriterMain, true},
 	[B_CHECKPOINTER] = {"checkpointer", CheckpointerMain, true},
+	[B_IO_WORKER] = {"io_worker", IoWorkerMain, true},
 	[B_STARTUP] = {"startup", StartupProcessMain, true},
 	[B_WAL_RECEIVER] = {"wal_receiver", WalReceiverMain, true},
 	[B_WAL_SUMMARIZER] = {"wal_summarizer", WalSummarizerMain, true},
diff --git a/src/backend/postmaster/pmchild.c b/src/backend/postmaster/pmchild.c
index 0d473226c3a..cde1d23a4ca 100644
--- a/src/backend/postmaster/pmchild.c
+++ b/src/backend/postmaster/pmchild.c
@@ -101,6 +101,7 @@ InitPostmasterChildSlots(void)
 
 	pmchild_pools[B_AUTOVAC_WORKER].size = autovacuum_worker_slots;
 	pmchild_pools[B_BG_WORKER].size = max_worker_processes;
+	pmchild_pools[B_IO_WORKER].size = MAX_IO_WORKERS;
 
 	/*
 	 * There can be only one of each of these running at a time.  They each
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index d13846298bd..a0c37532d2f 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -108,9 +108,12 @@
 #include "replication/logicallauncher.h"
 #include "replication/slotsync.h"
 #include "replication/walsender.h"
+#include "storage/aio_subsys.h"
 #include "storage/fd.h"
+#include "storage/io_worker.h"
 #include "storage/ipc.h"
 #include "storage/pmsignal.h"
+#include "storage/proc.h"
 #include "tcop/backend_startup.h"
 #include "tcop/tcopprot.h"
 #include "utils/datetime.h"
@@ -340,6 +343,7 @@ typedef enum
 								 * ckpt */
 	PM_WAIT_XLOG_ARCHIVAL,		/* waiting for archiver and walsenders to
 								 * finish */
+	PM_WAIT_IO_WORKERS,			/* waiting for io workers to exit */
 	PM_WAIT_CHECKPOINTER,		/* waiting for checkpointer to shut down */
 	PM_WAIT_DEAD_END,			/* waiting for dead-end children to exit */
 	PM_NO_CHILDREN,				/* all important children have exited */
@@ -402,6 +406,10 @@ bool		LoadedSSL = false;
 static DNSServiceRef bonjour_sdref = NULL;
 #endif
 
+/* State for IO worker management. */
+static int	io_worker_count = 0;
+static PMChild *io_worker_children[MAX_IO_WORKERS];
+
 /*
  * postmaster.c - function prototypes
  */
@@ -436,6 +444,8 @@ static void TerminateChildren(int signal);
 static int	CountChildren(BackendTypeMask targetMask);
 static void LaunchMissingBackgroundProcesses(void);
 static void maybe_start_bgworkers(void);
+static bool maybe_reap_io_worker(int pid);
+static void maybe_adjust_io_workers(void);
 static bool CreateOptsFile(int argc, char *argv[], char *fullprogname);
 static PMChild *StartChildProcess(BackendType type);
 static void StartSysLogger(void);
@@ -1365,6 +1375,11 @@ PostmasterMain(int argc, char *argv[])
 	 */
 	AddToDataDirLockFile(LOCK_FILE_LINE_PM_STATUS, PM_STATUS_STARTING);
 
+	UpdatePMState(PM_STARTUP);
+
+	/* Make sure we can perform I/O while starting up. */
+	maybe_adjust_io_workers();
+
 	/* Start bgwriter and checkpointer so they can help with recovery */
 	if (CheckpointerPMChild == NULL)
 		CheckpointerPMChild = StartChildProcess(B_CHECKPOINTER);
@@ -1377,7 +1392,6 @@ PostmasterMain(int argc, char *argv[])
 	StartupPMChild = StartChildProcess(B_STARTUP);
 	Assert(StartupPMChild != NULL);
 	StartupStatus = STARTUP_RUNNING;
-	UpdatePMState(PM_STARTUP);
 
 	/* Some workers may be scheduled to start now */
 	maybe_start_bgworkers();
@@ -2502,6 +2516,16 @@ process_pm_child_exit(void)
 			continue;
 		}
 
