diff --git a/src/backend/port/posix_sema.c b/src/backend/port/posix_sema.c
index 3370adf..bdd6552 100644
--- a/src/backend/port/posix_sema.c
+++ b/src/backend/port/posix_sema.c
@@ -29,6 +29,7 @@
 #include <semaphore.h>
 #include <signal.h>
 #include <unistd.h>
+#include <sys/stat.h>
 
 #include "miscadmin.h"
 #include "storage/ipc.h"
@@ -181,10 +182,6 @@ PGSemaphoreShmemSize(int maxSemas)
  * are acquired here or in PGSemaphoreCreate, register an on_shmem_exit
  * callback to release them.
  *
- * The port number is passed for possible use as a key (for Posix, we use
- * it to generate the starting semaphore name).  In a standalone backend,
- * zero will be passed.
- *
  * In the Posix implementation, we acquire semaphores on-demand; the
  * maxSemas parameter is just used to size the arrays.  For unnamed
  * semaphores, there is an array of PGSemaphoreData structs in shared memory.
@@ -196,8 +193,22 @@ PGSemaphoreShmemSize(int maxSemas)
  * we don't have to expose the counters to other processes.)
  */
 void
-PGReserveSemaphores(int maxSemas, int port)
+PGReserveSemaphores(int maxSemas)
 {
+	struct stat statbuf;
+
+	/*
+	 * We use the data directory's inode number to seed the search for free
+	 * semaphore keys.  This minimizes the odds of collision with other
+	 * postmasters, while maximizing the odds that we will detect and clean up
+	 * semaphores left over from a crashed postmaster in our own directory.
+	 */
+	if (stat(DataDir, &statbuf) < 0)
+		ereport(FATAL,
+				(errcode_for_file_access(),
+				 errmsg("could not stat data directory \"%s\": %m",
+						DataDir)));
+
 #ifdef USE_NAMED_POSIX_SEMAPHORES
 	mySemPointers = (sem_t **) malloc(maxSemas * sizeof(sem_t *));
 	if (mySemPointers == NULL)
@@ -215,7 +226,7 @@ PGReserveSemaphores(int maxSemas, int port)
 
 	numSems = 0;
 	maxSems = maxSemas;
-	nextSemKey = port * 1000;
+	nextSemKey = statbuf.st_ino;
 
 	on_shmem_exit(ReleaseSemaphores, 0);
 }
diff --git a/src/backend/port/sysv_sema.c b/src/backend/port/sysv_sema.c
index ac5106f..a1652cc 100644
--- a/src/backend/port/sysv_sema.c
+++ b/src/backend/port/sysv_sema.c
@@ -17,6 +17,7 @@
 #include <signal.h>
 #include <unistd.h>
 #include <sys/file.h>
+#include <sys/stat.h>
 #ifdef HAVE_SYS_IPC_H
 #include <sys/ipc.h>
 #endif
@@ -301,10 +302,6 @@ PGSemaphoreShmemSize(int maxSemas)
  * are acquired here or in PGSemaphoreCreate, register an on_shmem_exit
  * callback to release them.
  *
- * The port number is passed for possible use as a key (for SysV, we use
- * it to generate the starting semaphore key).  In a standalone backend,
- * zero will be passed.
- *
  * In the SysV implementation, we acquire semaphore sets on-demand; the
  * maxSemas parameter is just used to size the arrays.  There is an array
  * of PGSemaphoreData structs in shared memory, and a postmaster-local array
@@ -314,8 +311,22 @@ PGSemaphoreShmemSize(int maxSemas)
  * have clobbered.)
  */
 void
-PGReserveSemaphores(int maxSemas, int port)
+PGReserveSemaphores(int maxSemas)
 {
+	struct stat statbuf;
+
+	/*
+	 * We use the data directory's inode number to seed the search for free
+	 * semaphore keys.  This minimizes the odds of collision with other
+	 * postmasters, while maximizing the odds that we will detect and clean up
+	 * semaphores left over from a crashed postmaster in our own directory.
+	 */
+	if (stat(DataDir, &statbuf) < 0)
+		ereport(FATAL,
+				(errcode_for_file_access(),
+				 errmsg("could not stat data directory \"%s\": %m",
+						DataDir)));
+
 	/*
 	 * We must use ShmemAllocUnlocked(), since the spinlock protecting
 	 * ShmemAlloc() won't be ready yet.  (This ordering is necessary when we
@@ -332,7 +343,7 @@ PGReserveSemaphores(int maxSemas, int port)
 	if (mySemaSets == NULL)
 		elog(PANIC, "out of memory");
 	numSemaSets = 0;
-	nextSemaKey = port * 1000;
+	nextSemaKey = statbuf.st_ino;
 	nextSemaNumber = SEMAS_PER_SET; /* force sema set alloc on 1st call */
 
 	on_shmem_exit(ReleaseSemaphores, 0);
diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index 968506d..dab2920 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -390,11 +390,12 @@ PGSharedMemoryAttach(IpcMemoryId shmId,
 
