diff --git a/src/backend/port/sysv_shmem.c b/src/backend/port/sysv_shmem.c
index b28766c..5b7e5db 100644
--- a/src/backend/port/sysv_shmem.c
+++ b/src/backend/port/sysv_shmem.c
@@ -19,6 +19,7 @@
 #include <signal.h>
 #include <unistd.h>
 #include <sys/file.h>
+#include <sys/mman.h>
 #include <sys/stat.h>
 #ifdef HAVE_SYS_IPC_H
 #include <sys/ipc.h>
@@ -46,6 +47,8 @@ typedef int IpcMemoryId;		/* shared memory ID returned by shmget(2) */
 
 unsigned long UsedShmemSegID = 0;
 void	   *UsedShmemSegAddr = NULL;
+static Size AnonymousShmemSize;
+static PGShmemHeader *AnonymousShmem;
 
 static void *InternalIpcMemoryCreate(IpcMemoryKey memKey, Size size);
 static void IpcMemoryDetach(int status, Datum shmaddr);
@@ -220,6 +223,11 @@ IpcMemoryDetach(int status, Datum shmaddr)
 {
 	if (shmdt(DatumGetPointer(shmaddr)) < 0)
 		elog(LOG, "shmdt(%p) failed: %m", DatumGetPointer(shmaddr));
+
+	/* If we mapped anonymous memory to reduce sysv shm pressure, dump it. */
+	if (AnonymousShmem != NULL
+		&& munmap(AnonymousShmem, AnonymousShmemSize) < 0)
+		elog(LOG, "munmap(%p) failed: %m", AnonymousShmem);
 }
 
 /****************************************************************************/
@@ -357,10 +365,52 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 	PGShmemHeader *hdr;
 	IpcMemoryId shmid;
 	struct stat statbuf;
+	Size		allocsize = size;
 
 	/* Room for a header? */
 	Assert(size > MAXALIGN(sizeof(PGShmemHeader)));
 
+	/*
+	 * Many systems allow only very limited amounts of POSIX shm by default.
+	 * To avoid blowing out the limit, we allocate most of our shared memory
+	 * via anonymous mmap, keeping a small POSIX segment just as an interlock
+	 * to protect the data directory.  But we can't do this in the EXEC_BACKEND
+	 * case because such a mapping can't be reattached.
+	 */
+#ifndef EXEC_BACKEND
+	{
+		long	pagesize = sysconf(_SC_PAGE_SIZE);
+
+		/*
+		 * pagesize will, for practical purposes, always be a power of two.
+		 * But just in case it isn't, we do it this way instead of using
+		 * TYPEALIGN().
+		 */
+		AnonymousShmemSize = size;
+		if (size % pagesize != 0)
+			AnonymousShmemSize += pagesize - (size % pagesize);
+
+		/*
+		 * According to the fine manual, MAP_SHARED can be used with
+		 * MAP_ANONYMOUS only in Linux >= 2.4.  What do we do about that?
+		 */
+		AnonymousShmem = mmap(NULL, size, PROT_READ|PROT_WRITE,
+#ifdef MAP_ANONYMOUS
+							  MAP_SHARED|MAP_ANONYMOUS,
+#else
+							  MAP_SHARED|MAP_ANON,
+#endif
+							  -1, 0);
+		if (AnonymousShmem == NULL)
+			ereport(FATAL,
+			 (errmsg("could not map %lu bytes of anonymous shared memory: %m",
+				(unsigned long) AnonymousShmemSize)));
+
+		/* Now we can allocate a minimal SHM block. */
+		allocsize = sizeof(PGShmemHeader);
+	}
+#endif
+
 	/* Make sure PGSharedMemoryAttach doesn't fail without need */
 	UsedShmemSegAddr = NULL;
 
@@ -370,7 +420,7 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 	for (NextShmemSegID++;; NextShmemSegID++)
 	{
 		/* Try to create new segment */
-		memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
+		memAddress = InternalIpcMemoryCreate(NextShmemSegID, allocsize);
 		if (memAddress)
 			break;				/* successful create and attach */
 
@@ -409,7 +459,7 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 		/*
 		 * Now try again to create the segment.
 		 */
-		memAddress = InternalIpcMemoryCreate(NextShmemSegID, size);
+		memAddress = InternalIpcMemoryCreate(NextShmemSegID, allocsize);
 		if (memAddress)
 			break;				/* successful create and attach */
 
@@ -448,6 +498,17 @@ PGSharedMemoryCreate(Size size, bool makePrivate, int port)
 	UsedShmemSegAddr = memAddress;
 	UsedShmemSegID = (unsigned long) NextShmemSegID;
 
+	/*
+	 * If we're using an anonymous segment for offload purposes, we must
+	 * return a pointer to that rather than the small, mostly fake POSIX
+	 * segment.
+	 */
+	if (AnonymousShmem != NULL)
+	{
+		memcpy(AnonymousShmem, hdr, sizeof(PGShmemHeader));
+		return AnonymousShmem;
+	}
+
 	return hdr;
 }
 
@@ -516,6 +577,11 @@ PGSharedMemoryDetach(void)
 			elog(LOG, "shmdt(%p) failed: %m", UsedShmemSegAddr);
 		UsedShmemSegAddr = NULL;
 	}
+
+	/* If we mapped anonymous memory to reduce sysv shm pressure, dump it. */
+	if (AnonymousShmem != NULL
+		&& munmap(AnonymousShmem, AnonymousShmemSize) < 0)
+		elog(LOG, "munmap(%p) failed: %m", AnonymousShmem);
 }