+		/* Was it an IO worker? */
+		if (maybe_reap_io_worker(pid))
+		{
+			if (!EXIT_STATUS_0(exitstatus) && !EXIT_STATUS_1(exitstatus))
+				HandleChildCrash(pid, exitstatus, _("io worker"));
+
+			maybe_adjust_io_workers();
+			continue;
+		}
+
 		/*
 		 * Was it a backend or a background worker?
 		 */
@@ -2723,6 +2747,7 @@ HandleFatalError(QuitSignalReason reason, bool consider_sigabrt)
 		case PM_WAIT_XLOG_SHUTDOWN:
 		case PM_WAIT_XLOG_ARCHIVAL:
 		case PM_WAIT_CHECKPOINTER:
+		case PM_WAIT_IO_WORKERS:
 
 			/*
 			 * NB: Similar code exists in PostmasterStateMachine()'s handling
@@ -2905,20 +2930,21 @@ PostmasterStateMachine(void)
 
 		/*
 		 * If we are doing crash recovery or an immediate shutdown then we
-		 * expect archiver, checkpointer and walsender to exit as well,
-		 * otherwise not.
+		 * expect archiver, checkpointer, io workers and walsender to exit as
+		 * well, otherwise not.
 		 */
 		if (FatalError || Shutdown >= ImmediateShutdown)
 			targetMask = btmask_add(targetMask,
 									B_CHECKPOINTER,
 									B_ARCHIVER,
+									B_IO_WORKER,
 									B_WAL_SENDER);
 
 		/*
-		 * Normally walsenders and archiver will continue running; they will
-		 * be terminated later after writing the checkpoint record.  We also
-		 * let dead-end children to keep running for now.  The syslogger
-		 * process exits last.
+		 * Normally archiver, checkpointer, IO workers and walsenders will
+		 * continue running; they will be terminated later after writing the
+		 * checkpoint record.  We also let dead-end children to keep running
+		 * for now.  The syslogger process exits last.
 		 *
 		 * This assertion checks that we have covered all backend types,
 		 * either by including them in targetMask, or by noting here that they
@@ -2933,12 +2959,13 @@ PostmasterStateMachine(void)
 									B_LOGGER);
 
 			/*
-			 * Archiver, checkpointer and walsender may or may not be in
-			 * targetMask already.
+			 * Archiver, checkpointer, IO workers, and walsender may or may
+			 * not be in targetMask already.
 			 */
 			remainMask = btmask_add(remainMask,
 									B_ARCHIVER,
 									B_CHECKPOINTER,
+									B_IO_WORKER,
 									B_WAL_SENDER);
 
 			/* these are not real postmaster children */
@@ -3039,11 +3066,25 @@ PostmasterStateMachine(void)
 	{
 		/*
 		 * PM_WAIT_XLOG_ARCHIVAL state ends when there are no children other
-		 * than checkpointer, dead-end children and logger left. There
+		 * than checkpointer, io workers and dead-end children left. There
 		 * shouldn't be any regular backends left by now anyway; what we're
 		 * really waiting for is for walsenders and archiver to exit.
 		 */
-		if (CountChildren(btmask_all_except(B_CHECKPOINTER, B_LOGGER, B_DEAD_END_BACKEND)) == 0)
+		if (CountChildren(btmask_all_except(B_CHECKPOINTER, B_IO_WORKER,
+											B_LOGGER, B_DEAD_END_BACKEND)) == 0)
+		{
+			UpdatePMState(PM_WAIT_IO_WORKERS);
+			SignalChildren(SIGUSR2, btmask(B_IO_WORKER));
+		}
+	}
+
+	if (pmState == PM_WAIT_IO_WORKERS)
+	{
+		/*
+		 * PM_WAIT_IO_WORKERS state ends when there's only checkpointer and
+		 * dead_end children left.
+		 */
+		if (io_worker_count == 0)
 		{
 			UpdatePMState(PM_WAIT_CHECKPOINTER);
 
@@ -3171,10 +3212,14 @@ PostmasterStateMachine(void)
 		/* re-create shared memory and semaphores */
 		CreateSharedMemoryAndSemaphores();
 
+		UpdatePMState(PM_STARTUP);
+
+		/* Make sure we can perform I/O while starting up. */
+		maybe_adjust_io_workers();
+
 		StartupPMChild = StartChildProcess(B_STARTUP);
 		Assert(StartupPMChild != NULL);
 		StartupStatus = STARTUP_RUNNING;
-		UpdatePMState(PM_STARTUP);
 		/* crash recovery started, reset SIGKILL flag */
 		AbortStartTime = 0;
 