 	/*
 	 * Try to attach to the segment and see if it matches our data directory.
-	 * This avoids key-conflict problems on machines that are running several
-	 * postmasters under the same userid and port number.  (That would not
-	 * ordinarily happen in production, but it can happen during parallel
-	 * testing.  Since our test setups don't open any TCP ports on Unix, such
-	 * cases don't conflict otherwise.)
+	 * This avoids any risk of duplicate-shmem-key conflicts on machines that
+	 * are running several postmasters under the same userid.
+	 *
+	 * (When we're called from PGSharedMemoryCreate, this stat call is
+	 * duplicative; but since this isn't a high-traffic case it's not worth
+	 * trying to optimize.)
 	 */
 	if (stat(DataDir, &statbuf) < 0)
 		return SHMSTATE_ANALYSIS_FAILURE;	/* can't stat; be conservative */
@@ -617,12 +618,9 @@ AnonymousShmemDetach(int status, Datum arg)
  * we do not fail upon collision with foreign shmem segments.  The idea here
  * is to detect and re-use keys that may have been assigned by a crashed
  * postmaster or backend.
- *
- * The port number is passed for possible use as a key (for SysV, we use
- * it to generate the starting shmem key).
  */
 PGShmemHeader *
-PGSharedMemoryCreate(Size size, int port,
+PGSharedMemoryCreate(Size size,
 					 PGShmemHeader **shim)
 {
 	IpcMemoryKey NextShmemSegID;
@@ -631,6 +629,17 @@ PGSharedMemoryCreate(Size size, int port,
 	struct stat statbuf;
 	Size		sysvsize;
 
+	/*
+	 * We use the data directory's ID info (inode and device numbers) to
+	 * positively identify shmem segments associated with this data dir, and
+	 * also as seeds for searching for a free shmem key.
+	 */
+	if (stat(DataDir, &statbuf) < 0)
+		ereport(FATAL,
+				(errcode_for_file_access(),
+				 errmsg("could not stat data directory \"%s\": %m",
+						DataDir)));
+
 	/* Complain if hugepages demanded but we can't possibly support them */
 #if !defined(MAP_HUGETLB)
 	if (huge_pages == HUGE_PAGES_ON)
@@ -659,10 +668,10 @@ PGSharedMemoryCreate(Size size, int port,
 	/*
 	 * Loop till we find a free IPC key.  Trust CreateDataDirLockFile() to
 	 * ensure no more than one postmaster per data directory can enter this
-	 * loop simultaneously.  (CreateDataDirLockFile() does not ensure that,
-	 * but prefer fixing it over coping here.)
+	 * loop simultaneously.  (CreateDataDirLockFile() does not entirely ensure
+	 * that, but prefer fixing it over coping here.)
 	 */
-	NextShmemSegID = 1 + port * 1000;
+	NextShmemSegID = statbuf.st_ino;
 
 	for (;;)
 	{
@@ -748,11 +757,6 @@ PGSharedMemoryCreate(Size size, int port,
 	hdr->dsm_control = 0;
 
 	/* Fill in the data directory ID info, too */
-	if (stat(DataDir, &statbuf) < 0)
-		ereport(FATAL,
-				(errcode_for_file_access(),
-				 errmsg("could not stat data directory \"%s\": %m",
-						DataDir)));
 	hdr->device = statbuf.st_dev;
 	hdr->inode = statbuf.st_ino;
 