@@ -3198,6 +3243,7 @@ pmstate_name(PMState state)
 			PM_TOSTR_CASE(PM_WAIT_BACKENDS);
 			PM_TOSTR_CASE(PM_WAIT_XLOG_SHUTDOWN);
 			PM_TOSTR_CASE(PM_WAIT_XLOG_ARCHIVAL);
+			PM_TOSTR_CASE(PM_WAIT_IO_WORKERS);
 			PM_TOSTR_CASE(PM_WAIT_DEAD_END);
 			PM_TOSTR_CASE(PM_WAIT_CHECKPOINTER);
 			PM_TOSTR_CASE(PM_NO_CHILDREN);
@@ -3236,6 +3282,16 @@ LaunchMissingBackgroundProcesses(void)
 		StartSysLogger();
 
 	/*
+	 * The number of configured workers might have changed, or a prior start
+	 * of a worker might have failed. Check if we need to start/stop any
+	 * workers.
+	 *
+	 * A config file change will always lead to this function being called, so
+	 * we always will process the config change in a timely manner.
+	 */
+	maybe_adjust_io_workers();
+
+	/*
 	 * The checkpointer and the background writer are active from the start,
 	 * until shutdown is initiated.
 	 *
@@ -4120,6 +4176,7 @@ bgworker_should_start_now(BgWorkerStartTime start_time)
 		case PM_WAIT_DEAD_END:
 		case PM_WAIT_XLOG_ARCHIVAL:
 		case PM_WAIT_XLOG_SHUTDOWN:
+		case PM_WAIT_IO_WORKERS:
 		case PM_WAIT_BACKENDS:
 		case PM_STOP_BACKENDS:
 			break;
@@ -4270,6 +4327,99 @@ maybe_start_bgworkers(void)
 	}
 }
 
+static bool
+maybe_reap_io_worker(int pid)
+{
+	for (int id = 0; id < MAX_IO_WORKERS; ++id)
+	{
+		if (io_worker_children[id] &&
+			io_worker_children[id]->pid == pid)
+		{
+			ReleasePostmasterChildSlot(io_worker_children[id]);
+
+			--io_worker_count;
+			io_worker_children[id] = NULL;
+			return true;
+		}
+	}
+	return false;
+}
+
+/*
+ * Start or stop IO workers, to close the gap between the number of running
+ * workers and the number of configured workers.  Used to respond to change of
+ * the io_workers GUC (by increasing and decreasing the number of workers), as
+ * well as workers terminating in response to errors (by starting
+ * "replacement" workers).
+ */
+static void
+maybe_adjust_io_workers(void)
+{
+	if (!pgaio_workers_enabled())
+		return;
+
+	/*
+	 * If we're in final shutting down state, then we're just waiting for all
+	 * processes to exit.
+	 */
+	if (pmState >= PM_WAIT_IO_WORKERS)
+		return;
+
+	/* Don't start new workers during an immediate shutdown either. */
+	if (Shutdown >= ImmediateShutdown)
+		return;
+
+	/*
+	 * Don't start new workers if we're in the shutdown phase of a crash
+	 * restart. But we *do* need to start if we're already starting up again.
+	 */
+	if (FatalError && pmState >= PM_STOP_BACKENDS)
+		return;
+
+	Assert(pmState < PM_WAIT_IO_WORKERS);
+
+	/* Not enough running? */
+	while (io_worker_count < io_workers)
+	{
+		PMChild    *child;
+		int			id;
+
+		/* find unused entry in io_worker_children array */
+		for (id = 0; id < MAX_IO_WORKERS; ++id)
+		{
+			if (io_worker_children[id] == NULL)
+				break;
+		}
+		if (id == MAX_IO_WORKERS)
+			elog(ERROR, "could not find a free IO worker ID");
+
+		/* Try to launch one. */
+		child = StartChildProcess(B_IO_WORKER);
+		if (child != NULL)
+		{
+			io_worker_children[id] = child;
+			++io_worker_count;
+		}
+		else
+			break;				/* XXX try again soon? */
+	}
+
+	/* Too many running? */
+	if (io_worker_count > io_workers)
+	{
+		/* ask the IO worker in the highest slot to exit */
+		for (int id = MAX_IO_WORKERS - 1; id >= 0; --id)
+		{
+			if (io_worker_children[id] != NULL)
+			{
+				kill(io_worker_children[id]->pid, SIGUSR2);
+				break;
+			}
+		}
+	}
+}
+
+
 /*
  * When a backend asks to be notified about worker state changes, we
  * set a flag in its backend entry.  The background worker machinery needs
diff --git a/src/backend/storage/aio/Makefile b/src/backend/storage/aio/Makefile
index 89f821ea7e1..f51c34a37f8 100644
--- a/src/backend/storage/aio/Makefile
+++ b/src/backend/storage/aio/Makefile
@@ -15,6 +15,7 @@ OBJS = \
 	aio_io.o \
 	aio_target.o \
 	method_sync.o \
+	method_worker.o \
 	read_stream.o
 