diff --git a/src/backend/port/win32_sema.c b/src/backend/port/win32_sema.c
index 013c122..32cc697 100644
--- a/src/backend/port/win32_sema.c
+++ b/src/backend/port/win32_sema.c
@@ -44,7 +44,7 @@ PGSemaphoreShmemSize(int maxSemas)
  * process exits.
  */
 void
-PGReserveSemaphores(int maxSemas, int port)
+PGReserveSemaphores(int maxSemas)
 {
 	mySemSet = (HANDLE *) malloc(maxSemas * sizeof(HANDLE));
 	if (mySemSet == NULL)
diff --git a/src/backend/port/win32_shmem.c b/src/backend/port/win32_shmem.c
index ccd7b6b..6cb6328 100644
--- a/src/backend/port/win32_shmem.c
+++ b/src/backend/port/win32_shmem.c
@@ -194,7 +194,7 @@ EnableLockPagesPrivilege(int elevel)
  * standard header.
  */
 PGShmemHeader *
-PGSharedMemoryCreate(Size size, int port,
+PGSharedMemoryCreate(Size size,
 					 PGShmemHeader **shim)
 {
 	void	   *memAddress;
diff --git a/src/backend/postmaster/postmaster.c b/src/backend/postmaster/postmaster.c
index 62dc93d..a5446d5 100644
--- a/src/backend/postmaster/postmaster.c
+++ b/src/backend/postmaster/postmaster.c
@@ -385,7 +385,7 @@ static void getInstallationPaths(const char *argv0);
 static void checkControlFile(void);
 static Port *ConnCreate(int serverFd);
 static void ConnFree(Port *port);
-static void reset_shared(int port);
+static void reset_shared(void);
 static void SIGHUP_handler(SIGNAL_ARGS);
 static void pmdie(SIGNAL_ARGS);
 static void reaper(SIGNAL_ARGS);
@@ -1175,7 +1175,7 @@ PostmasterMain(int argc, char *argv[])
 	/*
 	 * Set up shared memory and semaphores.
 	 */
-	reset_shared(PostPortNumber);
+	reset_shared();
 
 	/*
 	 * Estimate number of openable files.  This must happen after setting up
@@ -2599,17 +2599,16 @@ InitProcessGlobals(void)
  * reset_shared -- reset shared memory and semaphores
  */
 static void
-reset_shared(int port)
+reset_shared(void)
 {
 	/*
 	 * Create or re-create shared memory and semaphores.
 	 *
 	 * Note: in each "cycle of life" we will normally assign the same IPC keys
-	 * (if using SysV shmem and/or semas), since the port number is used to
-	 * determine IPC keys.  This helps ensure that we will clean up dead IPC
-	 * objects if the postmaster crashes and is restarted.
+	 * (if using SysV shmem and/or semas).  This helps ensure that we will
+	 * clean up dead IPC objects if the postmaster crashes and is restarted.
 	 */
-	CreateSharedMemoryAndSemaphores(port);
+	CreateSharedMemoryAndSemaphores();
 }
 
 
@@ -3934,7 +3933,7 @@ PostmasterStateMachine(void)
 		/* re-read control file into local memory */
 		LocalProcessControlFile(true);
 
-		reset_shared(PostPortNumber);
+		reset_shared();
 
 		StartupPID = StartupDataBase();
 		Assert(StartupPID != 0);
@@ -4962,7 +4961,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores(0);
+		CreateSharedMemoryAndSemaphores();
 
 		/* And run the backend */
 		BackendRun(&port);		/* does not return */
@@ -4976,7 +4975,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitAuxiliaryProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores(0);
+		CreateSharedMemoryAndSemaphores();
 
 		AuxiliaryProcessMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -4989,7 +4988,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores(0);
+		CreateSharedMemoryAndSemaphores();
 
 		AutoVacLauncherMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -5002,7 +5001,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores(0);
+		CreateSharedMemoryAndSemaphores();
 
 		AutoVacWorkerMain(argc - 2, argv + 2);	/* does not return */
 	}
@@ -5020,7 +5019,7 @@ SubPostmasterMain(int argc, char *argv[])
 		InitProcess();
 
 		/* Attach process to shared data structures */
-		CreateSharedMemoryAndSemaphores(0);
+		CreateSharedMemoryAndSemaphores();
 
 		/* Fetch MyBgworkerEntry from shared memory */
 		shmem_slot = atoi(argv[1] + 15);
diff --git a/src/backend/storage/ipc/ipci.c b/src/backend/storage/ipc/ipci.c
index d7d7335..8853706 100644
--- a/src/backend/storage/ipc/ipci.c
+++ b/src/backend/storage/ipc/ipci.c
@@ -91,7 +91,7 @@ RequestAddinShmemSpace(Size size)
  * This is a bit code-wasteful and could be cleaned up.)
  */
 void
-CreateSharedMemoryAndSemaphores(int port)
+CreateSharedMemoryAndSemaphores(void)
 {
 	PGShmemHeader *shim = NULL;
 
@@ -163,14 +163,14 @@ CreateSharedMemoryAndSemaphores(int port)
 		/*
 		 * Create the shmem segment
 		 */
-		seghdr = PGSharedMemoryCreate(size, port, &shim);
+		seghdr = PGSharedMemoryCreate(size, &shim);
 
 		InitShmemAccess(seghdr);
 
 		/*
 		 * Create semaphores
 		 */
-		PGReserveSemaphores(numSemas, port);
+		PGReserveSemaphores(numSemas);
 
 		/*
 		 * If spinlocks are disabled, initialize emulation layer (which
diff --git a/src/backend/utils/init/postinit.c b/src/backend/utils/init/postinit.c
index 43b9f17..29c5ec7 100644
--- a/src/backend/utils/init/postinit.c
+++ b/src/backend/utils/init/postinit.c
@@ -445,12 +445,10 @@ InitCommunication(void)
 	if (!IsUnderPostmaster)		/* postmaster already did this */
 	{
 		/*
-		 * We're running a postgres bootstrap process or a standalone backend.
-		 * Though we won't listen on PostPortNumber, use it to select a shmem
-		 * key.  This increases the chance of detecting a leftover live
-		 * backend of this DataDir.
+		 * We're running a postgres bootstrap process or a standalone backend,
+		 * so we need to set up shmem.
 		 */
-		CreateSharedMemoryAndSemaphores(PostPortNumber);
+		CreateSharedMemoryAndSemaphores();
 	}
 }
 
diff --git a/src/include/storage/ipc.h b/src/include/storage/ipc.h
index e9b243f..d113813 100644
--- a/src/include/storage/ipc.h
+++ b/src/include/storage/ipc.h
@@ -76,6 +76,6 @@ extern void on_exit_reset(void);
 /* ipci.c */
 extern PGDLLIMPORT shmem_startup_hook_type shmem_startup_hook;
 
-extern void CreateSharedMemoryAndSemaphores(int port);
+extern void CreateSharedMemoryAndSemaphores(void);
 
 #endif							/* IPC_H */
diff --git a/src/include/storage/pg_sema.h b/src/include/storage/pg_sema.h
index b95efa1..1ac4f9a 100644
--- a/src/include/storage/pg_sema.h
+++ b/src/include/storage/pg_sema.h
@@ -41,7 +41,7 @@ typedef HANDLE PGSemaphore;
 extern Size PGSemaphoreShmemSize(int maxSemas);
 
 /* Module initialization (called during postmaster start or shmem reinit) */
-extern void PGReserveSemaphores(int maxSemas, int port);
+extern void PGReserveSemaphores(int maxSemas);
 
 /* Allocate a PGSemaphore structure with initial count 1 */
 extern PGSemaphore PGSemaphoreCreate(void);
diff --git a/src/include/storage/pg_shmem.h b/src/include/storage/pg_shmem.h
index ac24878..6bd5664 100644
--- a/src/include/storage/pg_shmem.h
+++ b/src/include/storage/pg_shmem.h
@@ -82,7 +82,7 @@ extern void PGSharedMemoryReAttach(void);
 extern void PGSharedMemoryNoReAttach(void);
 #endif
 
-extern PGShmemHeader *PGSharedMemoryCreate(Size size, int port,
+extern PGShmemHeader *PGSharedMemoryCreate(Size size,
 										   PGShmemHeader **shim);
 extern bool PGSharedMemoryIsInUse(unsigned long id1, unsigned long id2);
 extern void PGSharedMemoryDetach(void);
diff --git a/src/test/recovery/t/017_shm.pl b/src/test/recovery/t/017_shm.pl
index 7f10ff5..be35f90 100644
--- a/src/test/recovery/t/017_shm.pl
+++ b/src/test/recovery/t/017_shm.pl
@@ -4,16 +4,18 @@
 use strict;
 use warnings;
 use Config;
+use File::stat qw(stat);
 use IPC::Run 'run';
+use IPC::SharedMem;
+use IPC::SysV qw(IPC_CREAT S_IRUSR S_IWUSR);
 use PostgresNode;
 use Test::More;
 use TestLib;
 use Time::HiRes qw(usleep);
 
-plan tests => 5;
+plan tests => 4;
 
 my $tempdir = TestLib::tempdir;
-my $port;
 
 # Log "ipcs" diffs on a best-effort basis, swallowing any error.
 my $ipcs_before = "$tempdir/ipcs_before";
@@ -25,71 +27,40 @@ sub log_ipcs
 	return;
 }
 