 include $(top_srcdir)/src/backend/common.mk
diff --git a/src/backend/storage/aio/meson.build b/src/backend/storage/aio/meson.build
index 2c26089d52e..74f94c6e40b 100644
--- a/src/backend/storage/aio/meson.build
+++ b/src/backend/storage/aio/meson.build
@@ -7,5 +7,6 @@ backend_sources += files(
   'aio_io.c',
   'aio_target.c',
   'method_sync.c',
+  'method_worker.c',
   'read_stream.c',
 )
diff --git a/src/backend/storage/aio/method_worker.c b/src/backend/storage/aio/method_worker.c
new file mode 100644
index 00000000000..0ef9ef93e2b
--- /dev/null
+++ b/src/backend/storage/aio/method_worker.c
@@ -0,0 +1,88 @@
+/*-------------------------------------------------------------------------
+ *
+ * method_worker.c
+ *    AIO - perform AIO using worker processes
+ *
+ * Portions Copyright (c) 1996-2025, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * IDENTIFICATION
+ *	  src/backend/storage/aio/method_worker.c
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "libpq/pqsignal.h"
+#include "miscadmin.h"
+#include "postmaster/auxprocess.h"
+#include "postmaster/interrupt.h"
+#include "storage/aio_subsys.h"
+#include "storage/io_worker.h"
+#include "storage/ipc.h"
+#include "storage/latch.h"
+#include "storage/proc.h"
+#include "tcop/tcopprot.h"
+#include "utils/wait_event.h"
+
+
+/* GUCs */
+int			io_workers = 3;
+
+
+void
+IoWorkerMain(const void *startup_data, size_t startup_data_len)
+{
+	sigjmp_buf	local_sigjmp_buf;
+
+	MyBackendType = B_IO_WORKER;
+	AuxiliaryProcessMainCommon();
+
+	pqsignal(SIGHUP, SignalHandlerForConfigReload);
+	pqsignal(SIGINT, die);		/* to allow manually triggering worker restart */
+
+	/*
+	 * Ignore SIGTERM, will get explicit shutdown via SIGUSR2 later in the
+	 * shutdown sequence, similar to checkpointer.
+	 */
+	pqsignal(SIGTERM, SIG_IGN);
+	/* SIGQUIT handler was already set up by InitPostmasterChild */
+	pqsignal(SIGALRM, SIG_IGN);
+	pqsignal(SIGPIPE, SIG_IGN);
+	pqsignal(SIGUSR1, procsignal_sigusr1_handler);
+	pqsignal(SIGUSR2, SignalHandlerForShutdownRequest);
+
+	/* see PostgresMain() */
+	if (sigsetjmp(local_sigjmp_buf, 1) != 0)
+	{
+		error_context_stack = NULL;
+		HOLD_INTERRUPTS();
+
+		EmitErrorReport();
+
+		proc_exit(1);
+	}
+
+	/* We can now handle ereport(ERROR) */
+	PG_exception_stack = &local_sigjmp_buf;
+
+	sigprocmask(SIG_SETMASK, &UnBlockSig, NULL);
+
+	while (!ShutdownRequestPending)
+	{
+		WaitLatch(MyLatch, WL_LATCH_SET | WL_EXIT_ON_PM_DEATH, -1,
+				  WAIT_EVENT_IO_WORKER_MAIN);
+		ResetLatch(MyLatch);
+		CHECK_FOR_INTERRUPTS();
+	}
+
+	proc_exit(0);
+}
+
+bool
+pgaio_workers_enabled(void)
+{
+	/* placeholder for future commit */
+	return false;
+}
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
index 55ab2da299b..0554a4ae3c7 100644
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -3316,6 +3316,13 @@ ProcessInterrupts(void)
 					(errcode(ERRCODE_ADMIN_SHUTDOWN),
 					 errmsg("terminating background worker \"%s\" due to administrator command",
 							MyBgworkerEntry->bgw_type)));
+		else if (AmIoWorkerProcess())
+		{
+			ereport(DEBUG1,
+					(errmsg_internal("io worker shutting down due to administrator command")));
+
+			proc_exit(0);
+		}
 		else
 			ereport(FATAL,
 					(errcode(ERRCODE_ADMIN_SHUTDOWN),
diff --git a/src/backend/utils/activity/pgstat_backend.c b/src/backend/utils/activity/pgstat_backend.c
index a8cb54a7732..5518a18e060 100644
--- a/src/backend/utils/activity/pgstat_backend.c
+++ b/src/backend/utils/activity/pgstat_backend.c
@@ -375,6 +375,7 @@ pgstat_tracks_backend_bktype(BackendType bktype)
 		case B_LOGGER:
 		case B_BG_WRITER:
 		case B_CHECKPOINTER:
+		case B_IO_WORKER:
 		case B_STARTUP:
 			return false;
 