-# These tests need a $port such that nothing creates or removes a segment in
-# $port's IpcMemoryKey range while this test script runs.  While there's no
-# way to ensure that in general, we do ensure that if PostgreSQL tests are the
-# only actors.  With TCP, the first get_new_node picks a port number.  With
-# Unix sockets, use a postmaster, $port_holder, to represent a key space
-# reservation.  $port_holder holds a reservation on the key space of port
-# 1+$port_holder->port if it created the first IpcMemoryKey of its own port's
-# key space.  If multiple copies of this test script run concurrently, they
-# will pick different ports.  $port_holder postmasters use odd-numbered ports,
-# and tests use even-numbered ports.  In the absence of collisions from other
-# shmget() activity, gnat starts with key 0x7d001 (512001), and flea starts
-# with key 0x7d002 (512002).
-my $port_holder;
-if (!$PostgresNode::use_tcp)
-{
-	my $lock_port;
-	for ($lock_port = 511; $lock_port < 711; $lock_port += 2)
-	{
-		$port_holder = PostgresNode->get_new_node(
-			"port${lock_port}_holder",
-			port     => $lock_port,
-			own_host => 1);
-		$port_holder->init;
-		$port_holder->append_conf('postgresql.conf', 'max_connections = 5');
-		$port_holder->start;
-		# Match the AddToDataDirLockFile() call in sysv_shmem.c.  Assume all
-		# systems not using sysv_shmem.c do use TCP.
-		my $shmem_key_line_prefix = sprintf("%9lu ", 1 + $lock_port * 1000);
-		last
-		  if slurp_file($port_holder->data_dir . '/postmaster.pid') =~
-		  /^$shmem_key_line_prefix/m;
-		$port_holder->stop;
-	}
-	$port = $lock_port + 1;
-}
-
 # Node setup.
-sub init_start
-{
-	my $name = shift;
-	my $ret = PostgresNode->get_new_node($name, port => $port, own_host => 1);
-	defined($port) or $port = $ret->port;    # same port for all nodes
-	$ret->init;
-	# Limit semaphore consumption, since we run several nodes concurrently.
-	$ret->append_conf('postgresql.conf', 'max_connections = 5');
-	$ret->start;
-	log_ipcs();
-	return $ret;
-}
-my $gnat = init_start 'gnat';
-my $flea = init_start 'flea';
+my $gnat = PostgresNode->get_new_node('gnat');
+$gnat->init;
+
+# Create a shmem segment that will conflict with gnat's first choice
+# of shmem key.
+my $gnat_dir_stat = stat($gnat->data_dir);
+defined($gnat_dir_stat) or die('unable to stat ' . $gnat->data_dir);
+my $gnat_inode = $gnat_dir_stat->ino;
+note "gnat's datadir inode = $gnat_inode";
+
+my $gnat_conflict_shm =
+  IPC::SharedMem->new($gnat_inode, 1024, IPC_CREAT | S_IRUSR | S_IWUSR);
+
+$gnat->start;
+log_ipcs();
+
+$gnat->restart;    # should keep same shmem key
+log_ipcs();
 
 # Upon postmaster death, postmaster children exit automatically.
 $gnat->kill9;
 log_ipcs();
-$flea->restart;       # flea ignores the shm key gnat abandoned.
-log_ipcs();
 poll_start($gnat);    # gnat recycles its former shm key.
 log_ipcs();
 
-# After clean shutdown, the nodes swap shm keys.
-$gnat->stop;
-$flea->restart;
+note "removing the conflicting shmem ...";
+$gnat_conflict_shm->remove;
 log_ipcs();
-$gnat->start;
+
+# Upon postmaster death, postmaster children exit automatically.
+$gnat->kill9;
+log_ipcs();
+poll_start($gnat);    # gnat will now use its normal shmem key.
 log_ipcs();
 
 # Scenarios involving no postmaster.pid, dead postmaster, and a live backend.
@@ -152,24 +123,18 @@ print STDERR $single_stderr;
 like($single_stderr, $pre_existing_msg,
 	'single-user mode detected live backend via shared memory');
 log_ipcs();
-# Fail to reject startup if shm key N has become available and we crash while
-# using key N+1.  This is unwanted, but expected.  Windows is immune, because
-# its GetSharedMemName() use DataDir strings, not numeric keys.
-$flea->stop;    # release first key
-is( $gnat->start(fail_ok => 1),
-	$TestLib::windows_os ? 0 : 1,
-	'key turnover fools only sysv_shmem.c');
-$gnat->stop;     # release first key (no-op on $TestLib::windows_os)
-$flea->start;    # grab first key
-# cleanup
+
+# cleanup slow backend
 TestLib::system_log('pg_ctl', 'kill', 'QUIT', $slow_pid);
 $slow_client->finish;    # client has detected backend termination
 log_ipcs();
-poll_start($gnat);       # recycle second key
 
+# now startup should work
+poll_start($gnat);
+log_ipcs();
+
+# finish testing
 $gnat->stop;
-$flea->stop;
-$port_holder->stop if $port_holder;
 log_ipcs();