diff --git a/src/backend/utils/activity/pgstat_io.c b/src/backend/utils/activity/pgstat_io.c
index eb575025596..c8de9c9e2d3 100644
--- a/src/backend/utils/activity/pgstat_io.c
+++ b/src/backend/utils/activity/pgstat_io.c
@@ -376,6 +376,7 @@ pgstat_tracks_io_bktype(BackendType bktype)
 		case B_BG_WORKER:
 		case B_BG_WRITER:
 		case B_CHECKPOINTER:
+		case B_IO_WORKER:
 		case B_SLOTSYNC_WORKER:
 		case B_STANDALONE_BACKEND:
 		case B_STARTUP:
diff --git a/src/backend/utils/activity/wait_event_names.txt b/src/backend/utils/activity/wait_event_names.txt
index b44e4908b25..3f6dc3876b4 100644
--- a/src/backend/utils/activity/wait_event_names.txt
+++ b/src/backend/utils/activity/wait_event_names.txt
@@ -57,6 +57,7 @@ BGWRITER_HIBERNATE	"Waiting in background writer process, hibernating."
 BGWRITER_MAIN	"Waiting in main loop of background writer process."
 CHECKPOINTER_MAIN	"Waiting in main loop of checkpointer process."
 CHECKPOINTER_SHUTDOWN	"Waiting for checkpointer process to be terminated."
+IO_WORKER_MAIN	"Waiting in main loop of IO Worker process."
 LOGICAL_APPLY_MAIN	"Waiting in main loop of logical replication apply process."
 LOGICAL_LAUNCHER_MAIN	"Waiting in main loop of logical replication launcher process."
 LOGICAL_PARALLEL_APPLY_MAIN	"Waiting in main loop of logical replication parallel apply process."
diff --git a/src/backend/utils/init/miscinit.c b/src/backend/utils/init/miscinit.c
index dc3521457c7..43b4dbccc3d 100644
--- a/src/backend/utils/init/miscinit.c
+++ b/src/backend/utils/init/miscinit.c
@@ -293,6 +293,9 @@ GetBackendTypeDesc(BackendType backendType)
 		case B_CHECKPOINTER:
 			backendDesc = gettext_noop("checkpointer");
 			break;
+		case B_IO_WORKER:
+			backendDesc = gettext_noop("io worker");
+			break;
 		case B_LOGGER:
 			backendDesc = gettext_noop("logger");
 			break;
diff --git a/src/backend/utils/misc/guc_tables.c b/src/backend/utils/misc/guc_tables.c
index 4984d12606c..c89316ce294 100644
--- a/src/backend/utils/misc/guc_tables.c
+++ b/src/backend/utils/misc/guc_tables.c
@@ -75,6 +75,7 @@
 #include "storage/aio.h"
 #include "storage/bufmgr.h"
 #include "storage/bufpage.h"
+#include "storage/io_worker.h"
 #include "storage/large_object.h"
 #include "storage/pg_shmem.h"
 #include "storage/predicate.h"
@@ -3268,6 +3269,18 @@ struct config_int ConfigureNamesInt[] =
 	},
 
 	{
+		{"io_workers",
+			PGC_SIGHUP,
+			RESOURCES_IO,
+			gettext_noop("Number of IO worker processes, for io_method=worker."),
+			NULL,
+		},
+		&io_workers,
+		3, 1, MAX_IO_WORKERS,
+		NULL, NULL, NULL
+	},
+
+	{
 		{"backend_flush_after", PGC_USERSET, RESOURCES_IO,
 			gettext_noop("Number of pages after which previously performed writes are flushed to disk."),
 			gettext_noop("0 disables forced writeback."),
diff --git a/src/backend/utils/misc/postgresql.conf.sample b/src/backend/utils/misc/postgresql.conf.sample
index db44fa563b5..7d0bf1dc006 100644
--- a/src/backend/utils/misc/postgresql.conf.sample
+++ b/src/backend/utils/misc/postgresql.conf.sample
@@ -207,6 +207,7 @@
 					# can execute simultaneously
 					# -1 sets based on shared_buffers
 					# (change requires restart)
+#io_workers = 3				# 1-32;
 
 # - Worker Processes -